The existence of giant planets in sub-Mercurian orbits (see epistellar jovians) and in highly elongated orbits (see eccentric jovians) has come as a surprise and forced theorists to revise their understanding of how young planetary systems evolve (see planetary systems, formation). The challenge now is to develop more sensitive techniques capable of finding Earth-class planets, especially those in orbit within the habitable zones of their host stars. These are potentially the most important worlds in the search for life, although, as in the case of Europa and Callisto, large moons of giant planets may also prove to be biologically interesting.
The discovery of (a) extrasolar planets around roughly 12% of the target stars searched (remembering that many lower mass worlds around these stars almost certainly await detection), (b) a planetary system in the case of upsilon Andromedae, and (c) circumstellar disks around stars of widely differing spectral type, has led to heightened optimism that planetary systems are common throughout the Universe. However, some researchers have urged caution. In 1997, Guillermo Gonzalez of the University of Washington, Seattle, and his colleagues, pointed out1 that almost all of the stars around which planets have been detected to date are richer in heavy elements than is the Sun (see heavy element concentration, related to the occurrence of planets). The relative scarcity of such stars might imply strict limits, too, on the numbers of planetary systems. Moreover, argued Gonzalez, the phenomenon of inward orbital migration which might account for the presence of giant planets in orbits close to their host stars would tend to scatter any terrestrial planets that had formed between the migrating planet and the star, resulting in a dearth of Earthlike worlds.2 The next decade should see a clarification of such issues.
Related categories EXTRASOLAR PLANETS AND SUBSTELLAR OBJECTS
PLANETS AND MOONS
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