Two NASA space telescopes have teamed up to identify, for the first time, the detailed chemical “fingerprint” of a planet between the sizes of Earth and Neptune. No planets like this can be found in our own solar system, but they are common around other stars.
The planet, Gliese 3470 b (also known as GJ 3470 b), may be a cross between Earth and Neptune, with a large rocky core buried under a deep crushing hydrogen and helium atmosphere. Weighing in at 12.6 Earth masses, the planet is more massive than Earth, but less massive than Neptune (which is more than 17 Earth masses).
Many similar worlds have been discovered by NASA’s Kepler space observatory, whose mission ended in 2018. In fact, 80% of the planets in our galaxy may fall into this mass range. However, astronomers have never been able to understand the chemical nature of such a planet until now, researchers say.
By inventorying the contents of GJ 3470 b’s atmosphere, astronomers are able to uncover 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,” said Björn Benneke of the University of Montreal, Canada. “We don’t have anything like this in the solar system, and that’s what makes it striking.”
Astronomers enlisted the combined multi-wavelength capabilities NASA’s Hubble snd Spitzer space telescopes to do a first-of-a-kind study of GJ 3470 b’s atmosphere.
This was accomplished by measuring the absorption of starlight as the planet passed in front of its star (transit) and the loss of reflected light from the planet as it passed behind the star (eclipse). All totaled, 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. But he is at a loss for classification: Should it be called a “super-Earth” or “sub-Neptune?” Or perhaps something else?
Fortuitously, the atmosphere of GJ 3470 b turned out to be mostly clear, with only thin hazes, enabling the scientists to probe deep into the atmosphere.
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