Gliese 1214b is a so-called mini-Neptune, a smaller, denser version of Neptune that consists of a rocky core surrounded by a thick blanket of gas.
This artist’s concept depicts the mini-Neptune exoplanet Gliese 1214b. Image credit: NASA / JPL-Caltech / R. Hurt, IPAC.
Gliese 1214b is located approximately 52 light-years away in the constellation of Ophiuchus.
This exoplanet orbits a red dwarf star, Gliese 1214, every 38 hours at a distance of 1.3 million miles.
Discovered in 2009 by the MEarth Project, Gliese 1214b is about 2.7 times Earth’s diameter and weighs almost seven times as much.
And while the planet, also known as GJ 1214b, is too hot to harbor liquid-water oceans, water in vaporized form still could be a major part of its atmosphere.
“The planet is totally blanketed by some sort of haze or cloud layer,” said University of Maryland astronomer Eliza Kempton.
“The atmosphere just remained totally hidden from us until this observation.”
“If indeed water-rich, the planet could have been a water world, with large amounts of watery and icy material at the time of its formation.”
To penetrate such a thick barrier, Dr. Kempton and colleagues took a chance on a novel approach.
In addition to making the standard observation, they tracked Gliese 1214b through nearly its entire orbit around its parent star.
Using Webb’s Mid-Infrared Instrument (MIRI), they were able to create a kind of heat map of the planet.
The heat map revealed — just before the planet’s orbit carried it behind the star, and as it emerged on the other side — both its day and night sides, unveiling details of the atmosphere’s composition.
“The ability to get a full orbit was really critical to understand how the planet distributes heat from the day side to the night side,” Dr. Kempton said.
“There’s a lot of contrast between day and night. The night side is colder than the day side.”
“In fact, the temperatures shifted from 279 to 165 degrees Celsius (535 to 326 degrees Fahrenheit).”
Such a big shift is only possible in an atmosphere made up of heavier molecules, such as water or methane, which appear similar when observed by MIRI.
That means the atmosphere of Gliese 1214b is not composed mainly of lighter hydrogen molecules, which is a potentially important clue to the planet’s history and formation — and perhaps its watery start.
“This is not a primordial atmosphere. It does not reflect the composition of the host star it formed around,” Dr. Kempton said.
“Instead, it either lost a lot of hydrogen, if it started with a hydrogen-rich atmosphere, or it was formed from heavier elements to begin with — more icy, water-rich material.”
“And while the planet is hot by human standards, it is much cooler than expected.”
That’s because its unusually shiny atmosphere, which came as a surprise to the researchers, reflects a large fraction of the light from its parent star rather than absorbing it and growing hotter.
The new observations could open the door to deeper knowledge of mini-Neptunes — the most common type of planet in the galaxy, but mysterious to us because they don’t occur in our Solar System.
“For the last almost decade, the only thing we really knew about Gliese 1214b was that the atmosphere was cloudy or hazy,” said Dr. Rob Zellem, an astronomer at NASA’s Jet Propulsion Laboratory.
“This paper has really cool implications for additional detailed climate interpretations — to look at the detailed physics happening inside this planet’s atmosphere.”
The new work suggests Gliese 1214b might have formed farther from its star, then spiraled gradually inward to its present, close orbit.
“The simplest explanation, if you find a very water-rich planet, is that it formed farther away from the host star,” Dr. Kempton said.
The results appear in the journal Nature.
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E.MR. Kempton et al. A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve. Nature, published online May 10, 2023; doi: 10.1038/s41586-023-06159-5
