PARIS--Astronomers have for the first time analysed the atmosphere of a "super-Earth," the name given to rocky exoplanets only a few times larger than our own, according to a study released Wednesday.
The breakthrough is a key step in the quest to identify planets in other solar systems that could potentially host forms of life we might recognise, the researchers said.
"We've reached a milestone on the road toward characterising these worlds," said lead author Jacob Bean, a professor at the Harvard-Smithsonian Center for Astrophysics.
The exoplanet in question, dubbed GJ 1214b, is some 42 light years -- four hundred trillion kilometers, or 250 trillion miles -- from our corner of the universe, with a radius about 2.6 times that of Earth.
Discovered last year, GJ 1214b circles a small, faint star, making it that much easier for scientists to tease out data about the atmosphere by analysing starlight as it passes the rim of the planet on its way to us.
Depending on the chemical composition and weather of the atmosphere, specific signature wavelengths of light are absorbed.
Using the European Space Agency's Very Large Telescope in Chile, Bean and colleagues were able to narrow the range of possibilities from three to two.
The first is that GJ 1214b is shrouded by water which -- given the nearness to its star -- would be in the form of steam.
It could also be a rocky world with an atmosphere consisting mostly of hydrogen, but with high clouds or haze obscuring the view.
What the exoplanet is not, the observations prove, is a "mini-Neptune" with a small rocky core and a deep, hydrogen-rich atmosphere.
"Although we can't say yet 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.
In either case, it is more than unlikely that GJ 1214b hosts life forms.
"This planet is much too hot to be considered habitable," Bean told AFP. "In the regions of the atmosphere with pressures similar to what are seen at sea level on Earth, the temperatures are estimated to be more than 500 degree Celsius (930 degrees Fahrenheit)."
It circles its star every 38 hours at a distance of only two million kilometers, seventy times closer than Earth's orbit of the Sun.
Despite this, GJ 1214b is smaller, cooler and more Earth-like than any other known exoplanet.
Most of the more than 500 exoplanets discovered to date are "hot Jupiters", so-called because of their large, gaseous masses and extreme temperatures.
But as observational tools become more powerful, astronomers have begun to identify more and more rocky orbs similar to our own.
"We are working to discover and eventually characterise the atmospheres of planets that would be habitable," said Bean.
"We aren't there yet, but the goal is obtainable within the next decade," he said by email.
No exoplanet discovered so far falls within its solar system's "Goldilocks zone," where temperatures are not so hot that water evaporates, nor so cold that it freezes, but just right for the stuff of life to exist in liquid form.
Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen and one percent other gases, including carbon dioxide.