Telescope Teamwork Reveals Gas Dwarf Planet's Atmosphere

An artist's depiction of one hypothesis about the interior structure of GJ 3470 b.
An artist's depiction of one hypothesis about the interior structure of GJ 3470 b.
(Image: © NASA, ESA and L. Hustak (STScI))

Two NASA telescopes peered more closely than ever before at a planet shedding gas into space.

NASA's Hubble and Spitzer space telescopes together discovered the chemical "fingerprint" of the planet, called Gliese 3470 b or GJ 3470 b. The planet was already a puzzle because it orbits extremely close to its parent star that is literally evaporating. It's nearly 13 times the size of Earth, comparable to Neptune, which clocks in at 17 Earth masses. And what scientists found when they looked at the planet more closely was a big surprise.

The scientists predicted GJ 3470 b's atmosphere would be full of oxygen and carbon, the same elements that are at the root of the water vapor and methane gas observed at Neptune. "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," Björn Benneke, lead author on the new research and an astronomer at the University of Montreal, said in a statement.

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What's more, the team thinks they may know why GJ 3470 b's composition is so different than that of Neptune's: It has to do with the planet's origin story.

Astronomers have already spotted many much larger exoplanets that orbit extremely close to their parent stars, known as "hot Jupiters." Many scientists suspect that these Jupiter-size planets formed farther out in their respective solar systems before migrating in toward their stars.

That's not the case for GJ 3470 b, however. The star likely formed just where it is now, extremely close to a red dwarf star that is about half the size of our own sun. The planet probably started out as a dry rock, and then picked up hydrogen from the gas disk surrounding the star when the star was very young. (This is called a "protoplanetary disk" and is a common feature in young star systems.)

Commonly, planets such as GJ 3470 b rapidly grow into hot Jupiters, but that's clearly not the case here. "This is an intriguing regime," Benneke said. One explanation could be that the disk of protoplanetary gas somehow dispersed while the planet was still growing, stunting its size forever. 

The two NASA telescopes did their work by studying the planet closely as it orbited around the star. Fortunately for scientists, GJ 3470 b passes directly across the disk of its parent star, which allows scientists to pick out the chemical fingerprint of the planet through spectroscopy. The researchers looked at two features of the star's light: how much light was absorbed by the planet as it passed in front of its star (which they observed 12 times) and how much reflected light was lost from the planet when it went behind the star (which they observed 20 times).

Chances are, astronomers will be stuck on the GJ 3470 b puzzle until they have access to a more powerful observatory — but one is on its way to orbit in a few years. NASA's James Webb Space Telescope could probe the planet after the observatory launches in 2021 or so. Webb is more sensitive to the infrared wavelengths in which the planet shines brightly, so it will be able to see more detail about the world than Hubble or Spitzer can, the team said.

A paper based on the research was published July 1 in the journal Nature Astronomy.

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