European Southern Observatory | 01 Dec 2010
Super-Earth Has an Atmosphere, But Is It Steamy or Gassy?The atmosphere around a super-Earth exoplanet has been analysed for the first time by an international team of astronomers using ESO’s Very Large Telescope. The planet, which is known as GJ 1214b, was studied as it passed in front of its parent star and some of the starlight passed through the planet’s atmosphere. We now know that the atmosphere is either mostly water in the form of steam or is dominated by thick clouds or hazes. The results will appear in the 2 December 2010 issue of the journal Nature.
The planet GJ 1214b was discovered in 2009 using the HARPS instrument on ESO’s 3.6-metre telescope in Chile (eso0950). Initial findings suggested that this planet had an atmosphere, which has now been confirmed and studied in detail by an international team of astronomers, led by Jacob Bean (Harvard–Smithsonian Center for Astrophysics), using the FORS instrument on ESO’s Very Large Telescope.
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GJ 1214b has a radius of about 2.6 times that of the Earth and is about 6.5 times as massive, putting it squarely into the class of exoplanets known as super-Earths. Its host star lies about 40 light-years from Earth in the constellation of Ophiuchus (the Serpent Bearer). It is a faint star, but it is also small, which means that the size of the planet is large compared to the stellar disc, making it relatively easy to study. The planet travels across the disc of its parent star once every 38 hours as it orbits at a distance of only two million kilometres: about seventy times closer than the Earth orbits the Sun.
To study the atmosphere, the team observed the light coming from the star as the planet passed in front of it. During these transits, some of the starlight passes through the planet’s atmosphere and, depending on the chemical composition and weather on the planet, specific wavelengths of light are absorbed. The team then compared these precise new measurements with what they would expect to see for several possible atmospheric compositions.
Before the new observations, astronomers had suggested three possible atmospheres for GJ 1214b. The first was the intriguing possibility that the planet was shrouded by water, which, given the close proximity to the star, would be in the form of steam. The second possibility was that this is a rocky world with an atmosphere consisting mostly of hydrogen, but with high clouds or hazes obscuring the view. The third option was that this exoplanet was like a mini-Neptune, with a small rocky core and a deep hydrogen-rich atmosphere.
The new measurements do not show the telltale signs of hydrogen and hence rule out the third option. Therefore, the atmosphere is either rich in steam, or it is blanketed by clouds or hazes, similar to those seen in the atmospheres of Venus and Titan in our Solar System, which hide the signature of hydrogen.
Center for Astrophysics | 01 Dec 2010
NASA Aids in Characterizing Super-Earth AtmosphereIn December 2009, astronomers announced the discovery of a super-Earth known as GJ 1214b. At the time, they reported signs that the newfound world likely had a thick, gaseous atmosphere. Now, a team led by Jacob Bean (Harvard-Smithsonian Center for Astrophysics) has made the first measurements of GJ 1214b's atmosphere. However, the measurements raise as many questions about the planet's atmospheric composition as they answer.
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A super-Earth is a planet up to three times the size of Earth and weighing one to ten times as much. (GJ 1214b is 2.7 times the size of Earth and 6.5 times as massive.) They are likely to be mostly solid (some combination of rock or ices), unlike the hundreds of Jupiter-sized gas giants found to date around distant stars.
Researchers suggested three atmospheric possibilities for GJ 1214b. The most intriguing was a thick blanket of steam vaporized by the nearby star. (This option led to the nickname "waterworld," although it's too hot for an ocean.) The second option was a mini-Neptune with a rocky core surrounded by ices and a hydrogen/helium atmosphere. The third model has no equivalent in our solar system - a big, rocky world with a soupy mix of gases (mainly hydrogen) recently emitted by volcanoes.
To study the planet's atmosphere, the team observed it when it crossed in front of its star. During such transits, the star's light filters through the atmosphere. Gases absorb the starlight at particular wavelengths, or colors, leaving behind a chemical fingerprint detectable from Earth. Similar observations have found gases like hydrogen and sodium vapor in the atmospheres of distant "hot Jupiters."
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The spectrum of GJ 1214b proved to be featureless, which ruled out a cloud-free atmosphere composed primarily of hydrogen. If the atmosphere of GJ 1214b has abundant hydrogen, then it must be cloaked by a thick blanket of clouds (like Venus) or haze (like Saturn's moon Titan). A dense, steamy atmosphere also fits the data.
