Extrasolar ‘Neptunes’ are exoplanets with masses and radii similar to those of the ice giants in the Solar System. In 2015, astronomers using the NASA/ESA Hubble Space Telescope found that GJ 436b, one of the warmest known Neptunes, is losing its atmosphere. The gas giant isn’t expected to evaporate away, but hotter Neptunes might not have been so lucky. Now, astronomers have used Hubble to find a second ‘very warm’ Neptune, named GJ 3470b, that is losing its atmosphere at a rate 100 times faster than that of GJ 436b. The discovery is described in a paper in the journal Astronomy & Astrophysics (arXiv.org preprint).
This artist’s illustration shows a giant cloud of hydrogen streaming off the warm Neptune GJ 3470b. The host star’s intense radiation is heating the hydrogen in the planet’s upper atmosphere to a point where it escapes into space. GJ 3470b is losing hydrogen at a rate 100 times faster than a previously observed warm Neptune whose atmosphere is also evaporating away. Image credit: NASA / ESA / D. Player, STScI.
Planet hunters have found a large number of hot Jupiter-sized exoplanets and hot super-Earths (planets no more than 1.5 times Earth’s diameter). These planets are scorching hot because they orbit very close to their star.
But so-called ‘hot Neptunes,’ whose atmospheres are heated to more than 1,700 degrees Fahrenheit (927 degrees Celsius), have been much harder to find. In fact, only about a handful of hot Neptunes have been found so far.
Astronomers hypothesize that these gas giants get stripped of their atmospheres and ultimately become smaller planets.
It’s difficult, however, to actively witness them doing so because they can only be studied in UV light, which limits researchers to examining nearby stars no more than 150 light-years away from Earth, not obscured by interstellar material.
The warm Neptune GJ 3470b is some 96 light-years away and circles a red dwarf star in the general direction of the constellation Cancer.
This 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 atmosphere suggests that hotter Neptunes may have eroded down to smaller, rocky super-Earths. Image credit: NASA / ESA / A. Feild, STScI.
A team of astronomers led by University of Geneva’s Dr. Vincent Bourrier found that GJ 3470b had lost significantly more mass and had a noticeably smaller exosphere than the first Neptune-sized exoplanet studied, GJ 436b, due to its lower density and receipt of a stronger radiation blast from its host star.
GJ 3470b’s lower density makes it unable to gravitationally hang on to the heated atmosphere, and while the star hosting GJ 436b was between 4 billion and 8 billion years old, the star hosting GJ 3470b is only 2 billion years old. A younger star is more active and powerful, and, therefore, has more radiation to heat the planet’s atmosphere.
The researchers estimate that GJ 3470b may have already lost up to 35% of its total mass and, in a few billion years, all of its gas may be stripped off, leaving behind only a rocky core.
“Until now we were not sure of the role played by the evaporation of atmospheres in the formation of the desert,” Dr. Bourrier said.
“The discovery of several warm Neptunes at the edge of the desert losing their atmosphere supports the idea that the hotter version of these planets is short-lived. Hot Neptunes would have shrunk to become mini-Neptunes, or would have eroded completely to leave only their rocky core.”
“This is the smoking gun that planets can lose a significant fraction of their entire mass,” said team member Professor David Sing, an astronomer at Johns Hopkins University and the University of Exeter.
“GJ 3470b is losing more of its mass than any other planet we seen so far; in only a few billion years from now, half of the planet may be gone.”
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V. Bourrier et al. 2018. Hubble PanCET: an extended upper atmosphere of neutral hydrogen around the warm Neptune GJ 3470b. A&A 620, A147; doi: 10.1051/0004-6361/201833675
