The atmosphere surrounding a "super-Earth" extrasolar planet has been measured for the first time.
The planet, GJ 1214b, is three times larger than Earth and about seven times heavier, and is the first planet of its kind known to have an atmosphere.
Researchers reporting in Nature have said it appears to have either a dense atmosphere of water steam, or is wrapped in thick clouds like Venus.
More experiments measuring more colours of light will bear out which it is.
The method used could be used on many ground-based telescopes to yield atmospheric data on other exoplanets.
It works by canceling out the disruptive effects of the Earth's atmosphere while that of far-flung worlds is measured from the ground.
GJ 1214b was first discovered just 40 light-years away in November 2009 and has since become a focus for exoplanet research.
"'Super-Earths' are really interesting, they're at the forefront of what's going on in exoplanet research," said Jacob Bean, the Harvard-Smithsonian University Center for Astrophysics scientist who led the research.
"They're an interesting regime because they're defined as being the transition from terrestrial planets like Earth, Venus and Mars, up to the ice giants like Uranus and Neptune."
Dr Bean and his colleagues used the same "transit method" to probe the planet's atmosphere that was used to discover it.
When a planet passes between its host star and the Earth, it blocks some of the star's light - how much light is blocked gives an indication of the planet's radius.
Now the team has used the Very Large Telescope in Chile to see if specific colours of infrared light are blocked; the atmosphere should absorb specific colours that give away the chemicals from which it is made.
From what is known about GJ 1214b, three theories of its composition and atmosphere dominated. It could either be covered in a dense layer of steam, or it could be an icy, rocky world with a hydrogen or helium atmosphere, or perhaps a large rocky planet with a mixture of volcanic gases.
To measure the minuscule differences in light levels between the cases, Dr Bean and his colleagues improved upon a method to eliminate the effect of the Earth's atmosphere.
While studying the light from GJ 1214b during its transit in front of its star, the team detected the "squiggles" of light from a reference star - whose light should remain unchanged.
"You get squiggles on the reference star - those are mainly due to the Earth's atmosphere; you get squiggles on the target that you're looking at - that's also mainly due to the Earth's atmosphere," Dr Bean explained.
"You subtract the two and what you're left with is really what this extrasolar planet is doing."
The results showed, however, that the squiggles left from GJ 1214b did not have any significant absorptions that give away its atmosphere's makeup.
But the featureless data that the team gathered does not mean they cannot make some informed guesses, as Dr Bean explained.
"The most important parameter governing the strength of absorption features is the 'puffiness' of the planet's atmosphere - how extended, how low-density it is, not necessarily the abundance of certain things."
A light, extended "puffy" atmosphere - made of elements like hydrogen - might block more light than a dense one - made of, for example, water - that is drawn more to a planet's surface by gravity.
The denser atmosphere's "low profile" means it is exposed to less light and from Earth appears to absorb less of it.
Alternatively, a cloudy atmosphere would scatter all light, regardless of colour, leading to a similarly squiggle-free measurement.
For Dr Bean, that means that GJ 1214b must not be an icy "mini-Neptune" with a hydrogen or helium atmosphere.
But further measurements will be needed before astronomers find out if it has a thin, steamy atmosphere, or has a thicker but cloudier one.
Drake Deming, of Nasa's Planetary Systems Laboratory said the work "opens the door to characterisng these kinds of super-Earth atmospheres".
"We've been seduced by the abundance of exoplanet results into thinking that these are routine measurements, but they're actually extraordinarily difficult," he told BBC News.
"This is certainly the most significant atmospheric exoplanet result in the last year - and a year is a long time in this field."
The next step is to carry out the same measurement using different colours of light, further into the infrared where the distinction can be made; the Spitzer space telescope will do this first, but many groups have GJ 1214b in their sights.
"This is the most interesting exoplanet out there so there are a lot of people focusing on it," Dr Bean said.
"I think in the next year we should have a very clear picture about this planet, with all the different observations people are doing."