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The team examined GJ 1214b in the near-infrared region of the spectrum (780 - 1000 nanometers) using the ground-based Very Large Telescope at Paranal Observatory in Chile. Additional observations in the mid- or far-infrared might finally answer the question: Is the atmosphere of GJ 1214b steamy or gassy?
NASA JPL | 01 Dec 2010
A gaze at exoplanet hazeA team of astronomers, including two NASA Sagan Fellows, has made the first characterizations of a super-Earth's atmosphere, by using a ground-based telescope. A super-Earth is a planet up to three times the size of Earth and weighing up to 10 times as much. The findings, reported in the Dec. 2 issue of the journal Nature, are a significant milestone toward eventually being able to probe the atmospheres of Earth-like planets for signs of life.
The team determined the planet, GJ 1214b, is either blanketed with a thin layer of water steam or surrounded by a thick layer of high clouds. If the former, the planet itself would have an icy composition. If the latter, the planet would be rocky or similar to the composition of Neptune, though much smaller.
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GJ 1214b, first discovered in December 2009, is 2.7 times the size of Earth and 6.5 times as massive. Previous observations of the planet's size and mass demonstrated it has a low density for its size, leading astronomers to conclude the planet is some kind of solid body with an atmosphere.
The planet orbits close to its dim star, at a distance of 0.014 astronomical units. An astronomical unit is the distance between Earth and the sun, approximately 93 million miles. GJ 1214b circles too close to its star to be habitable by any life forms.
Bean and his team observed infrared light as the planet crossed in front of its star. During such transits, the star's light filters through the atmosphere. Gases absorb the starlight at particular wavelengths, leaving behind chemical fingerprints detectable from Earth. This same type of technique has been used to study the atmospheres of distant "hot Jupiters," or Jupiter-like planets orbiting close to their stars, and found gases like hydrogen, methane and sodium vapor.
In the case of the super-Earth, no chemical fingerprints were detected; however, this doesn't mean there are no chemicals present. Instead, this information ruled out some possibilities for GJ 1214b's atmosphere, and narrowed the scope to either an atmosphere of water steam or high clouds. Astronomers believe it's more likely the atmosphere is too thin around the planet to let enough light filter through and reveal chemical fingerprints.
Nature News | 01 Dec 2010
Planetary science: A cloudy view of exoplanets - Drake DemingAtmosphere of distant super-Earth may contain a Titan-like cloak.
Spectroscopic observations of a distant planet swooping across the face of its sun have given astronomers a first-ever look at the atmosphere of an Earth-like planet lying beyond our Solar System. The new data don't provide a precise list of gaseous constituents, but they do indicate that the atmosphere is not one of cloud-free hydrogen.
The planet, dubbed GJ 1214b when it was discovered late last year, tightly orbits a red-dwarf star about 13 parsecs from Earth. With a mass about 6.5 times that of Earth, it is one of the smallest exoplanets yet discovered, and previous observations suggest that the planet's density is far too low for it to be a bare solid object with no atmosphere.
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For about 50 minutes during each 38-hour orbit, GJ 1214b passes directly in front of its star as we view it from Earth, a fortuitous circumstance that offers researchers a chance to observe light filtering through its atmosphere and glean clues about the gases swaddling the orb.
During two transits of GJ 1214b earlier this year, Bean and his colleagues observed the planet at narrow bands of wavelength ranging from the red to the near infrared. If the planet were surrounded by a hydrogen-dominated atmosphere — and assuming that our view is not obscured by clouds — trace amounts of water vapour at high altitudes would absorb the light more strongly at some of the observed wavelengths than at others, says Bean.
That pattern would cause GJ 1214b to appear to have a larger diameter at some wavelengths than at others. But the planet seemed to be a consistent size at all wavelengths that the researchers scrutinized. That indicates another type of atmosphere altogether, they report in Nature today.
For example, GJ 1214b could be cloaked in layers of clouds that block red and near-infrared wavelengths equally effectively. No known major constituents of planetary atmospheres could condense in the temperature and pressure conditions inferred for GJ 1214b (1200-1400°C and about 100 Earth atmospheres, if the atmosphere is mainly hydrogen). But the planet could be enveloped in layers of photochemical haze, as are Titan and Venus in our Solar System.
Or, the researchers say, the atmosphere may be composed of at least 70% water vapour — rendering GJ 1214b a perpetual sauna planet
- Nature 468 636 (02 Dec 2010) DOI: 10.1038/468636a
- Nature 468 669 (02 Dec 2010) DOI: 10.1038/09596