Astronomers are curious about the two recently-discovered classes of planets not found in the Solar System: the so-called super-Earths and sub-Neptunes, alien worlds with masses and sizes intermediate between Earth and Neptune. Now, the NASA/ESA Hubble Space Telescope and NASA’s Spitzer Space Telescope have teamed up to identify, for the first time, the detailed chemical ‘fingerprint’ of a sub-Neptune.
This artist’s illustration shows a giant cloud of hydrogen streaming off Gliese 3470b. Image credit: NASA / ESA / D. Player, STScI.
Our Solar System contains two major classes of planets. Earth is a rocky terrestrial planet, as are Mercury, Venus, and Mars. At about the distance of the asteroid belt, there is a ‘frost line’ where space is so cold more volatile material, like water, can remain frozen. Out here live gas and ice giants — Jupiter, Saturn, Uranus, and Neptune — which have bulked up on hydrogen and helium and other volatiles.
A sub-Neptune exoplanet called Gliese 3470b is unlike anything found in the Solar System. Also known as GJ 3470b, the planet is approximately 96 light-years away and circles a red dwarf star in the general direction of the constellation Cancer.
Gliese 3470b 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.
Many similar worlds have been discovered by NASA’s Kepler Space Telescope. In fact, 80% of the planets in our Milky Way Galaxy may fall into this mass range. However, astronomers have never been able to understand the chemical nature of such a planet until now.
By inventorying the contents of Gliese 3470b’s atmosphere, University of Montreal astronomer Björn Benneke and colleagues were able to uncover clues about the planet’s nature and origin.
“This is a big discovery from the planet formation perspective,” Dr. Benneke said.
“Gliese 3470b orbits very close to the star and is far less massive than Jupiter, but has managed to accrete the primordial hydrogen/helium atmosphere that is largely ‘unpolluted’ by heavier elements.”
This artist’s illustration shows the theoretical internal structure of Gliese 3470b. Image credit: NASA / ESA / L. Hustak, STScI.
The team analyzed a combined Hubble/Spitzer dataset of 12 transits and 20 eclipses of Gliese 3470b.
“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,” Dr. 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.”
“Other exoplanets called hot Jupiters are thought to form far from their stars, and over time migrate much closer. But this planet seems to have formed just where it is today.”
The most plausible explanation is that Gliese 3470b was born precariously close to its red dwarf star, which is about half the mass of our Sun. The planet started out as a dry rock, and rapidly accreted hydrogen from a protoplanetary disk when its star was very young.
“We’re seeing an object that was able to accrete hydrogen from the protoplanetary disk, but didn’t runaway to become a hot Jupiter. This is an intriguing regime,” Dr. Benneke said.
“One explanation is that the disk dissipated before the planet could bulk up further. The planet got stuck being a sub-Neptune.”
The findings were published in the journal Nature Astronomy.
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Björn Benneke et al. A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds. Nature Astronomy, published online July 3, 2019; doi: 10.1038/s41550-019-0800-5
