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ScienceWeek
SCIENCEWEEK
ScienceWeek
February 7, 2003
Vol. 7 Number 6
An Online Digest of Research in the Sciences
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We may or may not be majestic as a species, but if one considers
an astronomer sitting alone on a cold night at a telescope on a
mountain top, one must conclude we are certainly obsessed with
knowing what and where we are. -- Anonymous.
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A note from the editors of ScienceWeek: The word "astronaut" is
derived from the Greek for "sailor to the stars". A few days ago
we lost seven of our sailors. We are indeed a lonesome species
with a will to learn and explore. We will go on. As stated many
years ago by the astronomer Edwin Hubble, "The urge is older than
history. It is not satisfied and it will not be suppressed."
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Section 1
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Thematic Issue: Extrasolar Planets
1. Introduction.
2. Planets: Solar and Extrasolar.
3. Formation of Planetary Systems.
4. Detection of Extrasolar Planets.
5. Giant Extrasolar Planets.
6. On Habitable Planets.
New Books Noted
Notices and Subscription Information
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Section 2
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1. INTRODUCTION
ON EXTRASOLAR PLANETS
"For 2000 years, prevailing scientific principles have provided
rationale both for and against the existence of other planetary
systems. In the fourth century BC, Aristotle and Epicurus argued
pro and con, respectively, on the uniqueness of the Earth. In the
late 1500s, Copernicus himself did not espouse an abundance of
planetary systems, but his disciple, Giordano Bruno, staked his
life on it. In the early 1900s, the "spiral nebulae," such as
M51, were widely interpreted as planetary systems in formation
(Moulton 1905). While those nebulae were apparently abundant, the
close-stellar encounter hypothesis to explain them implied that
planetary systems were rare. The rich 2400-year history of
theories on extrasolar planets is given by Dick (1996). Many
modern astrophysicists published articles on the existence and
formation of extrasolar planets (e.g. Eddington 1929, Spitzer
1939, Jeans 1942, Russell 1943). Eddington (1928) wrote that "not
one of the profusion of stars in their myriad clusters looks on
scenes comparable to those which are passing beneath the rays of
the sun". As recently as the early 1940s, the prevailing "Jeans-
Jeffreys" model for planet formation involved tidal stripping of
gas during a close stellar encounter. Russell (1935), Spitzer
(1939) developed dynamical and thermodynamic arguments against
this model, in which the streams of material would never reach
Saturn-like distances of ~10 astronomical units (AU), nor would
the hot gas condense into planets. At the same time, Aitken
(1938) showed that detecting extrasolar planets, either directly
or by the wobble induced in the host star, was beyond the
technical horizon."
G.W. Marcy and R.P. Butler: Annu. Rev. Astron. Astrophys. 1998
36:57
ON EXTRASOLAR PLANETS
"The existence of planets outside our solar system has been a
delicate matter in astronomy ever since the 16th-century
philosopher Giordano Bruno [1548-1600] was burned at the stake
for (among other things) proposing that the universe holds an
infinite number of other worlds. People are no longer set aflame
in the public square for proposing the existence of extrasolar
planets, but the field remains contentious. To date more than 70
planets have been found in orbit around other stars, generating a
considerable amount of excitement in the astronomical community.
Perhaps even more intriguing is the discovery of a few dozen
extrasolar planets that are not affiliated with any star at all.
These so-called "free-floating planets" are among the most
controversial objects yet found in the search for other worlds.
"The problem is that astronomers don't all agree on what it means
to be a planet. Some of the objects found orbiting other stars
are in fact much larger than any of the giant planets in our own
solar system, weighing in at more than 10 times the mass of
Jupiter (although most are less than 3 or 4 Jupiter masses). This
approaches the size threshold of another type of substellar
object known as a "brown dwarf", often described as a "failed
star" because its too small to ignite the fusion of hydrogen in
its core. Brown dwarfs are intermediate in size between planets
and true stars, and the boundary lines at the upper and lower
limits of brown dwarfdom are still a little fuzzy."
J.R. Hurley and M.M. Shara: American Scientist 2002 90:140
ON THE EVIDENCE FOR EXTRASOLAR PLANETS
"What are the characteristics of planetary systems around stars
other than the Sun? How many planets are typical? What are their
masses and compositions? What are the orbital parameters of
individual planets, and how are the paths of planets orbiting the
same star related to one another? These questions are difficult
to answer because planets are so faint that none have yet been
directly observed over interstellar distances. However, more than
two dozen extrasolar planets have been detected during the 1990s
by observations of the wobble that results from their
gravitational tugs on the stars to which they are bound. These
extrasolar planets show the large diversity of planetary systems.
Current research aims at detecting an even greater variety of
extrasolar planetary systems and at explaining systematically
their origins and the origin of our Solar System.
"Several research groups have successfully pursued an indirect
method of detecting extrasolar planets that makes use of Newton's
second law: 'For every action, there is an equal and opposite
reaction.' The stellar wobble betrays the existence of an
invisible orbiting planet. The greater the wobble, the more
massive the planet, and the time to complete one cycle is the
orbital period of the planet. The Doppler effect has been used to
detect these small stellar movements. As a star travels toward
the observer, the light waves are shortened toward the blue.
Conversely, as a star moves away from Earth, the wavelengths are
lengthened toward the red ('redshift'). These Doppler shifts are
quite tiny. The Sun wobbles by only 12.5 m/sec because of the
presence of Jupiter; Saturn induces variations of amplitude 2.7
m/sec on a longer time scale, and the effect of other planets is
substantially less. A reliable detection of this wobble requires
measurement precision of 3 m/sec, which is equivalent to
detecting changes in the wavelengths of starlight by 1 part in
108. The periodic wobble of a star, analyzed with Newton's laws,
gives us the planet's orbital period, the orbital distance, and
its mass multiplied by the unknown sin(i), where (i) is the
inclination of the planet's orbital plane to the line of sight.
"After a century of hopeful but dubious claims, evidence for
planets around other stars finally appears robust. Surveys of
normal stars show that 5% harbor planetary companions having
masses 0.5-8 times that of Jupiter and orbital periods of a few
years or less. Within that mass range, low-mass planets are more
common... To date, 28 extrasolar planet candidates are known.
Their orbits are either very small or quite elliptical, both
properties being different from those of planets within our Solar
System."
J.J. Lissauer et al: Proc. Nat. Acad. Sci. 2000 97:12405
DETECTION OF AN EXTRASOLAR PLANET BY STARLIGHT BLOCKADE
"Beyond the Sun's neighborhood, out in another spiral arm of the
Milky Way, scientists have found a strange planet [TR-56b] that
orbits so close to its parent star that a year passes every 29
hours. Temperatures on the planet, whose mass is similar to that
of Jupiter, are melting hot, and the skies may be cloudy from
time to time, with occasional showers of microscopic droplets of
iron. Of more than 100 planets found thus far around stars
other than the Sun, this one is the most distant: at 8000 light-
years from Earth, it is more than 30 times as far away as any of
the others. It is the first extrasolar planet detected outside
what astronomers call the local neighborhood, the Orion spiral
arm. Strange and remote though the newfound planet may be,
astronomers said today that they were more impressed with the way
it had been discovered. Since the first of the extrasolar planets
was detected in 1995, all had signaled their existence by the
effect of their gravitational tug on the star they orbit; that
tug makes the star wobble slightly. Now the presence of a
companion planet has been betrayed by the perceptible dimming of
its star's light. Like a mosquito flying in front of a
searchlight, this planet passing across the face of its star
causes a periodic blocking of the starlight -- slight, but just
enough to be detectable. Timing this occurrence yields accurate
information on the size of the planet and its orbital pattern."
J.N. Wilford: New York Times 7 Jan 2003 (data reported at Jan
2003 meeting of the American Astronomical Society, Seattle, WA
[US] by D.D. Sasselov et al, Harvard University, US).
EXTRASOLAR PLANETS: HOT JUPITERS
"Among the 100 or so extrasolar planets discovered to date, the
most bizarre are the dozen or so Jupiter-mass planets that orbit
their parent stars with periods between three and seven days.
