Astronomers have discovered an Earth-size rocky world, or "super-Earth," that's hot enough to melt gold and, as a result, may have no atmosphere.
By comparing these super-Earths to our planet, scientists can identify which of these terrestrial exoplanets might be capable of hosting life.
"We're just beginning to learn how often, and under what circumstances, rocky planets can keep their atmospheres," Laura Kreidberg, an exoplanet scientist at the Max Planck Institute for Astronomy in Germany and co-author of a new paper describing the discovery, said in a statement. "This measurement is an indication that for the hottest planets, it's unlikely that thick atmospheres typically survive."
The exoplanet, called GJ 1252 b, is located 65 light-years away. It is much closer to its star than Earth is to the sun, and one side — its "dayside" — permanently faces its star, driving up temperatures on the exoplanet.
When the astronomers used the now-retired Spitzer Space Telescope to measure infrared radiation from GJ 1252 b as the exoplanet passed behind its star — an arrangement called a "secondary eclipse" — they discovered that the planet's dayside temperatures reached a scorching 2,242 degrees Fahrenheit (1,228 degrees Celsius).
Not only are these temperatures so hot that gold, silver and copper would melt on the planet's surface, but this extreme heat would make it difficult for GJ 1252 b to hang on to a thick atmosphere. The team thinks this searing temperature is consistent with what would be expected for a planet with a bare, rocky surface.
Moreover, the team found that GJ 1252 b has a surface pressure of no more than 10 bar, which indicates that its atmosphere — if it exists — must be substantially thinner than that of Venus.
The astronomers calculated that on GJ 1252 b, an even atmosphere thick enough to cause a surface pressure 10 times greater than this would have been stripped from the planet over the course of a million years — much shorter than the exoplanet's estimated lifetime of 3.9 billion years.
As such, GJ 1252 b has an extremely limited atmosphere or possibly no atmosphere at all, the team concluded in their paper, which was published Sept. 23 in The Astrophysical Journal Letters.
This is the smallest planet yet for which scientists have determined tight atmospheric constraints, said Ian Crossfield, an astronomer at the University of Kansas who led the research.
Further investigation of GJ 1252 b with the James Webb Space Telescope (JWST) could place even tighter constraints on the planet's atmosphere — or lack thereof — over the coming years.
"At the time, Spitzer was the only facility in the known universe that could make these sorts of measurements," Crossfield said in the statement. "Now, Spitzer has been turned off, but JWST is there, and at these wavelengths, it's much more sensitive than Spitzer was."
That means the kind of work Crossfield and his team did with Spitzer will be much easier with JWST.
"JWST observations in the infrared have the potential to reveal the surface properties of hot, rocky planets like this," Kreidberg said. "Different types of rock have different spectral signatures, so we could potentially learn what type of rock GJ 1252b is made of."
Further study of GJ 1252 b could thus reveal the planet's composition, and this investigation could be spread over many more terrestrial worlds like GJ 1252b, thereby giving astronomers a better understanding of relatively small, hot exoplanets, the researchers said.
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