For the first time, using Hubble and Spitzer telescopes, scientists have identified the chemical signature of the atmosphere surrounding a mid-sized planet.

The mid-sized planet is called Gliese 3470 b (GJ 3470 b). It is between the sizes of Earth and Neptune. While similar-sized planets are common around other stars, there’s none in our solar system.

GJ 3470 b is so close to its red dwarf star that it completes one orbit in just three days. The exoplanet has a large rocky core under a deep crushing atmosphere of hydrogen and helium. It weighs 12.6 Earth masses.

Based on discoveries by NASA’s Kepler Space Telescope, about 80% of the planets in our galaxy are similar in size. But astronomers have never been able to understand the chemical nature of such a planet until now.

Recently, by studying the atmosphere of GJ 3470 b, scientists have uncovered clues about the planet’s nature and origin.

“This is a big discovery from the planet-formation perspective. 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,” Bj√∂rn Benneke, researcher at the University of Montreal in Canada, said in a news release.

“We don’t have anything like this in the solar system, and that’s what makes it striking.”

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Astronomers have used Hubble and Spitzer telescope to measure changes in the spectral signature of the host star’s light as the planet passed across. By observing which wavelengths were absorbed as the planet made its transits, scientists confirmed the dominance of hydrogen and helium in the exoplanet’s atmosphere.

The space telescopes observed 12 transits and 20 eclipses. The science of analyzing chemical fingerprints based on light is called “spectroscopy.”

“For the first time we have a spectroscopic signature of such a world,” said Benneke.

“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. Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium-rich composition of the sun.”

Usually, other exoplanets called “hot Jupiters” form far from their stars, and over time come much closer. But GJ 3470 b seems to have formed just where it is today, says Benneke.

Benneke estimates this world started as a rocky core. Then, it slowly accreted gas from the protoplanetary disk to form its atmosphere.

“We’re seeing an object that was able to accrete hydrogen from the protoplanetary disk but didn’t run away to become a hot Jupiter,” said Benneke. “This is an intriguing regime.”

One possibility is that the disk of gas dispersed before the alien world could get any bigger.

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