These so-called 'hot Jupiters' offer the most immediate chances
for direct detection and characterization. At present only one of
these planets has been found to transit the face of its parent
star. Its radius is that of a slightly inflated gas giant. Models
of the structure and evolution of these planets are beginning to
show how clues to their evolutionary history may be encoded in
their mass-radius-age relations. Over the next few years, the
prospects are good for discovery of many more transiting systems
that can be used to infer mass-radius relations for irradiated
planets. At the same time, new optical and infrared spectral
separation techniques will uncover the role played by the
chemistry and physics of irradiated gas-giant atmospheres in
regulating their cooling and contraction."
A.C. Cameron: Astron. & Geophys. 2002 43:4.21.
EXTRASOLAR PLANETOLOGY:
"Astronomers have had plenty of luck lately finding planets
circling other stars. But they've had no guarantees that the
greatest prizes--planetary systems like our own, with a potential
for life--are out there to be found. Due to limitations of the
searches so far, the 77 newly discovered extrasolar planets
either are gas giants orbiting much closer to their stars than
Jupiter or are far more massive than Jupiter. No one can yet
detect the most prominent hallmark of our solar system: planets
resembling Jupiter in mass (at 70% of the solar system's total
planetary mass) and in orbital distance (five times Earth's)."
R.A. Kerr: Science 2002 295:605
EXTRASOLAR PLANETS AND ASTROBIOLOGY
"Astrobiology, as originally conceived, addressed far more than
just the search for life in our Solar System. It involved
understanding the current states of planets; how they formed and
the nature of their initial states; the evolutionary processes
that, over 4.5 billion years, led to their current states; and
how those same processes might have operated in other planetary
systems. It was about understanding planetary habitability and
planetary non-habitability, as well as the actual distribution of
life in our Solar System and elsewhere in our Galaxy.
"To achieve these goals, we need to know what the 'building
blocks' were at time zero, before recognizable planets emerged
from the disk of gas and dust surrounding the young Sun. The
young Sun itself is the product of the collapse of an irregular,
cold, slowly rotating cloud of gas and dust that formed from the
debris from earlier generations of stars that lived during the
first 10 billion years of the Universe. What were the diverse
processes of planetary evolution? Why, for example, did Earth,
Venus and Mars end up so different from each other? The study of
planets and satellites that are hostile to life -- as well as
investigations of those that might support it -- provides insight
about the starting conditions and evolutionary processes that
cause some of them to be hospitable to life and others sterile.
"We need to know how oceans and atmospheres form and operate on
some planets, but also why they do not exist on others. We need
to find out why some planets have magnetic fields and some do
not, the longevity and stability of these fields, and their role
in the evolution of atmospheres or in protecting life from
hazardous cosmic rays. We need to know the bombardment history of
planets to understand whether massive impact events snuffed out
early attempts by living things to flourish. We need to
understand how planetary systems form -- for example, is a giant
planet, such as Jupiter, essential for complex life to evolve
because it is a 'cosmic vacuum cleaner', sweeping up comets that
might have had severe impact effects on smaller planets'
environments?
"Astrobiology requires a rich backdrop of diverse missions,
observations, laboratory experiments and theoretical calculations
to achieve its promise of understanding the connection between
life and planets. This broad picture is needed to understand how
these same processes might have occurred in other solar systems,
and whether there are likely to be habitable planets elsewhere."
M.J. Drake and B.M. Jakosky: Nature 2002 415:733.
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2. PLANETS: SOLAR AND EXTRASOLAR
Jack J. Lissauer (National Aeronautics and Space Administration,
US) discusses solar and extrasolar planets, the author making the
following points:
1) What is a planet? Five planets, or "wandering stars", were
known to the ancients: Mercury, Venus, Mars, Jupiter and Saturn.
The astronomical revolution brought about by Copernicus [1473-
1543], Kepler [1571-1630] and Newton [1642-1727] showed that
these objects were more akin to the Earth than to the Sun and
other stars. Thus our home orb was added to the list of known
planets. Then, in 1781, the scientific world was taken by
surprise when amateur telescope-maker William Herschel [1738-
1822] announced the discovery of a more distant planet,
subsequently named Uranus. In 1800, the small planetary object
Ceres was discovered in orbit between Mars and Jupiter, and tens
of thousands of even smaller minor planets asteroids have
since been detected in that region. The planet Neptune signaled
its presence through its gravitational effect on the orbit of
Uranus, and was first actually seen in 1846. And Pluto, the
furthermost Solar System planet known to us, appeared in a
careful optical search carried out by Clyde Tombaugh in 1930.
2) In the 1960s, with great fanfare, the discovery of first one,
and then two Jupiter-like planets in orbit around Barnard's star
was announced. Only 6 light years away -- but still too faint to
see with the unaided eye -- Barnard's star is one of the Sun's
nearest neighbors (only the Alpha Centauri system is closer). But
by the 1970s, the evidence for these purported planets was
discredited. More claims of the discovery of the first extrasolar
planet, or "exoplanet", continued to capture newspaper headlines,
but these too failed to stand up to scrutiny.
3) It was only after decades of false leads that in 1991 two bona
fide extrasolar planets were detected(1), and this discovery has
stood the test of time. Exoplanets are small, very faint objects,
located close to much brighter stars. The planets themselves have
not been seen, but instead they have been identified by the
gravitational tugs that they exert on their stars. About 100
exoplanets are now known; most are comparable in mass to Jupiter,
and have orbital periods of a few years or less. Astronomers are
amassing a variety of detection techniques to better assess the
diversity of planetary systems within our Galaxy. And the hunt is
on for a true analogue of our Solar System that has an Earth-like
planet, perhaps harboring life as we know it.
4) For exoplanets, the question is not whether an object is too
small to call a planet (small objects are difficult to detect),
but rather whether it is too large. A star maintains itself
against gravitational collapse using energy released by nuclear
fusion in its interior; only objects at least 7 8% as massive as
our Sun can maintain sufficiently high temperatures in their
interiors to become stars. In comparison, the most massive planet
in our Solar System, Jupiter, has less than 0.1% of the mass of
the Sun. Various definitions of a planet have been proposed, some
based on mass, or the origins of the body, or on its current
orbit. The provisional definition adopted by the International
Astronomical Union's working group on extrasolar planets is an
object that is in orbit about a star and that is smaller than the
limit for deuterium fusion to occur (about 13 times the mass of
Jupiter).(2-5)
References (abridged):
1. Wolszczan, A. & Frail, D. Nature 255, 145-147 (1992).
2. Alcock, C. et al. Nature 414, 617-619 (2001).
3. Peale, S. J. Icarus 127, 269-289 (1997).
4. Wolszczan, A. et al. Astrophys. J. 528, 907-912 (2000).
5. Mayor, M. & Queloz, D. Nature 378, 355-359 (1995).
Nature 2002 419:355
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3. FORMATION OF PLANETARY SYSTEMS
ON PLANET FORMATION
Jack J. Lissauer (National Aeronautics and Space Administration,
US) discusses planet formation, the author making the following
points:
1) Models of planet formation have been developed primarily to
explain the existence of planets and smaller bodies within our
Solar System. The major planets have almost circular orbits in
roughly the same plane, suggesting that they formed from a disk
of material orbiting the Sun. The inner planets, moons and
asteroids have a rocky composition, whereas most moons and small
bodies beyond the asteroid belt are rich in ice. All of these
bodies grew by condensing the material around them. The variation
in composition implies that the temperature of the disk material
decreased away from the centre of the Solar System.
2) The four jovian planets (Jupiter, Saturn, Uranus and Neptune)
have large masses but low densities, so they must mainly be
composed of light materials, such as hydrogen and helium. The
most popular models for the formation of giant planets begin with
the accumulation of a core of rock and ice roughly ten times the
mass of Earth. The core grows into a giant planet by
gravitationally accumulating hydrogen and helium gas from the
surrounding protoplanetary disk. These models generally require a
few million years to form a Jupiter-like planet. Although this is
rapid compared to the 20 100 million years believed necessary to
form Earth-like planets, giant planets require a substantial gas
reservoir to complete their growth, whereas rocky planets can
keep growing in a gas-free environment like that of our Solar
System today. So according to current models of planet formation,
these giant planets must accumulate gas fairly rapidly.
