Before 1995, most astronomers expected giant extrasolar planets to orbit their stars in quasicircular orbits at a distance of more than a few astronomical units. (1 AU is the mean distance between Earth and the Sun.) In our Solar System, the orbits of the four giant planets—Jupiter, Saturn, Uranus, and Neptune—have semimajor axes ranging from 5.2 to 30 AU and eccentricities no larger than 5.6%.

Since then, more than 100 extrasolar planets have been discovered, all of them giants with at least 10% the mass of Jupiter (0.1 MJ, about twice the mass of Uranus or Neptune). Much smaller Earthlike (“terrestrial”) extrasolar planets would not be massive enough to be detected by current methods.

The newly found planets are strange—not at all what we expected. A significant fraction of the extrasolar giants detected thus far orbit extremely close to their star, that is, at less than 0.1 AU....

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