Abstract
Several distinctly different techniques have detected almost 500 planets or- biting around main-sequence stars, 45 multiple planet systems, and a number of extrasolar planets have been the subject of direct study. Hundreds of other candidate planets de- tected by the Kepler spacecraft await confirmation of their existence. Planets are thus common phenomena around stars, and the prospects seem good in the next few years for establishing statistics on the occurrence of Earth-sized planets. Extension of the most successful technique of Doppler spectroscopy in sensitivity to detect Earth-mass planets around Sun-like stars will be limited by the noise generated by the stellar photospheres themselves. The James Webb Space Telescope will have the capability to measure at- mospheric abundances of certain gases and of liquid water on extrasolar planets, includ- ing superEarths within a factor of two of the radius of the Earth. The ultimate goal of measuring the atmospheric composition of an Earth-sized planet orbiting at 1 AU around a star like the Sun remains a daunting challenge that is perhaps twenty years in the future. Roughly 90% of all exoplanet discoveries are made through the radial velocity (also called doppler spectroscopic) technique. This technique is most effectively done from the ground, and requires very high precisions (4) in measurement of Doppler shifted spectral lines from the photosphere of a star orbiting around the common center-of-mass (barycentric motion) with its planet or planetary system (multiple planets); these precisions are discussed later. The quantity derived from the measurements is not the mass but rather the mass times the sine of the inclination of the normal to the plane of the
Highlights
The prediction that planets would be common, first on philosophical on scientific grounds, has been confirmed with the discovery of some 492 planets orbiting stars other than the Sun [1]
Astrometry measures the positions of stars in the sky and the transverse periodic motion of a star possessing planets
Ground-based radial velocity can be considered a strict replacement for a space-based astrometry mission in terms of detecting Earth-mass planets if and only if 0.1 m/s radial velocity can be achieved around at least 50–100 Sun-like stars, which is mainly contingent on overcoming the stellar noise sources to which radial velocity is intrinsically much more sensitive than astrometry
Summary
The prediction that planets would be common, first on philosophical on scientific grounds, has been confirmed with the discovery (at the time of writing) of some 492 planets orbiting stars other than the Sun [1]. At least one of every ten stars like the Sun has one or more planets, and with the latest Kepler data most of these will have sizes within a factor of a few that of the Earth. The first extrasolar planet, or exoplanet, residing in the liquid water “habitable zone” has been found [3]. There is every expectation that the number of known planets will grow, and with that the number of systems that resemble their own.
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