3) Do giant planets form only in unusually long-lived
protoplanetary disks, or can they accumulate around most stars?
The measurements of Thi et al (Nature 2001 409:60) suggest that
gas remains in many protoplanetary disks long enough to form
Jupiter-like planets. Massive dust disks are known to occur
around most stars less than 1 million years old, but few stars
older than 5 million years possess such disks. Thi et al (2001)
measured the amount of H2 gas in the smallish dust disks found
around three stars between 8 million and 30 million years old.
Previous observations of these stars, based on measurements of
carbon monoxide gas, suggested that there is surprisingly little
gas left around the star, giving a much lower gas-to-dust ratio
than found in the interstellar medium (which is mostly hydrogen).
But Thi et al's hydrogen measurements show that these stars have
around one Jupiter mass of H2 in their dust disks, with gas-to-
dust ratios similar to that in the interstellar medium. One of
the three disks may still have enough gas for the growth of
Jupiter-mass planets. The presence of H2 but not carbon monoxide
may be explained by carbon monoxide condensing onto dust grains
in icy conditions, or being destroyed by ultraviolet starlight.
Nature 2001 409:23
Related Background:
EVIDENCE FOR DUST GRAIN GROWTH IN YOUNG CIRCUMSTELLAR DISKS
H.B. Throop et al (University of Colorado Boulder, US) discuss
circumstellar disks, the authors making the following points:
1) The growth of dust grains orbiting young stars represents the
first stage of planet formation (1). However, stars born in
massive star-forming regions such as the Orion nebula are heated
by intense ultraviolet (UV) radiation from nearby O and B stars,
and the gas and dust in their disks can be lost in less than
10^(5) years (2). Planet formation in such environments may
therefore be inhibited if it requires substantially longer time
than this (3). But, if growth to large particles can occur before
removal of the gas and small particles, planets may nevertheless
form from these disks.
2) The authors present a study in which visual and near-infrared
(IR) wavelength images obtained with the Hubble Space Telescope
(HST) are used to demonstrate that particles in Orion's largest
disk have grown to radii larger than 5 microns. Furthermore, the
absence of millimeter-wavelength emission may provide evidence
that grains have grown to sizes larger than a few millimeters.
The authors develop a grain evolution model incorporating the
effects of photo-ablation that demonstrates that the time scale
for grain growth can be shorter than the photo-evaporation time.
It is thought that the majority of stars in the Galaxy form in
photo-evaporating regions such as the Orion nebula (4); if this
is true, then giant planets and Kuiper belts of icy bodies around
stars are probably rare unless they are formed very rapidly.
3) Solar system-sized circumstellar disks in the Orion nebula
were first inferred from radio observations of dense ionized
regions surrounding young low-mass stars (5). HST subsequently
yielded images of extended circumstellar material surrounding
over half of the observed 300 young low-mass stars in the core of
the Orion nebula. Most of these "proplyds" consist of comet-
shaped ionized envelopes pointing directly away from the
brightest stars in the nebula. Proplyds are believed to contain
evaporating circumstellar disks, and over 40 disks have been
resolved on HST images. More than 25 are found inside ionized
envelopes, whereas 15 are seen purely in silhouette against the
background light of the nebula.
4) In summary: Hundreds of circumstellar disks in the Orion
nebula are being rapidly destroyed by the intense ultraviolet
radiation produced by nearby bright stars. These young, million-
year-old disks may not survive long enough to form planetary
systems. Nevertheless, the first stage of planet formation -- the
growth of dust grains into larger particles -- may have begun in
these systems. Observational evidence for these large particles
in Orion's disks is presented. A model of grain evolution in
externally irradiated protoplanetary disks has been developed by
the authors and predicts rapid particle size evolution and sharp
outer disk boundaries. The authors discuss implications for the
formation rates of planetary systems.
References (abridged):
1. S. V. W. Beckwith, T. Henning, Y. Nakagawa, in Protostars and
Planets IV, V. Mannings, A. P. Boss, S. S. Russell, Eds. (Univ.
of Arizona Press, Tucson, AZ, 2000), pp. 533-558.
2. C. J. Henney, C. R. O'Dell, Astron. J. 118, 2350 (1999).
3. H. Stoerzer and D. Hollenbach, Astrophys. J. 495, 853 (1998).
4. F. M. Walter, J. M. Alcala, R. Neuhauser, M. Sterzik, S. J.
Wolk, in Protostars and Planets IV, V. Mannings, A. P. Boss, S.
S. Russell, Eds. (Univ. of Arizona Press, Tucson, AZ, 2000), pp.
273-298.
5. E. Churchwell, M. Felli, D. O. S. Wood, M. Massi, Astrophys.
J. 321, 516 (1987).
Science 2001 292:1686
Related Background:
FORMATION OF GIANT PLANETS BY FRAGMENTATION OF PROTOPLANETARY
DISKS
L. Mayer et al (University of Washington, US) discuss giant
planet formation, the authors making the following points:
1) Approximately 100 extrasolar planets have been detected by the
wobble they induce on their star (1,2). Their masses range from
about one Jupiter mass (MJ) to more than 10 MJ and have orbits
ranging from nearly circular to very eccentric. In the standard
core-accretion model, giant planets might require longer than
10^(6) years to form (3,4), which could exceed observed disk
lifetimes (5). In particular, more than 80% of the stars in the
Galaxy probably formed in dense clusters like those in the Orion
nebula, where the ultraviolet radiation of bright stars can
ablate the gaseous disk in far less than a million years (5).
2) Hence giant planet formation must occur quickly, or such
planets would be rare. Even in the case where a large solid core
is assembled rapidly enough, torques acting between the disk and
the protoplanets are believed to induce its complete inward
migration in a few thousand years. Planets could therefore sink
toward the star before being able to accrete the large gaseous
masses observed.
3) Alternatively, giant planets could coagulate directly in the
gas component as a result of gravitational instabilities in a
cold disk with a mass comparable to that adopted in the core-
accretion model. Simulations done with codes that solve the
hydrodynamical equations on a fixed grid show that slightly
perturbed disks form strong spiral arms and overdensities at R >
10 astronomical units (AU), where the temperature can be lower
than 60 K. The trigger of the instability might come from
material of the protostellar cloud infalling onto the disk. If
these condensations are long-lasting and can contract to
planetary densities, gravitational instability would be the
prevailing formation mechanism for giant planets because it takes
less than a thousand years. Solid cores with masses as low as
currently estimated for Jupiter (between 0 and 10 Earth masses)
could then form inside the gaseous protoplanets due to dust and
planetesimals driven there by local pressure gradients in a few
thousand years.
4. In summary: The authors report they have studied the evolution
of gravitationally unstable protoplanetary gaseous disks with the
use of three-dimensional smoothed particle hydrodynamics
simulations with unprecedented resolution. The authors have
considered disks with initial masses and temperature profiles
consistent with those inferred for the protosolar nebula and for
other protoplanetary disks. The authors demonstrate that long-
lasting, self-gravitating protoplanets arise after a few disk
orbital periods if cooling is efficient enough to maintain the
temperature close to 50 K. The resulting bodies have masses and
orbital eccentricities similar to those of detected extrasolar
planets.
References (abridged):
1. G. W. Marcy and R. P. Butler, Publ. Astron. Soc. Pac. 112, 137
(2000).
2. ___, Annu. Rev. Astron. Astrophys. 36, 57 (1998).
3. J. B. Pollack, et al., Icarus 124, 62 (1996).
4. J. Lissauer, Nature 409, 23 (2001).
5. J. Bally, L. Testi, A. Sargent, J. Carlstrom, Astron. J. 116,
854 (1998).
