Fishermen would be puzzled if they netted only big and little fish, but few medium-sized fish. Astronomers likewise have been perplexed in conducting a census of star-hugging extrasolar planets. They have found hot Jupiter-sized planets 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, have been much harder to find. In fact, only about a handful of hot Neptunes have been found so far.
In fact, most of the known Neptune-sized exoplanets are merely "warm," because they orbit farther away from their star than those in the region where astronomers would expect to find hot Neptunes. The mysterious hot-Neptune deficit suggests that such alien worlds are rare, or, they were plentiful at one time, but have since disappeared.
A few years ago astronomers using NASA's Hubble Space Telescope found that one of the warmest known Neptunes (GJ 436b) is losing its atmosphere. The planet isn't expected to evaporate away, but hotter Neptunes might not have been so lucky.
Now, astronomers have used Hubble to nab a second "very warm" Neptune (GJ 3470b) that is losing its atmosphere at a rate 100 times faster than that of GJ 436b. Both planets reside about 3.7 million miles from their star. That's one-tenth the distance between our solar system's innermost planet, Mercury, and the Sun.
"I think this is the first case where this is so dramatic in terms of planetary evolution," said lead researcher Vincent Bourrier of the University of Geneva in Sauverny, Switzerland. "It's one of the most extreme examples of a planet undergoing a major mass-loss over its lifetime. This sizable mass loss has major consequences for its evolution, and it impacts our understanding of the origin and fate of the population of exoplanets close to their stars."
As with the previously discovered evaporating planets, the star's intense radiation heats the atmosphere to a point where it escapes the planet's gravitational pull like an untethered hot air balloon. The escaping gas forms a giant cloud around the planet that dissipates into space. One reason why GJ 3470b may be evaporating faster than GJ 436b is that it is not as dense, so it is less able to gravitationally hang on to the heated atmosphere.
What's more, the star hosting GJ 3470b is only 2 billion years old, compared to the 4-billion- to 8-billion-year-old star that planet GJ 436b orbits. The younger star is more energetic, so it bombards the planet with more blistering radiation than GJ 436b receives. Both are red dwarf stars, which are smaller and longer-lived than our Sun.
Uncovering two evaporating warm Neptunes reinforces the idea that the hotter version of these distant worlds may be a class of transitory planet whose ultimate fate is to shrink down to the most common type of known exoplanet, mini-Neptunes — planets with heavy, hydrogen-dominated atmospheres that are larger than Earth but smaller than Neptune. Eventually, these planets may downsize even further to become super-Earths, more massive, rocky versions of Earth.
"The question has been, where have the hot Neptunes gone?" said Bourrier. "If we plot planetary size and distance from the star, there's a desert, a hole, in that distribution. That's been a puzzle. We don't really know how much the evaporation of the atmospheres 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 confirmation 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 surrounding the planet as it passed in front of its star. The intervening cocoon of hydrogen filters out some of the starlight. These results are interpreted 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 radiation. If the planet continues to rapidly lose material, it will shrink down to a mini-Neptune in a few billion years.
Hydrogen probably isn't the only element evaporating away: it may be a tracer for other material streaming off into space. The researchers 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 elements such as carbon, which reside deeper in the atmosphere, upward and out into space," Bourrier said.
The observations are part of the Panchromatic Comparative Exoplanet Treasury (PanCET) survey, a Hubble program to look at 20 exoplanets, mostly hot Jupiters, in the first large-scale ultraviolet, visible, and infrared comparative study of distant worlds.
Observing the evaporation of these two warm Neptunes is encouraging, but team members know they need to study more of them to confirm predictions. Unfortunately, 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 Neptunes 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 Neptune'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 interstellar material in space.
"Looking for helium could expand our survey range," Bourrier said. "Webb will have incredible sensitivity, so we would be able to detect helium escaping from smaller planets, such as mini-Neptunes."
The researcher's paper will appear in the Dec. 13 issue of Astronomy and Astrophysics.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
The hot Neptunes, planets that shrink
Astronomers from the UNIGE explain the rarity of the hot Neptunes by their evaporation which transforms them into super-Earths.
“But where did the hot Neptunes go?” This is the question astronomers have been asking for a long time, faced with the mysterious absence of planets the size of Neptunes very close to their star. A team of researchers, led by astronomers from the University of Geneva (UNIGE), Switzerland, has just discovered that one of these planets is losing its atmosphere at a frantic pace. This observation strengthens the theory that hot Neptunes have lost much of their atmosphere and turned into smaller planets called super-Earths, which are much more numerous. Results to read in the journal Astronomy & Astrophysics.
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 the European project FOUR ACES** and first author of the study. The team estimates that GJ 3470b has already lost more than a third of its mass.
“Until now we were not sure of the role played by the evaporation of atmospheres in the formation of the desert”, states Vincent Bourrier. 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 could explain the abundance of hot super-Earths that have been discovered,” says David Ehrenreich, associate professor in the astronomy department of the science faculty at UNIGE and co-author of the study.
The evolution of the hot Neptun hunt
Observing the evaporation of two warm Neptunes is encouraging, but team members know they need to study more of them to confirm their predictions. Unfortunately, the hydrogen that escapes from these planets cannot be detected if they are more than 150 light-years from Earth (GJ 3470b is 97 light-years away), because hydrogen is then hidden by interstellar gas. Researchers thus plan to use Hubble to look for other traces of atmospheric escape, because hydrogen could drag upward heavier elements such as carbon. The solution could also come from helium, whose infrared radiation isn’t blocked by interstellar medium. “Helium will expand the range of our surveys,” said Vincent Bourrier, “the high sensitivity of the James Webb space telescope should allow us to detect helium escaping small planets, such as mini-Neptunes, and complete our observations of the edge of the desert.”
Quelle: UNIVERSITY OF GENEVA