Free Astronomy Magazine March-April 2019

54 MARCH-APRIL 2019 SPACE CHRONICLES shrink down to the most common type of known exoplanet, mini- Neptunes — plan- ets with heavy, hydrogen-domi- nated atmos- pheres that are larger than Earth but smaller than Neptune. Eventually, these planets may down- size even further to become super- Earths, more mas- sive, rocky ver- sions of Earth. “The question has been, where have the hot Neptunes gone?” said Bour- rier. “If we plot planetary size and distance from the star, there’s a desert, a hole, in that distribution. That’s been a puz- zle. We don’t re- ally know how much the evaporation of the atmos- pheres played in forming this desert. But our Hubble observations, which show a large amount of mass loss from a warm Neptune at the edge of the desert, is a direct confirma- tion that atmospheric escape plays a major role in forming this desert.” The researchers used Hubble’s Space Telescope Imaging Spectrograph to detect the ultraviolet-light signature of hydrogen in a huge cocoon sur- rounding the planet as it passed in front of its star. The intervening co- coon of hydrogen filters out some of the starlight. These results are inter- preted as evidence of the planet’s atmosphere bleeding off into space. The team estimates that the planet has lost as much as 35 percent of its material over its lifetime, because it was probably losing mass at a faster rate when its red-dwarf star was younger and emitting even more ra- diation. If the planet continues to rapidly lose material, it will shrink down to a mini-Neptune in a few bil- lion years. Hydrogen probably isn’t the only element evaporating away: it may be a tracer for other material streaming off into space. The re- searchers plan to use Hubble to hunt for elements heavier than hydrogen and helium that have hitched a ride with the hydrogen gas to escape the planet. “We think that the hydrogen gas could be dragging heavy ele- ments such as carbon, which reside deeper in the atmosphere, upward and out into space,” Bourrier said. The observations are part of the P a n c h r oma t i c Comparative Exo- planet Treasury (PanCET) survey, a Hubble program to look at 20 exo- planets, mostly hot Jupiters, in the first large- scale ultraviolet, visible, and in- frared compara- tive study of distant worlds. Observing the evaporation of these two warm Neptunes is en- couraging, but team members know they need to study more of them to confirm predictions. Un- fortunately, there may be no other planets of this class residing close enough to Earth to observe. The problem is that hydrogen gas cannot be detected in warm Nep- tunes farther away than 150 light- years from Earth because it is obscured by interstellar gas. GJ 3470b resides 97 light-years away. However, helium is another tracer for material escaping a warm Nep- tune’s atmosphere. Astronomers could use Hubble and the upcoming NASA James Webb Space Telescope to search in infrared light for helium, because it is not blocked by interstel- lar material in space. “Looking for helium could expand our survey range,” Bourrier said. “Webb will have incredible sensitiv- ity, so we would be able to detect helium escaping from smaller plan- ets, such as mini-Neptunes.” T his graphic plots exoplanets based on their size and distance from their star. Each dot represents an exoplanet. Planets the size of Jupiter (located at the top of the graphic) and planets the size of Earth and so-called super-Earths (at the bottom) are found both close to and far from their star. But planets the size of Neptune (in the middle of the plot) are scarce close to their star. This so- called desert of hot Neptunes shows that such alien worlds are rare, or, they were plentiful at one time, but have since disappeared. The detection that GJ 3470b, a warm Neptune at the border of the desert, is fast losing its atmos- phere suggests that hotter Neptunes may have eroded down to smaller, rocky super-Earths. [NASA, ESA, and A. Feild (STScI)] !