Science 2002 298:1756
Related Background Brief:
A PAIR OF RESONANT PLANETS ORBITING GJ 876. Precise Doppler
measurements during 6 yr from the Lick and Keck observatories
reveal two planets orbiting GJ 876 (M4V). The orbital fit yields
companion masses of M sin i = 0.56 and 1.89 M-J, orbital periods
of P = 30.1 and 61.0 days, semimajor axes of a = 0.13 and 0.21
AU, and eccentricities of e = 0.28 and 0.10, respectively. The
orbital periods are nearly in the ratio of 2: 1, unprecedented
among major planets but common among moons and asteroids.
Moreover, the axes of the elliptical orbits appear to be nearly
aligned. The inner companion was not recognized previously owing
to the 2: 1 ratio of periods, which allowed its signature to
masquerade as added orbital eccentricity of the outer planet.
Dynamical simulations show that the system is stable within a
subset of the observed orbital parameters. The stability may be
provided by a mean-motion resonance and the apparent alignment of
the major axes. These planets pose unsolved questions about their
formation and dynamical evolution, which brought them within 0.08
AU of each other and locked them in resonance. G.W. Marcy et al:
Astrophys. J. 2001 556:296.
Related Background Brief:
FORMATION OF THE GIANT PLANETS BY CONCURRENT ACCRETION OF SOLIDS
AND GAS. New numerical simulations of the formation of the giant
planets are presented by the authors, a study in which for the
first time both the gas and planetesimal accretion rates are
calculated in a self-consistent, interactive fashion. The
simulations combine three elements: (1) three-body accretion
cross sections of solids onto an isolated planetary embryo, (2) a
stellar evolution code for the planet's gaseous envelope, and (3)
a planetesimal dissolution code within the envelope, used to
evaluate the planet's effective capture radius and the energy
deposition profile of accreted material. Major assumptions
include: The planet is embedded in a disk of gas and small
planetesimals with locally uniform initial surface mass density,
and planetesimals are not allowed to migrate into or out of the
planet's feeding zone. All simulations are characterized by three
major phases. During the first phase, the planet's mass consists
primarily of solid material. The planetesimal accretion rate,
which dominates that of gas, rapidly increases owing to runaway
accretion, then decreases as the planet's feeding zone is
depleted. During the second phase, both solid and gas accretion
rates are small and nearly independent of time. The third phase,
marked by runaway gas accretion, starts when the solid and gas
masses are about equal. It is engendered by a strong positive
feedback on the gas accretion rates, driven by the rapid
contraction of the gaseous envelope and the rapid expansion of
the outer boundary, which depends on the planet's total mass. The
overall evolutionary time scale is generally determined by the
length of the second phase. The actual rates at which the giant
planets accreted small planetesimals is probably intermediate
between the constant rates assumed in most previous studies and
the highly variable rates used here. Within the context of the
adopted model of planetesimal accretion, the joint constraints of
the time scale for dissipation of the solar nebula and the
current high-Z masses of the giant planets lead to estimates of
the initial surface density (sigma(init)) of planetesimals in the
outer region of the solar nebula. The authors suggest their
results demonstrate that sigma(init) approximate to 10 g cm^(-2)
near Jupiter's orbit and that sigma(init) proportional to a^(-2),
where a is the distance from the Sun. These values are a factor
of 3 to 4 times as high as that of the "minimum-mass" solar
nebula at Jupiter's distance and a factor of 2 to 3 times as high
at Saturn's distance. The estimates for the formation time of
Jupiter and Saturn are 1 to 10 million years, whereas those for
Uranus fall in the range 2 to 16 million years. These estimates
follow from the properties of our Solar System and do not
necessarily apply to giant planets in other planetary systems.
J.B. Pollack et al: Icarus 1996 124:62.
Related Background Brief:
CO AND H3+ IN THE PROTOPLANETARY DISK AROUND THE STAR HD141569.
Massive planets have now been found orbiting about 80 stars. A
long outstanding question critical to theories of planet
formation has been the timescale on which gas-giant planets form;
in particular, stars more massive than the Sun may blow away the
surrounding gas associated with their formation more quickly than
it can be accumulated by the protoplanetary cores. Evidence for a
protoplanet around a Herbig AeBe star (such stars are 2 3 times
more massive than the Sun) would constrain the timescale of
planet formation. The authors report the detection of CO and H3+
emission from the 5 10-million-year-old Herbig AeBe star
HD141569. The authors interpret the CO data as indicating that
the inner disk surrounding the star is past the early phase of
accretion and planetesimal formation, and that most of the gas
has been cleared out to a distance of more than 17 astronomical
units. CO effectively destroys H3+, so their presence in the same
source is surprising. Moreover, H3+ line emission has previously
been detected only from the atmospheres of the giant planets in
the Solar System. The H3+ and CO may therefore be distributed in
the disk at different circumstellar distances, or, alternatively,
H3+ may be located in the extended envelope of a protoplanet.
S.D. Brittain and T.W. Rettig: Nature 2002 418:57.
Related Background:
ON TERRESTRIAL PLANET FORMATION
Q. Yin et al (Harvard University, US) discuss terrestrial planet
formation, the authors making the following points:
1) Determining the chronology for the assembly of planetary
bodies in the early Solar System is essential for a complete
understanding of star- and planet-formation processes. It has
been argued that the rate of terrestrial core formation was
limited by accretion, that it started very early, and that it was
largely completed within the first 10 20 Myr or less of Earth
history. Various radionuclide chronometers (applied to
meteorites) have been used to determine that basaltic lava flows
on the surface of the asteroid Vesta formed within 3 million
years (3 Myr) of the origin of the Solar System(1-3). Such rapid
formation is broadly consistent with astronomical observations of
young stellar objects, which suggest that formation of planetary
systems occurs within a few million years after star
formation(4,5). Some hafnium tungsten isotope data, however,
require that Vesta formed later (16 Myr after the formation of
the Solar System) and that the formation of the terrestrial
planets took a much longer time (62-14+4504 Myr).
2) Astronomical observations place a severe constraint on the
formation time of a gas-giant planet (possibly with a solid core
of ten Earth masses) to within a few million years after central
star formation and before complete dissipation of nebula
gas(4,5). It is now well accepted from dynamical models that
Mars-sized bodies will form within 0.1 Myr of the origin of the
Solar System. Dynamic accretion models favor the main growth
stage (60%) of the terrestrial planets (from Mars-sized to Earth-
sized bodies) taking about 10 20 Myr, but the "tail" of accretion
may arguably continue for another 80 90 Myr.
3) The authors report measurements of tungsten isotope
compositions and hafnium tungsten ratios of several meteorites.
The authors suggest their measurements indicate that, contrary to
previous results, the bulk of metal silicate separation in the
Solar System was completed within <30 Myr. These results are
completely consistent with other evidence for rapid planetary
formation(1-5), and are also in agreement with dynamic accretion
models that predict a relatively short time (10 Myr) for the main
growth stage of terrestrial planet formation.
References (abridged):
1. Lugmair, G. & Shukolyukov, A. Early solar system timescales
according to 53Mn-53Cr systematics. Geochim. Cosmochim. Acta 62,
2863-2886 (1998)
2. Srinivasan, G., Papanastassiou, D. A., Wasserburg, G. J.,
Bhandari, N. & Goswami, J. N. Re-examination of 26Al-26Mg
systematics in the Piplia Kalan eucrite. Lunar Planet. Sci. XXXI,
A1795 (2000)
3. Nyquist, L. E., Reese, Y., Wiesmann, H., Shih, C.-Y. & Takeda,
H. Live 53Mn and 26Al in an unique cumulate eucrite with very
calcic feldspar (An 98). Meteor. Planet. Sci. Suppl. 36, A151-
A152 (2001)
4. Brice¤o, C. et al. The CIDA-QUEST large-scale survey of Orion
OB1: Evidence for rapid disk dissipation in a dispersed stellar
population. Science 291, 93-96 (2001)
5. Bodenheimer, P. & Lin, D. N. C. Implications of extrasolar
planets for understanding planet formation. Annu. Rev. Earth
Planet. Sci. 30, 113-148 (2002)
Nature 2002 418:951
Related Background Brief:
EARLY SOLAR SYSTEM TIMESCALES ACCORDING TO MN-53-CR-53
SYSTEMATICS. The authors present results of a study of the Mn-53-
Cr-53 systematics in various solar system objects: angrites,
eucrites, chondrites, diogenites, pallasites, the Earth and the
Moon, and SNC meteorites. The primary goal of this study was to
explore the capabilities of the Mn-53-Cr-53 isotope system as a
chronometer and as a tracer for events in the early solar system,
to obtain chronological information on different classes of
meteorites, and to investigate the indigenous distribution of Mn-
53 in the late nebula. These studies have shown that all
meteorite groups investigated so far have excess Cr-53 relative
to the terrestrial value. A lunar sample exhibits Cr-53/Cr-52.
