ScienceWeek

Science 12 October 2007

Vol. 318. no. 5848, pp. 210 - 213

DOI: 10.1126/science.1144358

New Worlds on the Horizon: Earth-Sized Planets Close to Other Stars

Eric Gaidos et al.

The search for habitable planets like Earth around other stars fulfills an ancient imperative to understand our origins and place in the cosmos. The past decade has seen the discovery of hundreds of planets, but nearly all are gas giants like Jupiter and Saturn. Recent advances in instrumentation and new missions are extending searches to planets the size of Earth but closer to their host stars. There are several possible ways such planets could form, and future observations will soon test those theories. Many of these planets we discover may be quite unlike Earth in their surface temperature and composition, but their study will nonetheless inform us about the process of planet formation and the frequency of Earth-like planets around other stars.

The ancients looked at the night sky and wondered what the lights wandering among the fixed stars were. After the Copernican revolution, humanity asked whether any of them—the planets of the solar system—are worlds like ours and support life. The search for habitable worlds now extends to the other stars, around which more than 200 planets have been discovered. Until recently, all discoveries were of planets far more massive than Earth, that is, like Jupiter or Saturn. This is because most of them were discovered by Doppler velocimetry, a technique that measures the motion of the star around the system's center of mass by detecting the alternating Doppler shift of starlight toward red or blue wavelengths (Fig. 1). The shift is proportional to a planet's mass, and thus more-massive planets are easier to detect. The orbits of a few such planets are observed edge-on, and the planets periodically pass in front of (transit) their host star (Fig. 1). The masses of planets on such orbits are known unambiguously, and their diameters and mean densities can be calculated from the small (~1%) fraction of starlight that is occulted as well as knowledge of their stars' diameters. These planets turn out to have densities similar to those of the gas giants in our solar system and are presumed to be made mostly of hydrogen and helium gas.

Such objects fascinate astronomers, but our quest to find Earth-like planets with solid surfaces and conditions suitable for life continues. One condition is the presence of liquid water, and orbits on which a planet's surface temperature permits stable liquid water describe a circumstellar "habitable zone" (1). The detection of a planet like Earth in the habitable zone of even the nearest Sun-like stars is an enormous challenge because its Doppler signature is only 0.3% of a Jupiter-mass planet, it can occult only 0.01% of the star, and its distant orbit means the likelihood of a transit is less than 1 in 200. But the discovery of planets not too unlike the Earth may not be far off: Doppler velocimetry with more stable instruments has recently discovered several objects much less massive than Saturn and as small as five times larger than Earth (2–4); one of these has now been observed to transit its star (5). Like many of the giant planets detected by this method, they are much closer to their host stars than Earth is to the Sun (1 astronomical unit or AU), and so the Doppler shift they induce is larger and more detectable. New instruments, on the ground and in space, will discover still smaller planets. These worlds will also be on close orbits, many will be much hotter than the Earth, and some may have very different compositions. All will help us understand how planets form and the propensity for that process to yield planets like Earth.

BOOK SOURCES:

Planetary Science
Astronomy
Astrophysics
Extra-Solar Planets