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 than Neptune (which is more than 17 Earth masses.) Many similar worlds have been discovered at 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 the planet's nature and origin. [1
Astronomers enlisted the combined multi-wavelength capabilities of NASA's Hubble and Spitzer space telescopes or 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 customs, 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 is it a loss for classification: Should it be called "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. elements like oxygen and carbon which are forming abundant water vapor and methane gas, similar to what we see on Neptune, "said Benneke. "Instead, we found an atmosphere that is so poor in heavy elements that its composition resembles the hydrogen / helium-rich composition of the Sun."
and over time migrate much closer. But this planet seems to have formed just where it is today, said Benneke
The most plausible explanation, according to Benneke, is that GJ 3470 was born precariously close to its red dwarf star, which is about half the mass our Sun. He hypothesizes that it started out as a dry rock and rapidly accredited hydrogen from a primordial disk of gas when its star was very young. The disk is called a "protoplanetary disk."
"Not seeing an object that was able to get hydrogen from the protoplanetary disk but didn't run away to become a hot Jupiter," said Benneke. "This is an intriguing regime."
One explanation is that the disk dissipated before the planet could bulk up further. "The planet got stuck as a sub-Neptune," said Benneke.
The new results have already spawned great interest from American and Canadian teams developing the instruments on Webb. They will observe the transits and eclipses of GJ 3470 b at light wavelengths where the atmospheric hazes become increasingly transparent. [TheHubbleSpaceTelescopeisaprojectofinternationalcooperationbetweenNASAandESA(EuropeanSpaceAgency)NASA'sGoddardSpaceFlightCenterinGreenbeltMarylandmanagesthetelescopeTheSpaceTelescopeScienceInstitute(STScI)inBaltimoreMarylandconductsHubblescienceoperationsSTScIisoperatedforNASAattheAssociationofUniversitiesforResearchinAstronomyinWashingtonDC
The Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Space operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech Manages JPL for NASA.