ratios which are the same as the terrestrial normal. The
angrites, several eucrites, and the pallasites show clear
evidence for the existence of 53Mn during their formation while
other meteorites were isotopically equilibrated after essentially
all 53Mn had decayed. A well defined whole-rock Mn-53-Cr-53
isochron for the HED (Howardite-Eucrite-Diogenite) parent body
was obtained. The isochron indicates that this planetesimal was
essentially totally molten and differentiated similar to 7 Ma
before the angrites crystallized. Using the absolute age of the
angrites as a time marker, this event occurred 4565 Ma, within
present uncertainties, at the same time when high temperature
meteorite inclusions (CAI) were formed in the nebula. The first
basalts were deposited onto its surface within less than 3 Ma.
The bulk Mn/Cr ratios of the HED parent body (presumably Vesta),
the angrites, and the pallasites are consistent with a chondritic
Mn/Cr ratio. The results from the SCN meteorites show that their
Cr-53. excesses are less than half of those found in the other
meteorites. Thus, the characteristic Cr-53/Cr-52 ratio of Mars
(assuming SNCs originate from this planet) are intermediate
between that of the Earth-Moon system and those of the other
meteorites. When these 53Cr excesses are plotted as a function of
the heliocentric distance of the place of origin of the samples
then a linear relationship is indicated. Provided that this
variation is due to the decay of Mn-53 then a radial
heterogeneous distribution of Mn-53 must have existed in at least
the inner early solar system. It is argued that radial
fractionation within the nebula, based on the slightly higher
volatility of Mn as compared to that of Cr, is an unlikely cause
for this distribution. Thus, it must be an intrinsic feature of
the late solar nebula. Stochastic mixing processes at the
planetary embryo stage did obviously not eradicate this
heterogeneity. Based on the Mn-53-Cr-53 systematics in HED
meteorites and in chondrites rather narrow limits are inferred
for the age of the solar system or, more accurately, for the
start of the decay of 53Mn within the solar nebula; the range of
possible values is 4568-4571 Ma. The lower limit is consistent
with the Pb-Pb ages of CAI's. G.W Lugmair and A. Shukolyukov:
Geochimica Et Cosmochimica Acta 1998 62:2863.
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4. DETECTION OF EXTRASOLAR PLANETS
Currently, the identification and study of extrasolar planets
depends for the most part on indirect methods such as those
involving the measurement of perturbations of the observed
brightness or motions of their parent stars. The ideal method
would be direct imaging of extrasolar planets, and this would
considerably enhance the possibilities for understanding their
nature. A major problem in direct imaging of extrasolar planets
is that the bright light from the parent star (more particularly,
its diffracted halo in the imaging apparatus) can easily
overwhelm nearby faint light sources such as orbiting planets.
ON THE DETECTION OF EXTRASOLAR TERRESTRIAL PLANETS
E.B. Ford et al (Princeton University, US) discuss extrasolar
terrestrial planets:
1) The detection of massive planets orbiting nearby stars has
become almost routine, but current techniques are as yet unable
to detect terrestrial planets with masses comparable to that of
Earth. Future space-based observatories to detect Earth-like
planets are being planned. Terrestrial planets orbiting the
habitable zones of stars -- where planetary surface conditions
are compatible with the presence of liquid water --are of
enormous interest because they might have global environments
similar to Earth's and may even harbor life. The light scattered
by such a planet will vary in intensity and color as the planet
rotates, and the resulting light curve will contain information
about the planet's surface and atmospheric properties.
2) The authors report a model that predicts features that should
be discernible in the light curve obtained by low-precision
photometry. For extrasolar planets similar to Earth, the authors
expect daily flux variations of up to hundreds of percent,
depending sensitively on ice and cloud cover as well as on
seasonal variations. The authors suggest this indicates that the
meteorological variability, composition of the surface (e.g.,
ocean versus land fraction), and rotation period of an Earth-like
planet could be derived from photometric observations. Even
signatures of Earth-like plant life could be constrained, or
possibly with further study, even uniquely determined.
Nature 2001 412:885
Related Background:
EXTRASOLAR PLANETS: IMAGING VIA A NULLING-INTERFEROMETER
Approximately 24 years ago, R.N. Bracewell proposed a method for
direct imaging of extrasolar planets that eliminates or at least
reduces the problem of overwhelmed faint light, the method based
on the selective removal of starlight before detection by the
superposition of light from 2 telescopes so that the stellar
wave-fronts destructively interfere. In principle, such a
"nulling interferometer" could be used to search for extrasolar
Earth-like planets through their thermal emission, with
spectroscopic analysis used to identify planets that possess the
atmospheric signatures of life.
P.M. Hinz et al report mid-infrared observations using 2 co-
mounted telescopes of the *Multiple Mirror Telescope that
demonstrate the feasibility of the Bracewell technique. The
authors report that interfering light of unresolved stars is seen
to disappear almost completely, while light from a nearby source
as close as 0.2 *arc seconds remains, as shown by images of
*Betelgeuse. With this star canceled, there remains the thermal
image of its surrounding small dust nebula. The authors suggest
that in the future larger ground-based interferometers that
correct for atmospheric distortions should achieve better
cancellation, and thus allow direct detection of warm Jupiter-
size planets and detection of the faint *zodiacal dust around
other nearby stars.
Nature 1998 395:251
Text Notes:
... ... *Multiple Mirror Telescope: Completed and operational in
1980, this telescope near Tucson, Arizona (US) originally
comprised 6 identical mirrors, each 1.8 meters in diameter,
arranged symmetrically about a central axis, with a complex
alignment and electronic guidance to bring all 6 images to a
common focus.
... ... *arc seconds: (arcsec) A unit of angular measure equal to
1/3600 of a degree. 60 arcsec = 1 arc minute. The full Moon is
approximately 30 arc minutes in diameter.
... ... *Betelgeuse: (Alpha orionis) 10th brightest star in the
sky. Distance estimated at 400 light years. This is an extremely
large semiregular variable star, hundreds of times the diameter
of the Sun, with variations in brightness as swells and contracts
in size.
... ... *zodiacal dust: This refers to particles of 1 to 300
microns in size and originating from decaying comets and
asteroids spiraling inward to a star. In our Solar System, this
is the dust cloud primarily responsible for the "zodiacal light",
a permanent faint sky glow visible from Earth. The glow is
apparently caused by dust particle-scattering of sunlight.
Related Background:
ON MICROLENSING
"The microlensing method is being used to investigate the
distribution of faint, stellar and substellar bodies within our
Galaxy. Microlensing arises from the general relativistic bending
of the light from a distant star by a massive object (the lens)
passing between the source and the observer. Lensing causes the
source to appear to brighten gradually to a few times its usual
intensity over a period of weeks or months. If the lensing star
has planetary companions, then these less massive bodies would
produce brief enhancements in the brightness, provided that the
line of sight from Earth passes close to the planet. Under
favorable circumstances, planets as small as Earth could be
detected. But we would only be able to make statistical estimates
of the properties of individual planets, and often even of the
stars that they orbit, because of the many parameters that
influence microlensed light."
J.J. Lissauer: Nature 2002 419:355
EXPECTATIONS FROM A MICROLENSING SEARCH FOR PLANETS.
