'Hot Neptune' sports a tail millions of kilometers long

Mark Garlick/University of Warwick

'Hot Neptune' sports a tail millions of kilometers long

The Hubble Space Telescope has discerned signs of distress in a world 33 light-years away. The planet is as large as Neptune but hotter than Mercury, whirling around a red dwarf in the constellation Leo named Gliese 436 every 2.64 days. When the planet passes in front of its sun, astronomers report online today in Nature, hydrogen atoms spewing from the planet's atmosphere obscure 56% of the star's disk and absorb its ultraviolet light. As shown in this artist’s conception, the hydrogen atoms form a tail resembling a comet's that stretches millions of kilometers behind the planet, which the astronomers estimate has lost up to a tenth of its original atmosphere. In more extreme cases—when a Neptune-sized planet is closer to its sun and thus even hotter—it may shed so much material that it becomes a rocky world like Earth, albeit far hotter than the planet we call home.