So far we've spotted more than 4,000 planets orbiting stars other than our Sun, but we know very little about their atmospheres. Now, NASA astronomers have managed to probe the atmosphere of GJ 3470 b, a planet unlike any in our solar system. It marks the first time for this kind of world, and could be a step towards identifying potentially habitable planets.

GJ 3470 b is a type of planet commonly referred to as a "Super-Earth", or perhaps a "mini-Neptune." The reason why is pretty clear – with a mass about 12.6 times that of Earth, it tips the scales between Earth and Neptune, which has a mass of over 17 Earths. Although our solar system has missed out on this class of object, they may actually make up the majority of planets in the galaxy.

But until now, astronomers had never been able to take stock of what was in the atmosphere of this kind of world. To investigate, the Hubble and Spitzer space telescopes conducted a spectroscopic study of the planet, by analyzing the signatures of light that filtered through the air as it passed in front of its host star, as well as what was lost when the planet went behind the star.

The results were unexpected. The atmosphere was clear with thin hazes, and mostly made up of hydrogen and helium, surrounding a large rocky core. Strangely enough, no heavier elements were picked up.

"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," says Björn Benneke, corresponding author of the study. "Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium-rich composition of the Sun."

An artist's illustration of the atmosphere and position of the exoplanet GJ 3470 b NASA, ESA, and L. Hustak (STScI)

That makes GJ 3470 b a bit of a puzzle. The planet orbits relatively close to its host star, and is only the second so-called "hot Neptune" to be discovered. Exactly how it came to be at that size and that proximity to the star is unknown.

Previous studies of GJ 3470 b showed that the star's radiation was rapidly blasting the exoplanet's atmosphere away into space, suggesting that it was originally much bigger – a "hot Jupiter" you might say – but had since shrunk down a mere "hot Neptune."

But with the new atmospheric data taken into account, the team now believes that the planet was born from the rocky lump in the middle, which then scooped up hydrogen from a protoplanetary disk of material that once surrounded the star. But the disk dissipated before it could get much bigger.

"This is a big discovery from the planet-formation perspective," says 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 research was published in the journal Nature Astronomy.

Source: NASA