R.J. Peale (University of California Santa Barbara, US) discusses
microlensing in the search for planets, the author making the
following points:
1) The statistical distribution of the masses of planets about
stars between the Sun and the center of the Galaxy is constrained
to within a factor of 3 by an intensive search for planets during
microlensing events. Projected separations in terms of the lens
Einstein ring radius yield a rough estimate of the distribution
of planetary semimajor axes with planetary mass. The search
consists of following ongoing stellar microlensing events
involving sources in the center of the Galaxy lensed by
intervening stars with high time resolution 1% photometry in two
colors in an attempt to catch any short-time-scale planetary
perturbations of the otherwise smooth light curve. It is assumed
that 3000 events are followed over an 8-year period, but with
half of the lenses, those that are members of binary systems,
devoid of planets. The remaining 1500 lenses have Solar System-
like distributions of four or five planets.
2) The expectations from the microlensing search are extremely
assumption dependent, with 56, 138, and 81 planets being detected
for three sets of assumptions involving how the planetary masses
and separations vary with lens mass. The events can be covered
from 54 to 62% of the time on average by high-time-resolution
photometry from a system of three or four dedicated 2-m
telescopes distributed in longitude, so 38 to 46% of the
detectable small-mass planets (very short perturbations of the
light curve) will be missed. But perturbations comparable to a
day in length mean all of the detectable Jupiters and Saturns
will in fact be detected as will a large fraction of the
Uranuses.
3) The ground-based observational technique is robust, and
although no follow-up studies can be made, meaningful statistics
on planetary masses and separations can be inferred from such an
intensive search. These statistics, like the inferred data set,
will also be dependent on the assumptions about the nature of the
set of planetary systems. More precisely, the detection of few
Earths gives meaningful statistics for the occurrence of Earths
in the lensing zone, but does not exclude Earths very close to
their stars. But, finding few Jupiters, Saturns, and Uranuses
would have profound implications independent of reasonable
assumptions about the nature of the systems. Finding most of many
giant and sub-giant planets outside the Einstein ring radii of
their respective stars may be a better indicator of the frequency
of Earth-mass planets than direct detection of a few of the
latter.
Icarus 1997 127:269
Related Background Brief:
SHORT-TERM DYNAMICAL INTERACTIONS AMONG EXTRASOLAR PLANETS. The
authors demonstrate that short-term perturbations among massive
planets in multiple planet systems can result in radial velocity
variations of the central star that differ substantially from
velocity variations derived assuming the planets are executing
independent Keplerian motions. The authors discuss two fitting
methods that can lead to an improved dynamical description of
multiple planet systems. In the first method, the osculating
orbital elements are determined via a Levenberg-Marquardt
minimization scheme driving an N-body integrator. The second
method is an improved analytic model in which orbital elements
are allowed to vary according to a simple model for resonant
interactions between the planets. Both of these methods can
determine the true masses for the planets by eliminating the
degeneracy inherent in fits that assume independent Keplerian
motions. The authors apply their fitting methods to the sin(i) GJ
876 radial velocity data and argue that the mass factors for the
two planets are likely in the 1.25-2.0 range. G. Laughlin and
J.E. Chambers: Astrophys. J. 2001 551:L109.
Related Background Brief:
A SEARCH FOR SOLAR-LIKE OSCILLATIONS IN THE STARS OF M67 WITH CCD
ENSEMBLE PHOTOMETRY ON A NETWORK OF 4-M TELESCOPES. The authors
present results from a large observational project directed
toward the detection of solar-like oscillations in an ensemble of
open cluster stars. Seven groups collaborated in 1992 January to
observe twelve stars in M67 with 4 m class telescopes for a one
week period. High quality time series were collected on 22
telescope nights for a total of 156 h. The technique of CCD
ensemble photometry allowed precisions of about 250 mumag per
minute to be reached in the best cases, and provided robust
results in conditions that sometimes were far from "photometric".
The longitude-distributed network, coupled with generally low
noise levels, provided a good window function and yielded
detection thresholds of about 20 mumag (five times solar) for
solar-like oscillations in the best ensemble stars. Sensitivity
to solar-like oscillations over the twelve ensemble stars ranges
from 30% to a factor of three better than obtained previously by
any group. When the simultaneous results for 12 stars (prior most
sensitive result followed from photoelectric photometry on a
single star) is taken into account this project provides a
(multiplexed) factor of 20 to 30 gain over previous experiments.
For two stars the authors derive interesting upper limits for
oscillation amplitudes that are near the lower range predicted by
theory. Over half the stars in the ensemble show suggestive
evidence for oscillations; the authors develop the evidence for,
and the cautions against, claiming detections in these cases.
Given the unique aspects of this project the authors describe in
detail the observation planning process, data acquisition,
reductions, and ensuing analyses. The authors argue that a more
aggressive network campaign could provide a factor of two
sensitivity gain with a resulting high probability of attaining
unambiguous oscillation detections on most of the stars in the
M67 ensemble. R.L. Gilliland et al: Astron. J. 1993 106:2441.
Related Background Brief:
PLANETS ORBITING OTHER SUNS. After a century fraught with false
claims, evidence for planets around other stars finally appears
robust. Infrared imaging and spectroscopy of disks around stars
foreshadow detailed models of the formation and evolution of
planetary systems. Surveys of main-sequence stars show that 5%
harbor companions of (0.5-8)M-JUP within 3 AU, peaked at lowest
masses. Their orbits are either within 0.2 AU or eccentric, and
occasionally both. These odd orbits suggest that dynamics with
gas and planetesimals yield diverse systems and that stable,
coplanar orbits of about nine giant and rocky planets may require
special initial conditions. Far fewer stars (<1%) harbor (5-80)M-
JUP companions. This brown dwarf desert for companions stands in
contrast to the abundant brown dwarfs that are freely floating.
G.W. Marcy and R.P. Butler: Publ Astron Soc Pacific 2000 112:137.
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5. GIANT EXTRASOLAR PLANETS
NEW GIANT EXTRASOLAR PLANETS
In 1995, astronomers reported the first tentative evidence of
planets orbiting stars outside our Solar System, and since then
astronomers have detected perturbations in the motions of dozens
of nearby stars, these perturbations presumably due to the
gravity of planets. Currently, the identification and study of
extrasolar planets depends for the most part on indirect methods
such as those involving the measurement of perturbations of the
observed brightness or motions of their parent stars. One
emerging problem is the classification of a number of giant
extrasolar planets: Are they planets or brown dwarf stars?
Brown dwarf stars are formed by the contraction of a lump of gas
with a mass too small for nuclear reactions to begin in the core.
Such a star has a relatively short-lived luminosity
(approximately 100 million years) as the result of conversion of
gravitational energy to radiation. The surface temperature of a
brown dwarf is below 2500 kelvins. As recently as 1994, brown
dwarfs were "theoretical" stars, with no brown dwarfs considered
to be unambiguously identified; at present, a number of stars
have been recognized as brown dwarfs. In addition to the problem
of classifying apparently supermassive extrasolar giant planets,
there is the even more important problem of explaining their
origin.
Alan P. Bass (Carnegie Institution of Washington, US) presents a
commentary on evidence reported at a meeting (American
Astronomical Society San Diego, US 7-11 January 2001) by Geoff
Marcy (University of California Berkeley, US) and Paul Butler
(Carnegie Institution of Washington, US). Bass makes the
following points:
1) Marcy and Butler have discovered approximately two-thirds of
the 50-plus extrasolar planets known to date. The newest objects
are in orbit around a Sun-like star known as HD168443, the star
located approximately 123 light-years away in the direction of
the constellation Serpens. Previous measurements (1999) indicated
that HD168443 had at least one planet. The latest data show that
the original planet has a mass of at least 7.5 Jupiter-masses,
and an orbital period of 58 days, placing it closer to its star
than Mercury is to our Sun (approximately 0.3 astronomical
units). The second object has an apparent mass of approximately
17 Jupiter-masses and orbits the star with a period of 4.8 years
at a distance of 3 astronomical units, corresponding to the
distance to the Sun of the asteroid belt in our Solar System.
