Extrasolar planets

Abstract

The first known extrasolar planet in orbit around a Sun-like star was discovered in 1995. This object, as well as over two dozen subsequently detected extrasolar planets, were all identified by observing periodic variations of the Doppler shift of light emitted by the stars to which they are bound. All of these extrasolar planets are more massive than Saturn is, and most are more massive than Jupiter. All orbit closer to their stars than do the giant planets in our Solar System, and most of those that do not orbit closer to their star than Mercury is to the Sun travel on highly elliptical paths. Prevailing theories of star and planet formation, which are based on observations of the Solar System and of young stars and their environments, predict that planets should form in orbit about most single stars. However, these models require some modifications to explain the properties of the observed extrasolar planetary systems.

Figure 1

Measured values of Msini for planet candidates. The rising mass distribution from 8 to 0.5 Jupiter masses shows that lower-mass planets are more common than more massive ones.

Figure 2

Orbital eccentricity vs. semimajor axis (orbital distance) for extrasolar planet candidates (Earth is included for comparison). Beyond 0.2 Earth–Sun distances [0.2 astronomical units (AU)], all orbits are noncircular, in contrast to the nearly circular orbits of the giant planets within our own Solar System.