Where did the hot Neptunes go? A shrinking planet holds the answer

ScienceDaily | 12/13/2018 | Staff
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Fishermen would be puzzled if they netted only big and little fish, but few medium-sized fish. This is similar to what happens to astronomers hunting exoplanets. They found a large number of hot planets the size of Jupiter and numerous others a little larger than the Earth (called super-Earths whose diameter does not exceed 1.5 times that of the Earth), but no planets close to their star the size of Neptune. This mysterious "desert" of hot Neptunes suggests two explanations: either such alien worlds are rare, or, they were plentiful at one time, but have since disappeared.

A few years ago, UNIGE astronomers using NASA's Hubble Space Telescope discovered that a warm Neptune on the edge of the desert, GJ 436b, was losing hydrogen from its atmosphere. This loss is not enough to threaten the atmosphere of GJ 436b, but suggested that Neptunes receiving more energy from their star could evolve more dramatically. This has just been confirmed by the same astronomers, members of the national research center PlanetS. They observed with Hubble that another warm Neptune at the edge of the desert, named GJ 3470b, is losing its hydrogen 100 times faster than GJ 436b. The two planets reside about 3.7 million kilometres from their star, one-tenth the distance between Mercury and the Sun, but the star hosting GJ 3470b is much younger and energetic. "This is the first time that a planet has been observed to lose its atmosphere so quickly that it can impact its evolution," says Vincent Bourrier, researcher in the Astronomy Department of the Faculty of Science of the UNIGE, member of...
(Excerpt) Read more at: ScienceDaily
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