ScienceShot: Hot Jupiters Are Loners

J. H. Steffen et al., PNAS Early Edition (2012)

In October 1995 when astronomers announced the first planet orbiting another star like the sun, Ted Koppel was so impressed he featured the discovery on Nightline. The new world was unlike any ever seen: a "hot Jupiter" bearing roughly the same mass as the sun's largest planet but lying so close to its star that it revolved every 4.23 days. However, Koppel may be less jazzed about a new announcement. In a study published online today in the Proceedings of the National Academy of Sciences, astronomers analyzing 63 hot Jupiters (depicted above) detected by NASA's Kepler spacecraft have found no planets comparable in size to Earth orbiting nearby. In contrast, many hot Neptunes—close-in giant worlds with roughly 5% of Jupiter's mass—do have planetary neighbors. The findings may mean that hot Jupiters assume their peculiar orbits after far-off giant planets kick them close to their suns. As the hot Jupiter dashes inward, its gravity ejects any smaller planets near the star, both explaining the absence of close planetary neighbors and suggesting that solar systems with hot Jupiters are unlikely to host life-bearing worlds resembling Earth.

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