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Someday in the coming years, if astronomers finally succeed in locating a virtual Earth twin outside the solar system—a tiny dot of a world at a temperate, life-enabling distance from a sunlike star—the achievement will hardly be cause for resting on observational laurels. Instead another race will begin: to characterize the planet and its atmosphere and to determine if the world is truly habitable or, tantalizingly, if it is already inhabited by some extraterrestrial life-form.
In the meantime, astronomers are honing their techniques on the closest thing available—so-called super-Earths, just a few times the mass of our own planet, which are too hot to be habitable but are interesting in their own right. To that end, a team of researchers has managed to capture the light spectrum of a super-Earth backlit by its host star. The measurements provide an unprecedented glimpse of the atmosphere of a relatively small planet; most worlds that have been located in other planetary systems are behemoths more massive than Jupiter. The research appears in the December 2 issue of Nature. (Scientific American is part of Nature Publishing Group.)
The few known super-Earths whose orbits are fortuitously aligned so that they transit—pass in front of their host star—from Earth's vantage point provide a unique laboratory for planetary investigations. As a transiting planet moves through its orbit, starlight filtering through its atmosphere or reflected off its dayside can be spectroscopically measured to identify individual molecular species. In the case of a potentially habitable transiting planet, the same technique could be used to look for the signatures of biological activity. (The potentially habitable super-Earth announced by a team of astronomers in September and later questioned by a rival team does not appear to transit, limiting the possibility of such detailed study.)
"This is the most accessible way to study these planets' atmospheres," says lead study author Jacob Bean of the Harvard-Smithsonian Center for Astrophysics. "The light from the host star passes through part of the atmosphere, and the chemical species imprint their signature on the light."
Bean and his colleagues took advantage of a favorable planetary alignment to measure the spectrum of GJ 1214 b, a transiting super-Earth orbiting a small star about 40 light-years away. (The world is named by convention for its host star, GJ 1214.) The exoplanet, which is about 2.6 times as wide as Earth and about six times as massive, was discovered in 2009.
Even though it is a small planet, GJ 1214 b blots out a relatively large fraction of its star's light when it transits, thanks to the host star's diminutive size, just one-fifth the diameter of the sun. And, as an added bonus, the planet appears relatively broad for its mass, indicating the presence of a substantial atmosphere. "1214 b is like the perfect super-Earth for study," Bean says.
GJ 1214 b transits every 38 hours or so, passing in front of its host star and revealing itself by shading the star's light for about an hour. Bean tracked GJ 1214 b through two of those planetary transits using one of the 8.2-meter telescopes at the Very Large Telescope atop Cerro Paranal in Chile, parsing the observed light into its individual wavelengths. The resulting spectrum was essentially smooth, without any sharp peaks indicative of absorption by specific molecules. "It just looks like a flat line, but that's a very powerful constraint on the planet's atmosphere," Bean says.
The new research indicates two plausible explanations for the atmosphere of GJ 1214 b, each of which has implications for the planet's interior makeup. The lack of absorption features means that GJ 1214 b cannot have a diffuse hydrogen atmosphere unless it also has a high cloud layer that blocks the starlight from streaming through. That could indicate that the planet is a sort of mini-Neptune—a rocky core sheathed in ice and gas—or a terrestrial world that spewed out a hydrogen atmosphere from molten rock. "That would be kind of fantastic," Bean says of the latter option. The alternative explanation is a dense steam atmosphere that hugs tightly to GJ 1214 b, probably stemming from a planet that began as a ball of ice before drifting closer to its star, where the heat vaporized that ice to steam.
"We cannot distinguish between these two scenarios—puffy with clouds or dense water vapor," Bean says, although he notes that looking at the planet in longer infrared wavelengths could allow a glimpse inside the cloud layer, if indeed it is there. "I think within the next year we'll begin to solve the puzzle of this planet," he says.
But the most tantalizing glimpses will have to wait until a more habitable world is found, perhaps by the same search campaign, known as MEarth, that last year located GJ 1214 b. If the project can turn up a more temperate exoplanet nearby, astronomers could use the James Webb Space Telescope, which NASA plans to launch as early as 2015, to scan its atmosphere for molecular constituents and maybe even signs of life. "If we can find these kinds of planets, we're going to have the facilities available in the next decade to study them in detail," Bean says.
