This artist's illustration shows a giant cloud of hydrogen streaming off
a warm, Neptune-sized planet just 97 light-years from Earth. The
exoplanet is tiny compared to its star, a red dwarf named GJ 3470. The
star's intense radiation is heating the hydrogen in the planet's upper
atmosphere to a point where it escapes into space. The alien world is
losing hydrogen at a rate 100 times faster than a previously observed warm Neptune whose atmosphere is also evaporating away. Credits: NASA, ESA, and D. Player (STScI)
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. Credits: NASA, ESA, and A. Feild (STScI). Science: NASA, ESA, and V. Bourrier (University of Geneva, Switzerland)
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.
Related Links
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Related Links
This site is not responsible for content found on external links
- The science paper by V. Bourrier et al.
- NASA's Hubble Portal
- Astronomy and Astrophysics Journal
- University of Geneva's Release
- Johns Hopkins University's Release
Contact
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Vincent Bourrier
University of Geneva, Sauverny, Switzerland
011-41-22-379-2449
vincent.bourrier@unige.ch
Source: HubbleSite/News
