First super-Earth atmosphere analyzed
Scientists now know that the atmosphere is either mostly water in the form of steam or dominated by thick clouds or hazes.
December 1, 2010
The atmosphere around a super-Earth exoplanet has been analyzed for the first time by an international team of astronomers using the European Southern Observatory’s (ESO) Very Large Telescope. The scientists studied the planet, which is known as GJ 1214b, as it passed in front of its parent star, and some of the starlight passed through the planet's atmosphere. They now know that the atmosphere is either mostly water in the form of steam or dominated by thick clouds or hazes.
This artist’s impression shows the super-Earth exoplanet orbiting the nearby star GJ 1214.
Photo by ESO/L. Calçada
Astronomers discovered GJ 1214b in 2009 using the HARPS instrument on ESO's 3.6-meter telescope in Chile. Initial findings suggested that this planet had an atmosphere, which has now been confirmed. The international team led by Jacob Bean from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used the Focal Reducer and low dispersion Spectrograph (FORS) instrument on ESO's Very Large Telescope (VLT) for their study.
"This is the first super-Earth to have its atmosphere analyzed. We've reached a real milestone on the road toward characterizing these worlds," said Bean.
GJ 1214b has a radius of about 2.6 times that of 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 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 disk, making it relatively easy to study. The planet travels across the disk of its parent star once every 38 hours as it orbits at a distance of only 1.2 million miles (2 million kilometers) — about 70 times closer than 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 thus 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, which hide the signature of hydrogen.
"Although we can't yet say exactly what that atmosphere is made of, it is an exciting step forward to be able to narrow down the options for such a distant world to either steamy or hazy," said Bean. "Follow-up observations in longer wavelength infrared light are now needed to determine which of these atmospheres exists on GJ 1214b."