Probing A New Class of Exoplanets

For the first time, a team of researchers has probed the chemical composition of the atmosphere of a new class of exoplanet. The team, led by Université de Montréal astronomy professor Björn Benneke,included Caltech planetary scientist Heather Knutson.

The planet, dubbed GJ 3470 b, is unlike anything found in our solar system and resembles a cross between Earth and Neptune, with a large rocky core buried under a deep, crushing hydrogen and helium atmosphere. Its mass is 12.6 times that of Earth, but somewhat less than that of Neptune, which weighs in at 17 Earth masses. An analysis of the new planet was published in Nature Astronomy on July 1.

Many similarly sized worlds have been discovered by NASA’s Kepler space telescope. In fact, 80 percent of the planets in our galaxy fall into this mass and size range, but astronomers have never been able to understand the chemical nature of the atmosphere of such a planet until now. Astronomers enlisted the combined multi-wavelength capabilities of NASA’s Hubble and Spitzer space telescopes to do a first-of-its-kind probe of GJ 3470 b’s atmosphere.

“We expected to find an atmosphere strongly enriched in heavier elements like oxygen and carbon that create abundant water vapor and methane gas,” says Benneke, who worked as a postdoctoral researcher at Caltech during much of this project and is the lead author of the Nature Astronomy paper. “Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen- and helium-rich composition of the sun.”

Caltech’s Knutson advised Benneke on this project and provided a second data set used to detect molecular features in light emitted from the planet’s day side. “We understand the planet better when we observe its atmosphere using more than one technique; this is particularly true for planets with high-altitude clouds, which usually only affect select regions of the atmosphere,” says Knutson.

Read the full story from the Space Telescope Science Institute here.

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