Astronomers have long wondered if the process of planetary formation has accompanied the birth of stars other than the Sun. The discovery of extrasolar planets—planets circling other stars—would help clarify their ideas of the formation of Earth’s solar system by removing the handicap of being able to study only one example. Extrasolar planets were not expected to be easy to see directly with Earth-based telescopes because such small and dim objects would usually be obscured in the glare of the stars that they orbit. Instead, efforts were made to observe them indirectly by noting the gravitational effects that they exerted on their parent stars—for example, slight wobbles produced in the parent star’s motion through space or, alternately, small periodic changes in some property of the star’s radiation, caused by the planet’s tugging the star first toward and then away from the direction of Earth. Extrasolar planets also could be detected indirectly by measuring the change in a star’s apparent brightness as the planet passed in front of (transited) the star.

After decades of searching for extrasolar planets, astronomers in the early 1990s confirmed the presence of three bodies circling a pulsar—i.e., a rapidly spinning neutron star—called PSR B1257+12. The first discovery of a planet revolving around a less-exotic, more-sunlike star took place in 1995, when the existence of a massive planet moving around the star 51 Pegasi was announced. By the end of 1996 astronomers had indirectly identified several more planets in orbit around other stars, but only in 2005 did astronomers obtain the first direct photographs of what appeared to be an extrasolar planet. Hundreds of planetary systems are known.

Included among these many discoveries were systems comprising giant planets the size of several Jupiters orbiting their stars at distances closer than that of the planet Mercury to the Sun. Totally different from Earth’s solar system, they appeared to violate a basic tenet of the formation process discussed above—that giant planets must form far enough from the hot central condensation to allow ice to condense. One solution to this dilemma has been to postulate that giant planets can form quickly enough to leave plenty of matter in the disk-shaped solar nebula between them and their stars. Tidal interaction of the planet with this matter can cause the planet to spiral slowly inward, stopping at the distance at which the disk material no longer is present because the star has consumed it. Although this process has been demonstrated in computer simulations, astronomers remain undecided whether it is the correct explanation for the observed facts.

In addition, as discussed above with regard to Earth’s solar system, the enrichment of argon and molecular nitrogen detected on Jupiter by the Galileo probe is at odds with the relatively high temperature that must have existed in the vicinity of the snow line during the planet’s formation. This finding suggests that the snow line may not be crucial to the formation of giant planets. The availability of ice is certainly key to their development, but perhaps this ice formed very early, when the temperature at the nebula’s midplane was less than 25 K. Although the snow line at that time may have been much closer to the Sun than Jupiter is today, there simply may not have been enough matter in the solar nebula at those distances to form a giant planet.

Most of the extrasolar planets discovered in the first decade or so following the initial discoveries have masses similar to or greater than that of Jupiter. As techniques are developed for detecting smaller planets, astronomers will gain a better understanding of how planetary systems, including the Sun’s, form and evolve.