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,” [Jacob] 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.