Planet-Building on the Grandest Scales

Mercury, March/April 2004 Table of Contents

by Ken Rice and Phil Armitage

The extrasolar planets discovered to this time are believed to be giant gaseous planets similar to Jupiter and Saturn. Understanding how large planets form will help us construct an overall picture of how entire planetary systems form.

Research into the formation of planets and planetary systems is largely driven by the desire to understand the origins of life and the possibility of life elsewhere in the Universe. Until very recently the only known planetary system was our own solar system. Apart from the numerous comets and asteroids, it consists of four rocky planets (Mercury, Venus, Earth, and Mars), two giant gaseous planets (Jupiter and Saturn), two ice giant planets (Uranus and Neptune), and one icy planet with a comparably sized companion (Pluto and Charon).

In 1995, however, Michel Mayor and Didier Queloz, working at the Geneva Observatory, reported the discovery of an object with a mass approximately half that of Jupiter orbiting the star 51 Pegasi. This object was detected using the radial velocity, or “Doppler wobble,” technique; the method uncovers the small radial motion imposed by the orbiting planet on its parent star and manifest as small-scale, periodic shifts of features in the star’s spectrum. Although it can be used to calculate accurately the planet’s orbital period, the Doppler-wobble technique can only determine a lower limit to the planet’s mass. To date, in excess of 100 such extrasolar planets, with minimum masses between 1/10 and just over 10 Jupiter masses, have been discovered, and it is now safe to say that in most cases the actual planet mass is close to that calculated.

In one particular case, the extrasolar planet HD209458b was observed to eclipse its central star. Not only did this allow astronomers to determine the planet’s mass almost exactly, but they also found that this planet has a radius 1.4 times that of Jupiter. This, together with the large masses of the extrasolar planets discovered to date, has led researchers to conclude that all the extrasolar worlds are giant gaseous planets similar to Jupiter and Saturn. Although it is highly unlikely that any of these planets could harbour life, an understanding of the formation and evolution of such planets will aid our understanding of the formation of planets and planetary systems in general.

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