Both objects have non-circular (eccentric) orbits, as do nearly
all the known extrasolar planets orbiting at these distances.
2) Bass points out that HD168443 and its two giant planets form a
3-body system with the larger of the planets massive enough to be
called a brown dwarf star. "Beyond simple classification, there
is the looming question of how a system like HD168443 might be
created." Bass concludes: "Theorists have their work cut out to
explain HD168443's unexpected companions. The planet hunters,
meanwhile, are discovering bizarre solar systems at an alarming
rate."
Nature 2001 409:462
Related Background Brief:
A THEORY OF EXTRASOLAR GIANT PLANETS. The authors present a broad
suite of models of extrasolar giant planets (EGPs), ranging in
mass from 0.3 to 15 Jupiter masses. The models predict luminosity
(both reflected and emitted) as a function of age, mass,
deuterium abundance, and distance from parent stars of various
spectral types. The authors also explore the effects of helium
mass fraction, rotation rate, and the presence of a rock-ice
core. The models incorporate the most accurate available equation
of state for the interior, including a new theory for the
enhancement of deuterium fusion by electron screening, which is
potentially important in these low-mass objects, The authors
suggest the results of their calculations reveal enormous
sensitivity of EGPs to the presence of the parent star,
particularly for G and earlier spectral types. The calculations
also show a strong sensitivity of the flux contrast in the mid-
infrared, between parent star and EGP, to the mass and age of the
EGPs. The authors interpret their results in terms of search
strategies for ground- and space-based observatories in place or
anticipated in the near future. D. Saumon et al: Astrophys. J.
1996 460:993.
Related Background:
GIANT PLANETS VS. BROWN DWARFS
The term "astrometric measurement" refers to a method of
detection infers the presence of a companion to a star by
measuring the position of the star as it orbits the center of
mass of the entire system. From the orbital inclination, the real
mass of the companion can be derived.
Filipe D. Santos (Centro de Fisica da Universidade de Lisboa, PT)
presents a short review of current ideas concerning giant
extrasolar planets and brown dwarf stars. The author makes the
following points:
1) The recent discoveries of planets orbiting nearby Sun-like
stars have revealed that planetary systems can be surprisingly
diverse. The initial discovery in 1995 of the planet around the
star 51 Pegasi was a surprise because it is apparently a planet
with mass about that of Jupiter (at least 0.44 Jupiter-mass) and
an orbital period of only 4.2 days, which implies that the planet
is 20 times closer to its star than Earth is to the sun.
2) Seven additional planets around solar-type stars have since
been discovered, with Jupiter-mass values ranging from 0.44 to
6.84.
3) Two critical questions are, a) Where should we set the
dividing line that distinguishes massive planets from brown
dwarfs? and, b) What are the mechanisms leading to the formation
of massive planets and brown dwarfs?
4) Brown dwarfs are expected to have masses smaller than the
hydrogen-burning limit of approximately 0.075 solar-mass
(approximately 75 Jupiter-mass), but probably larger than the
deuterium-burning limit of 0.013 solar-mass (approximately 13
Jupiter-mass).
5) Like the companion massive planets mentioned, several
companion brown dwarfs to solar-type stars have also been
identified. One method of investigating brown dwarfs involves
astrometric measurements, and in all cases of brown dwarfs
investigated by the astrometric method, the masses are above or
very close to the hydrogen-burning limit. The extant data thus
suggest that the distribution of mass of brown dwarfs does not
extend to masses as small as giant planets. Also, the new
measurements indicate that brown dwarfs orbiting solar-type stars
are very rare.
6) The discovery of Jupiter-mass planets with orbits very close
to their stars poses a considerable problem, because it is
difficult to understand how such planets could form in place.
(Five known Jupiter-mass planets have orbital radii smaller than
the distance from Mercury to the Sun.) The suggestion has been
made that these planets formed at larger distances and migrated
inward, but the proposed migration mechanisms are not yet
empirically distinguishable. The author concludes: "Clearly the
discovery of planetary systems outside our solar system has
opened a Pandora's box of startling phenomena and new questions."
Science 1998 281:359
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6. ON HABITABLE PLANETS
Jack J. Lissauer discusses habitable planets, the author making
the following points:
1) The intellectual and technological advances of the past
century leave us poised at the turn of the millennium to
investigate the possibility of extraterrestrial life along
numerous paths of experimental, observational and theoretical
studies in both the physical and life sciences. Concerning
prerequisites for habitability, the author assumes that
extraterrestrial life would be carbon-based and use liquid water
(characteristics common to all life found on Earth), and the
author defines a "habitable planet" as one capable of supporting
such life.
2) Life on Earth has been able to evolve and thrive thanks to
billions of years of benign climate. Mars seems to have had a
climate sufficiently mild for liquid water to have flowed on its
surface when the Solar System was roughly one-tenth its current
age, but at present, its low atmospheric pressure means that
liquid water is not stable on the martian surface. Venus is too
hot, with a massive atmosphere dominated by carbon dioxide; we
cannot say whether or not young Venus had a mild Earth-like
climate. Indeed, as models of stellar evolution predict that the
young Sun was about 25 per cent less luminous than at present, we
do not understand why Earth, much less Mars, was warm enough to
be covered by liquid oceans 4 billion years ago, when life is
thought to have originated.
3) Carbon dioxide is important for carbon-based life. On Earth,
this compound cycles -- on a wide range of timescales -- between
the atmosphere, the oceans, living organisms, fossil fuels and
carbonate rocks. The carbonate rocks form the largest reservoir,
and are produced by reactions involving water (and in some cases
living organisms). Carbon dioxide is recycled from carbonates
back into the atmosphere as tectonic plates descend into the
Earth's mantle and are heated. Carbonates are not readily
recycled on a geologically inactive planet such as Mars, and they
are not formed on planets like Venus, which lack surface water.
Larger planets of a given composition remain geologically active
for longer, as they have smaller ratios of surface area to mass,
and thus retain heat from accretion and radioactive decay for
longer.
4) Concerning stellar properties and habitability, stars are huge
balls of plasma that radiate energy from their surfaces and
liberate energy through thermonuclear fusion reactions in their
interiors. During a star's long-lived "main-sequence phase",
hydrogen in its core is gradually "burned up" to maintain
sufficient pressure to balance gravity. The star's luminosity
(energy output) grows slowly during this phase, as fusion
increases the mean particle mass in the core and greater
temperature is required to achieve pressure balance. Once the
hydrogen in the core is used up, the star's structure and
luminosity change much more rapidly. Both Sun-like stars and
larger stars expand and become "red giants"; those stars with an
initial mass greater than about eight solar masses can end their
lives in spectacular supernova explosions.
5) In summary: The Earth is teeming with life, which occupies a
diverse array of environments; other bodies in our Solar System
offer fewer, if any, niches that are habitable by life as we know
it. Nonetheless, astronomical studies suggest that many habitable
planets may be present within our Galaxy.(1-5)
References (abridged):
1. Lewis, J. S. Worlds Without End: The Exploration of Planets
Known and Unknown (Helix, Reading, Massachusetts, 1998).
2. Joshi, M. M., Haberle, R. M. & Reynolds, R. T. Icarus 129,
450-465 (1997).
3. Lissauer, J. J. Rev. Mod. Phys. 71, 835-845 (1999).
4. Weidenschilling, S. J. & Cuzzi, J. N. in Protostars and
Planets III (eds Levy, E. H. & Lunine, J. I.) 1031-1060 (Univ.
Arizona Press, Tucson, 1993).
5. Safronov, V. S. Evolution of the Protoplanetary Cloud and
Formation of the Earth and Planets (Nauka Press, Moscow, 1969)
(in Russian); Publication TTF-677, NASA, 1972 (English transl.).
Nature 1999 402:C11
Related Background:
PROSPECTS FOR THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
The conjured image is poignant: intelligent life sprinkled
throughout our Galaxy, each sprinkle separated from the others by
1000 light years, each sprinkle searching for the others with
radio transmitters and receivers, small robotic spacecraft sent
beeping into empty space between the stars, the beeping like a
faint bleating in the dark as the sprinkles search for each
other. Of course, the conjured image may be wrong: there may be
intelligent life dense in the Galaxy; or we may be alone. It does
not matter. For the human species on this planet Earth, the quest
is part of our destiny, part of what we do as a species, and it
will go on as long as we remain civilized.
