Astronomers probe atmosphere of alien world that’s a cross between Earth and Neptune

Gliese 3470 b isn’t like anything in our solar system. The strange world—midway between Earth and Neptune in mass—orbits a star about half the mass of the sun roughly 100 light-years away. Now, astronomers have taken a detailed look at Gliese 3470 b’s atmosphere, the first time researchers have done so for a planet like this.

Astronomers used NASA’s Hubble and Spitzer space telescopes to measure which frequencies of starlight Gliese 3470 b absorbs and reflects as it circles around its star. The planet has a relatively thin atmosphere comprised primarily of hydrogen and helium, NASA announced yesterday. That’s similar to the atmosphere of the sun, with the exception of heavy elements such as oxygen and carbon. The planet (above) also has a hefty rocky core, the analysis reveals.

Gliese 3470 b appears to have formed close to its star, where it still sits today. This might explain why the planet was able to develop its unconventional atmosphere. One hypothesis is that it was able to corral gases from a primordial disk of gas surrounding its star. Typically when this happens, planets become giant gas worlds known as “hot Jupiters.” But Gliese 3470 b stayed relatively small, perhaps because the disk of gas dissipated before the planet was able to bulk up, the team speculates.

NASA’s new James Webb Space Telescope—Hubble’s successor set to launch in 2021—will penetrate the planet’s atmosphere to greater depths. Until then, astronomers have to solve another mystery about Gliese 3470 b: whether to call it a “super-Earth” or a “sub-Neptune.”