Water world found in deep space?

Artist’s impression of super-Earth GJ 1214b

This artist’s impression shows the super-Earth exoplanet orbiting the nearby star GJ 1214. The planet, GJ 1214b appears to be surrounded by an atmosphere that is either dominated by steam or blanketed by thick clouds or hazes.

  • Atmosphere of “super-Earth” GJ 1214b analysed
  • Planet air is either full of steam or clouds and hazes
  • First “super-Earth” to have it’s atmosphere studied

THE ATMOSPHERE of a “super-Earth” exoplanet has been analysed for the first time by an international team of astronomers using the European Southern Observatory’s (ESO) Very Large Telescope.

Exoplanet are ones that orbit stars beyond our Solar System. Over 500 have been detected so far. A “super-Earth” is one that is not much bigger than our own planet.

This particular planet, discovered in 2009 and 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.

The research team, led by Jacob Bean (Harvard–Smithsonian Centre for Astrophysics) say the atmosphere is either mostly water in the form of steam or is dominated by thick clouds or hazes.

“This is the first super-Earth to have its atmosphere analysed. We’ve reached a real milestone on the road toward characterising these worlds,” said Bean.

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 super-Earth class. Its orbits a star located about 40 light-years from Earth.

The star is faint, but it is also small, which means that the size of the planet is large compared to the star’s disc, making it relatively easy to study.

Although the planet is too far away for astronomers to take pictures of it, they can

The planet passes across the face of the star once every 38 hours as it orbits at a distance of only two million kilometres—about 70 times closer than the Earth orbits the Sun.

Initial discovery

GJ 1214b was discovered as a “transiting object” by the MEarth project, which focuses on about 2,000 low-mass stars in the hope of finding exoplanets. MEarth uses an armada of eight small telescopes each with a diameter of 40cm, located on top of Mount Hopkins, Arizona, USA.

Artist's impression of an exoplanet transit

As a planet passes in front of its star, part of the starlight passes through the planet's atmosphere. Astronomers can tell what's in the atmosphere by analysing that light. (Artist's impression)

MEarth looks for stars that change brightness as a planet crosses in front of, or transits, its star. During such a mini-eclipse, the planet blocks a small portion of the star’s light, making it dimmer.

NASA’s Kepler space telescope also uses transits, to look for Earth-sized planets orbiting Sun-like stars.

However, Earth-sized planets transiting Sun-like stars dim the starlight by only one part in ten thousand. The higher precision required to detect the drop means that such worlds can only be found from space.

In contrast, a super-Earth such as GJ 1214b—orbiting a smaller, dimmer star—yields a greater proportional decrease in brightness and a stronger signal that is detectable from telescopes on the ground.

To confirm the planetary nature of GJ 1214b and to gauge its mass (using the so-called Doppler method), the astronomers needed the full precision of the HARPS spectrograph, attached to ESO’s 3.6-metre telescope at La Silla in Chile.

An atmosphere analysed

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 following video shows an artist’s impression of the GJ 1214 system:

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. So 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.

“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,” says Bean.

“Follow-up observations in longer wavelength infrared light are now needed to determine which of these atmospheres exists on GJ 1214b.”

Adapted from information issued by ESO / L. Calçada.

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