Hell, it may turn out, is about 30 light years away, on a smallish planet orbiting a smallish star. Its name isn’t Hades — it’s GJ 367b.
Imagine a planet around three-quarters the size of Earth — larger than Mercury, but smaller than Mars — and as dense as pure iron. It’s barely more than the metallic core of a large planet. Now set this planet so close to its star that it takes a little less than eight hours to complete an orbit. Close enough to present the same face to its star as our Moon does to Earth, the sunny side heated to a scorching 2,200 degrees Fahrenheit.
“That side of the planet we think is just molten rock,” Caltech Atrophysicst and NASA Exoplanet Science Institute Scientist Jessie Christiansen tells Inverse. She’s one of the authors on a new paper published Thursday in the journal Science describing the discovery of the exoplanet GJ 367b, a world of “big lava lakes, or lava oceans.”
Scientists don’t yet understand just how GJ 367b got to be where it is, and as weird as it is, or whether it was always a little hellish ball of molten hell or was once a larger planet, but they plan to find out. And the answers could wind up telling us as much about our place in the universe as they reveal about the unique situation of GJ 367b.
What’s new?— GJ 367b is not the first exoplanet discovered to orbit so close to its star — “some of us call them lava worlds,” Christiansen adds — but it is one of the weirdest and may soon become one of the best-studied.
The planet’s position around a smaller star allowed scientists to very accurately estimate its density, which is close to that of iron. It suggests GJ 367b’s internal structure is a more extreme version of Mercury in our Solar System. Mercury’s metallic core makes up almost 85 percent of the planet’s total volume.
GJ 367b may likewise consist of a metallic core covered by a thin mantle and the faintest whispers of a hydrogen and helium atmosphere. Of course, one-half of the planet may exist in a persistent, molten state, with only the night side growing cold enough to maintain a solid structure.
The planet may also sport clouds of silica mantel materials blasted from the surface by the nearby star. “That’s how intense the radiation is,” Christiansen says.
How did they make this discovery?— Ultrashort-period exoplanets like GJ 367b are generally relatively small, rocky planets that exist at the very limits of scientists’ ability to detect them. But the parent star, GJ 367, is a small red dwarf, making the planet easier to detect.
“Almost all of the ways we measure exoplanets rely on actually measuring their impact on the star that they’re orbiting,” Christiansen says. “The smaller the star is, the bigger the effects that that planet has on the star, either in terms of a gravitational pull or in terms of blocking the light from the star” as the planet passed in front of it.
Scientists used Transiting Exoplanet Survey Satellite, or TESS space telescope, to discover GJ 367b. Launched in 2018, TESS is optimized to search for exoplanets around M-type stars, Christiansen says, “because they’re the stars that offer the best opportunities to follow up on the planet.”
Scientists cannot characterize all exoplanets discovered by TESS in as much detail as they have GJ 367b, but its short orbital period allowed researchers to so well characterize its size and mass that follow-up studies may be able to reveal more exciting details about this bizarre world.
“This will be another example of a planet where we might be able to determine something about its surface even, which is very rare,” Christiansen says. “We’ll go now, and we’ll try and see if we can find anything about its atmosphere. Like, does it have this cloud of silicate? Does it have a hydrogen-helium atmosphere?”
Why does it matter?— Before the discovery and characterizations of exoplanets, scientists based theories of planetary system formation on observations of our Solar System. “If your theory didn’t produce something that looked vaguely like the Solar System, then you threw it out,” Christiansen says. “When we went out and started to find real exoplanets around other the stars, the first thing we discovered is that they don’t look anything like our solar system as a general rule.”
Solar Systems like our own, with rocky planets in the habitable zone of their stars, could be pretty rare. Understanding why and how planets like GJ 367b came to exist may help scientists determine just how standard, or freakish, our Solar System really is.
When it comes to ultrashort-period planets, the exciting thing is they really shouldn’t exist. “It’s difficult to come up with ways to get a planet really, really, close to the star, but not all the way into the star,” Christiansen says.
Scientists have two theories.
In one, planets like GJ 367b may migrate close to their star very early in the life of their solar system. “We think planets form in a disk of material, a protoplanetary disk — like a thick pancake of dust and gas,” Christiansen says. If enough of that disk persists after rocky planets have formed, it could slow the worlds down so that they migrate toward their star.
In the second theory, a third body, another star, or another planet, perturbs the motion of a planet like GJ 367b, causing it to move closer to its star. Christiansen says that a third body could push a larger ice giant planet like Neptune close to its star under this scenario. The solar wind blows the planet’s outer layers away, leaving only the dense core we see today.
Which of those paths explain GJ 367b took is important, she says, because scientists would like to use ultrashort-period rocky planets as a proxy count to estimate how common rocky planets like Earth might be in the universe. Planets like GJ 367b are much easier to find than planets like Earth, and if planets like GJ 367b formed early as rocky planets in the habitable zone of their star, then scientists can infer something about how common rocky planets are in general.
“But if the short period rocky planets are actually stripped cores of much larger planets, then you can’t make the same inference,” Christiansen says.
What’s next?— The next steps then for scientists studying GJ 367b, in addition to trying to better characterize its atmosphere, will be to hunt for other planets or stars sharing the distant solar system with GJ 367b.
“If there’s a third body that could be interacting with that inner planet and causing it to migrate so close to its star, then that lends credence to the theory that it’s it’s a three-body interaction that put it where it is,” Christiansen says.
There will also be studies of GJ 367b’s red dwarf star to determine how active or quiet it is in terms of solar flares and other activity. “If this is a super active star with all these flares, then it’s much more likely that if [GJ 367b] ever had an atmosphere that it would have been stripped,” Christiansen says. “If it’s a very quiet M-star, then maybe it never had an atmosphere to start with.”
Abstract: Ultrashort-period (USP) exoplanets have orbital periods shorter than 1 day. Precise masses and radii of USP exoplanets could provide constraints on their unknown formation and evolution processes. We report the detection and characterization of the USP planet GJ 367b using high-precision photometry and radial velocity observations. GJ 367b orbits a bright (V-band magnitude of 10.2), nearby, and red (M-type) dwarf star every 7.7 hours. GJ 367b has a radius of 0.718 ± 0.054 Earth-radii and a mass of 0.546 ± 0.078 Earth-masses, making it a sub-Earth planet. The corresponding bulk density is 8.106 ± 2.165 grams per cubic centimeter—close to that of iron. An interior structure model predicts that the planet has an iron core radius fraction of 86 ± 5%, similar to that of Mercury’s interior.