J.C. Tarter and C.F. Chyba (SETI Institute, US) present a review
of current and future efforts in the search for extraterrestrial
intelligence, the authors making the following points:
1) During the past 40 years, researchers have conducted searches
for radio signals from an extraterrestrial technology, sent
spacecraft to all but one of the planets in our Solar System, and
expanded our knowledge of the conditions in which living systems
can survive. The public perception is that we have looked
extensively for signs of life elsewhere. But in reality, we have
hardly begun to search. Assuming our current, comparatively
robust space program continues, by 2050 we may finally know
whether there is, or ever was, life elsewhere in our Solar
System. At a minimum, we will have thoroughly explored the most
likely candidates, a task not yet accomplished. We will have
discovered whether life exists on Jupiter's moon Europa, or on
Mars. And we will have undertaken the systematic exobiological
exploration of planetary systems around other stars, seeking
traces of life in the spectra of planetary atmospheres. These
surveys will be complemented by expanded searches for intelligent
signals.
2) The authors point out that although the current language is
that of a "search for extraterrestrial intelligence" (SETI), what
is being sought is evidence of extraterrestrial technologies.
Until now, researchers have concentrated on only one specific
technology -- radio transmissions at wavelengths with weak
natural backgrounds and little absorption. No verified evidence
of a distant technology has been found, but the null result may
have more to do with limitations in range and sensitivity than
with actual lack of civilization. The most distant star probed
directly is still less than 1 percent of the distance across our
Galaxy.
3) The authors conclude: "If by 2050 we have found no evidence of
an extraterrestrial technology, it may be because technical
intelligence almost never evolves, or because technical
civilizations rapidly bring about their own destruction, or
because we have not yet conducted an adequate search using the
right strategy. If humankind is still here in 2050 and still
capable of doing SETI searches, it will mean that our technology
has not yet been our own undoing -- a hopeful sign for life
generally. By then we may begin considering the active
transmission of a signal for someone else to find, at which point
we will have to tackle the difficult questions of who will speak
for Earth and what they will say."
Scientific American December 1999
Related Background:
ON CARBON IN THE UNIVERSE
Carbon is a major factor in the evolutionary scheme of the
Universe because of its abundance and its ability to form complex
chemical entities. It is apparently also a key element in the
evolution of prebiotic molecules. The different forms of cosmic
carbon range from carbon atoms and carbon-bearing molecules to
complex solid-state carbonaceous structures, and evidence
gathered during the past decade has considerably enhanced our
understanding of the physical and chemical properties of carbon
materials in space.
Th. Henning and F. Salama (2 installations, DE US) present a
detailed review of the subject, the authors making the following
points:
1) More than 75 percent of the 118 *interstellar and
circumstellar molecules identified to date are carbon-bearing
molecules, and one component of interstellar dust is evidently
carbonaceous. The cosmic evolution of carbon from the
interstellar medium into *protoplanetary disks and
*planetesimals, and finally into habitable bodies, is intrinsic
to the study of the origin of life.
2) Carbon plays an important role in the physical evolution of
the interstellar medium because it is the main supplier of free
electrons in diffuse interstellar clouds, thus contributing to
the heating of interstellar gas.
3) The observation of unidentified ubiquitous molecular and
solid-state features in astronomical spectra, and the realization
that these features are linked to carbonaceous materials, have
resulted in major scientific progress in the past decade.
Laboratory and theoretical studies stimulated by these
astronomical observations have led to a better understanding of
the various forms of cosmic carbon such as polycyclic aromatic
hydrocarbons, carbon-chain molecules, carbon clusters, and
carbonaceous solids. These investigations have also led to the
detection of novel forms of carbon and laid the foundations for
the chemistry of *fullerenes.
4) The authors present the following categorization of carbon in
space:
... a) Carbon-rich circumstellar envelopes around *red giant and
*asymptotic giant branch (AGB) stars: CO, C(sub2)H(sub2), complex
hydrocarbons, gas-phase polycyclic aromatic hydrocarbons.
... b) Diffuse interstellar medium: C+, simple diatomic
molecules, gas-phase polycyclic aromatic hydrocarbons and carbon
chains.
... c) Dense interstellar medium: CO, complex hydrocarbons.
... d) Interstellar material in primitive meteorites: polycyclic
aromatic hydrocarbons.
5) The authors suggest that the widespread distribution of
complex organics in the interstellar medium has profound
implications for our understanding of a) the chemical complexity
of the interstellar medium, b) the evolution of prebiotic
molecules, c) the impact of this evolution on the origin and
evolution of life on early Earth through the exogenous delivery
(by cometary encounters and meteoritic bombardments) of prebiotic
organics.
Science 1998 282:2204
Notes:
*interstellar and circumstellar molecules: In this context, an
interstellar molecule is any molecule that occurs naturally in
clouds of gas and dust in space. In general, a circumstellar
molecule is any molecule that occurs in gas and dust surrounding
a star.
*protoplanetary disks: These are dust disks surrounding young
stars; it is from these disks that planets presumably form.
*planetesimals: Planetesimals are bodies with dimensions of 10^(-
3) to 10^(3) meters that are believed to form planets by a
process of accretion. The term "accretion" refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.
*fullerenes: Fullerenes are large molecules composed entirely of
carbon, with the chemical formula C(sub n), where (n) is any even
number from 32 to over 100. They apparently have the structure of
a hollow spheroidal cage with a surface network of carbon atoms
connected in hexagonal and pentagonal rings.
*red giant: A red giant star is a star in a late stage of
evolution. Having exhausted the hydrogen fuel in its core, the
star is burning elements heavier than hydrogen. It has a surface
temperature of less than 4700 kelvins and a diameter 10 to 100
times that of the Sun.
*asymptotic giant branch (AGB) stars: These are stars that occupy
a strip in the *Hertzsprung-Russell diagram that is almost
parallel to and just above what is called the "giant branch" off
the *Main Sequence. Stars evolve from the horizontal H-R branch
to the asymptotic giant branch when they have exhausted the
helium in their cores and are instead burning helium in a shell.
*Hertzsprung-Russell diagram: The Hertzsprung-Russell diagram is
a plot of stellar absolute magnitude against spectral type, and
is perhaps the most useful diagrammatic aid in astrophysics. It
allows the portrayal of the evolution of a star as occurring
along various paths in the diagram.
*Main Sequence: The Main Sequence is a region on the Hertzsprung-
Russell diagram where most stars lie, including our own Sun. The
evolution of a star can be diagrammed as a movement along the
Main Sequence and an eventual branching off the Main Sequence to
regions associated with various types of old stars.
Related Background Brief:
DETECTION OF CARBONATES IN DUST SHELLS AROUND EVOLVED STARS.
Carbonates on large Solar System bodies like Earth and Mars (the
latter represented by the meteorite ALH84001) form through the
weathering of silicates in a watery (CO3)2-solution. The presence
of carbonates in interplanetary dust particles and asteroids
(again, represented by meteorites) is not completely understood,
but has been attributed to aqueous alteration on a large parent
body, which was subsequently shattered into smaller pieces.
Despite efforts, the presence of carbonates outside the Solar
System has hitherto not been established. The authors report the
discovery of the carbonates calcite and dolomite in the dust
shells of evolved stars, where the conditions are too primitive
for the formation of large parent bodies with liquid water. These
carbonates, therefore, are not formed by aqueous alteration, but
perhaps through processes on the surfaces of dust or ice grains
or gas phase condensation. The authors suggest that the presence
of carbonates which did not form by aqueous alteration indicates
that some of the carbonates found in Solar System bodies no
longer provide direct evidence that liquid water was present on
large parent bodies early in the history of the Solar System. F.
Kemper et al: Nature 2002 415:295
ScienceWeek http://www.scienceweek.com
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