NASA combined two of its space telescopes to identify the “chemical fingerprint” of an exoplanet between the sizes of Earth and Neptune. So far nothing like this has yet been discovered in our own solar system. However, scientists have now detected an exoplanet with an unusual atmosphere that is more common in other planet-hosting star systems.
The planet is known by the names of Gliese 3470 b and GJ 3470 b, and it appears to be a hybrid of Earth and Neptune. It boasts a huge rocky core that is hidden in seemingly endless layers of hydrogen and helium. It weighs 12.6 times Earth’s weight, although it’s less massive than Neptune, which is more than 17 masses of Earth.
NASA’s Kepler space telescope discovered many worlds similar to the exoplanet with the unusual atmosphere. Interestingly, more than 80% of planets fall into the mass range of the studied planet.
“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,” Björn Benneke of the University of Montreal said in a statement. “We don’t have anything like this in the solar system, and that’s what makes it striking.”
The researchers used the multi-wavelength properties of the Hubble and Spitzer Space Telescopes to conduct the first-of-its-kind study on an exoplanet with an unusual atmosphere. They were able to learn more about the planet’s atmosphere by measuring how much starlight the planet absorbs as it passes by its host star, using the popular transit method. The loss of reflected light from the planet helped them determine its atmosphere.
“For the first time we have a spectroscopic signature of such a world,” said Benneke.
However, he is at a loss for classification. Should it be called a “super-Earth” or “sub-Neptune?” Or perhaps something else?
Despite having some of the properties of Neptune, GJ 3470 b has a clear atmosphere with only thin hazes, which allowed scientists to learn more about it.
“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,” Benneke said. “Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen/helium rich composition of the Sun.”
Another type of planet known as a “hot Jupiter” is believed to form much farther from its host star, and as the star system becomes less chaotic, it appears to migrate closer to its host star. However, the newly-studied planet seems to have formed in the same location where it is now.
According to Benneke, it’s likely that GJ 3470 b was formed close to a red dwarf star which is half of the size of our sun. Benneke believes the exoplanet rapidly accreted hydrogen from a primordial disk of gas during the early stages of the star.
“We’re seeing an object that was able to accrete hydrogen from the protoplanetary disk, but didn’t runaway to become a hot Jupiter,” Benneke said. “This is an intriguing regime.”
The exoplanet with the unusual atmosphere offers a lot of insight into planet formation. However, to be able to survey the planet with more detail, scientists will have to wait for the James Webb Space Telescope to become operational.