ScienceWeek

PLANETARY SCIENCE: ON THE CORES OF JUPITER AND SATURN

The following points are made by William B. Hubbard (Nature 2004 431:32:):

1) In July of 2004, NASA announced that it will fund further study of a proposed mission to Jupiter, as part of its New Frontiers program(1). A prime objective of this orbiter mission, called "Juno", would be to measure Jupiter's gravitational and magnetic fields at very close range, to discern whether the planet has a dense core. Meanwhile, Saumon and Guillot(2) have reported that the existence of a massive core in Jupiter may depend on the validity of an experiment here on Earth.

2) Over the past decade, it has become possible in the laboratory to squeeze hydrogen to pressures more than a million times greater than atmospheric pressure, while simultaneously heating it to temperatures exceeding 10,000 kelvin. Compression experiments on the hydrogen isotope deuterium -- driven by NOVA, a huge laser-implosion device at the US Lawrence Livermore National Laboratory -- seem to show that under such conditions deuterium's density increases as much as sixfold(3). Other experiments, however, find that this hydrogen isotope is less compressible(4). Theoretical calculations have yet to weigh in fully to help sort out the discrepancy. But Saumon and Guillot(2) find that if the NOVA results are right, then their models suggest that Jupiter does indeed have a sizeable core. The implications extend beyond the planned NASA mission to the very definition of what constitutes a planet.

3) Although Jupiter is 300 times more massive than Earth, it has only one-thousandth of the mass of the Sun; Saturn comprises about 100 Earth masses and is the only other hydrogen-rich planet in our Solar System. The formation of either planet by the spontaneous gravitational collapse of such a small mass of hydrogen from the Sun's primordial nebula -- a swirling disk of gas and dust -- has long been considered unlikely, especially because temperatures in the nebula could never be low enough to condense either hydrogen or helium. Furthermore, the Galileo probe spacecraft, which entered Jupiter's atmosphere in 1995, found that the outermost layers (at least) of Jupiter have a "metallicity" about three times that of the Sun. To astrophysicists, the metallicity of a star refers to its enrichment in elements heavier than hydrogen and helium, relative to the Sun. "Metals" comprise a bit more than 1% of the Sun's mass. The high metallicity of the jovian atmosphere would not be expected if the planet were formed by direct collapse from a solar-metallicity nebula(5).

4) It is more likely that the formation of Jupiter and Saturn was initiated by the collapse of primordial nebular gas into the gravitational well created by a solid dense core of refractory (readily condensed) elements. Estimates vary of the minimum mass of core needed to trigger the collapse, but typical values are around 15 Earth masses. To put the problem in perspective, that mass of rock and ice would correspond to about 1000 Earth masses of hydrogen and helium in a solar-metallicity nebula. About a third of that would need to be captured to make a Jupiter, and another tenth to make a Saturn.

References (abridged):

1. http://www.nasa.gov/lb/home/hqnews/2004/jul/

2. Saumon, D. & Guillot, T. Astrophys. J. 609, 1170-1180 (2004)

3. Collins, G. W. et al. Science 281, 1178-1181 (1998)

4. Knudson, M. D., Hanson, D. L., Bailey, J. E., Hall, C. A. & Asay, J. R. Phys. Rev. Lett. 87, 225501 (2001)

5. Hubbard, W. B., Burrows, A. & Lunine, J. I. Annu. Rev. Astron. Astrophys. 40, 103-136 (2002)

Nature http://www.nature.com/nature

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ASTROPHYSICS: INTERIORS OF SOLAR AND EXTRASOLAR GIANT PLANETS

Notes by ScienceWeek:

The planets Jupiter, Saturn, Uranus, and Neptune, often called the "giant planets", have diameters between 3.9 and 11.3 times that of the Earth, and masses of between 14 and 318 times Earth-mass. The giant planets orbit the Sun at a mean distance ranging from 5.21 astronomical units (AU) (1 AU = mean Earth-Sun distance) for Jupiter to 30.06 AU for Neptune, in orbital periods from 11.86 to 164.79 Earth years. All the giant planets have low densities, from 0.7 to 1.8 times that of water, and these planets are apparently composed largely of hydrogen in its molecular or metallic state. The visible surfaces of these planets are believed to be clouds of ammonia or methane. All four giant planets have planetary ring systems and share more than 57 satellites between them.

The following points are made by Tristan Guillot (Science 1999 286:72):

1) An understanding of the structure and composition of the giant planets is rapidly evolving because of a) high pressure experiments that make it possible to study metallic hydrogen and define the properties of its thermodynamic equation of state, and b) spectroscopic and in situ measurements made by telescopes and satellites that allow an accurate determination of the chemical composition of the deep atmosphere of the giant planets. However, the total amount of heavy elements that the solar giant planets contain remains only a broad approximation.

2) The discovery of extrasolar giant planets with masses ranging from that of Saturn to a few times the mass of Jupiter has opened new possibilities for understanding planet composition and formation. Planet evolutionary models predict that gaseous extrasolar giant planets should have a variety of atmospheric temperatures and chemical compositions, with radii estimated to be 0.9 to 1.7 Jupiter-radius, if it is assumed these extrasolar planets contain mostly hydrogen and helium.

3) Prospects for improving our knowledge of the composition of giant planets and their formation will depend on the following future developments: a) the calculation of a new hydrogen and helium equation of state consistent with recent compression experiments; b) accurate measurement by the Cassini orbiter of the chemical composition and temperature structure of the atmosphere of Saturn, and hopefully also of the atmosphere of Jupiter; c) a polar Jupiter orbiter would yield an accurate determination of the gravitational field of the planet and constrain its global composition and dynamic structure; d) important steps in understanding planet formation will come from spectroscopic measurements of the atmospheres of extrasolar giant planets, and from transit detections that would allow the determination of their radii and thus estimates of their global compositions.

Science http://www.sciencemag.org

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EXTRASOLAR PLANETS VS. BROWN DWARFS

Notes by ScienceWeek:

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.

The term "astrometric measurement" refers to a method of detection inferring 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.

The following points are made by Filipe D. Santos (Science 1998 281:359):

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."

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