Lesson 9: Other Worlds, Other Life 9.2 Planets of "Nearby" Stars Evidence for planets around other stars has been sought for many years. Many of the closest stars had their positions (against background stars) measured carefully for signs of wiggles as they rotated about their center of mass (barycenter) with a planetary companion. This astrometric technique was simply not precise enough to do the job, so astronomers turned to a Doppler shift technique where stellar spectral lines were monitored for evidence of a frequency shift, again induced by the star's rotation about the barycenter. The leading U.S. group headed by Marcy and Butler were searching for a Jupiter-sized object in a Jupiter-like (5 AU radius) orbit. They were beat out by the University of Geneva (Switzerland) team of Queloz and Mayor, who found a multi-Jupiter mass planet orbiting at a fraction of 1 AU (which, in our solar system, would put it inside the orbital radius of Mercury)! This type of exoplanet was completely unexpected, to say the least. Many more have been found like it, plus some further out in non-circular orbits. This has thrown the planetary system formation theorists into temporary disarray. It was thought that only rocky, terrestrial-type planets would be located close to a star. The theories predicted that the so-called "gas giants" would orbit farther out. A wealth of new theories have been proposed to explain the recent detections.Some of these have very dire consequences for small inner planets like ours: massive Jupiter-like planets may come cruising through our part of the solar system, ejecting smaller planets in to the star or out into space. Can we live there? Can anything else? We humans cannot visit the surface of a planet 150 times more massive than the Earth. (This is still less than half the mass of Jupiter). That is, assuming there is an accessible surface. These planets are probably more like our gas giant planets. In any case, the gravity and perhaps the gas pressure would crush us. In addition, very few of these planets are near our comfort zone, temperature-wise. They mostly orbit too close to their stars, and thus their temperatures are too hot for us. For reasons discussed earlier, we believe that liquid water may be a near-universal requirement for the development of life, so we wouldn't expect to find any other life on these planets. A zone in a gas giant's atmosphere might accommodate liquid water, possibly at high pressure, but the vertical winds do not make this a stable environment. So none of the planets discovered to date or expected to be found for some time should be capable of harboring life, but... Jupiters in distant enough orbits from their suns might have Europa analogs. We still expect to find terrestrial planets in the future. Have we found any Earth-like planets yet? No, sensitivities must improve to detect smaller planets such as our own. The lightest of the 50 or so planets discovered through mid-2001 is still more than 150 times the mass of our Earth. This corresponds to a velocity induced on the light from the star of some 3m (10ft) per second, a brisk walk. The signal must first be processed to remove the effects of the much faster rotation of the Earth about the Sun, the solar systems' motion through space, and other effects. It may be difficult to improve enough to detect small planets , so astronomers are looking at another technique, where the planet crosses in front of the star, causing a 2% or less dip in the signal strength. This only works when the Earth is in or very near the exoplanet's orbital plane, so many more stars will have to be monitored; thousands in all. This will require looking further afield, beyond present limits of instrumental sensitivity. It is currently expected that Earth-sized planets may become detectable in sufficient numbers for some statistical purposes by the year 2020, if space-borne instruments are funded.