For the first time, researchers have been able to observe the atmosphere of a planet between the sizes of Earth and Neptune. Gliese 3470 b is a world 96 light-years away, orbiting a star roughly half the size of the Sun. It is 12.6 times the mass of our planet and slightly smaller than Neptune, which weighs 17 times the mass of the Earth.

As reported in Nature Astronomy, this planet delivered quite the surprise to astronomers. Using the combined power of the Hubble Space Telescope and NASA’s Spitzer infrared observatory, they discovered a clear atmosphere of hydrogen and helium, the main components of stars.

"We expected an atmosphere strongly enriched in heavier elements like oxygen and carbon which are forming abundant water vapor and methane gas, similar to what we see on Neptune," lead author Björn Benneke, of the University of Montreal, said in a statement. "Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium-rich composition of the Sun."

And this is quite peculiar. We would only expect the largest planets to be capable of stealing primordial hydrogen and helium. On paper, Gliese 3470 b seemed a bit too minute (relatively) to manage this feat.

"This is a big discovery from the planet formation perspective," explained Benneke. "The planet orbits very close to the star and is far less massive than Jupiter – 318 times Earth's mass – but has managed to accrete the primordial hydrogen/helium atmosphere that is largely 'unpolluted' by heavier elements. We don't have anything like this in the Solar System, and that's what makes it striking."

The team believes that a likely explanation for this planet is that it formed right where it is now, orbiting its star in just three days. It quickly assembled into a large rocky core and began to steal hydrogen and helium that would have either ended up on the star or been pushed away by the stellar winds. But some of the primordial gas must have been pushed away, otherwise, the team believes, the planet would have gotten much bigger.

NASA’s replacement for both Hubble and Spitzer, the James Webb Space Telescope, will be able to probe this planet's atmosphere in even more detail and look at many more exoplanets. Planets of similar sizes to Gliese 3470 b are extremely common, so knowing their composition gives us a better idea of how these and other planetary systems formed.