Extrasolar Planet Space Missions

In Chapter 2 we reviewed the current theories of formation of the solar system and put forward explanations for the current position, composition, and mass of the planets. If such theories are sound then they should apply equally well to the planetary systems of other stars, but until relatively recently no "extrasolar" planets had ever been observed and thus there was no way of testing these ideas. Then, in 1995, the first extrasolar planet was observed about the solar-type star 51 Pegasi by Mayor and Queloz (1995) and then about 70Virginis by Marcy and Butler (1996). The technique used involved measuring the frequency of light emitted by the star and observing how this was modulated by the Doppler effect caused by the star's motion about the stellar system barycenter. The planets inferred by these two early studies were both large (with a mass of the order of Jupiter's) and were both very close to their stars (within 0.5 AU), leading them to become known as "Hot Jupiters'' and to the conclusion that these planetary systems are obviously nothing like our own solar system! Since then, many more extrasolar planets have been detected, not just with this radial velocity technique, but also by transits (where the reduction in light from the star as the planet passes in front of it is observed) and gravitational lensing, and the number of known exoplanets currently stands at more than 300. Most of the exoplanets discovered so far lie within a few astronomical units of their star, but the maximum detectable orbital radius is steadily increasing as the techniques become more precise and extend over longer timeframes. Although the radial velocity technique is biased to find heavy, short-period planets, most exoplanets discovered so far have a mass <10 Mj. However, the close "Hot Jupiters'' are by no means uncommon and preliminary studies suggest that approximately 6% of solar-type stars have close Jupiter-sized companions (Marcy and Butler, 1998) within 2AU (Irwin, 2008).

There is a clear need to establish just how typical our own solar system is, and on a more philosophical level to determine whether there are other planets in the galaxy which are more Earth-like and fall within the so-called "habitable zone'', where liquid water may exist on the surface. Such planets would be suitable for the evolution of life and it would be of profound significance indeed if life could be detected to have evolved elsewhere in the universe independently of the life that has evolved here on the Earth. The search for terrestrial extrasolar planets is one of the most exciting fields of research in planetary physics and at this time there are a number of missions on the drawing board which hope to address these questions. Of course a system capable of detecting terrestrial planets is also likely to detect giant planets as well, and thus one of the spin-offs from this program will be determination of the true "average" planetary system and variants, which will provide much stronger constraints on stellar and planetary formation theories than can be gleaned from consideration of our solar system alone. The major new space missions planned in the next 10-20 years to search for extrasolar terrestrial planets will now be reviewed.

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