We cannot understand nature without understanding its extent. This is evident in exoplanetary science and in our theories of planetary formation. Outliers and outliers in nature put pressure on our models and motivate scientists to dig deeper.
There are nearly 200 other planets in the list of more than 5,000 exoplanets, so Gliese 367 b is not unique in this regard. But it’s strange in another way: It’s also a superdense planet, about twice as dense as Earth.
This means that it must be almost pure iron.
“You can compare GJ 367 b to an Earth-like planet with its lithosphere removed.”
Elisa Goffo, lead author, University of Turin.
Astronomers found Tahay in TESS (Transiting Exoplanet Survey Satellite) data from 2021. But the new research in Astrophysical Journal Letters It improves the alien planet’s mass and radius through improved measurements. I also found two brothers of this planet.
Search is “Company for very high density and very short period underground GJ 367 b: discovery of two additional low-mass planets at 11.5 and 34 days.“The lead author is Elisa Gofo, a PhD student in the Department of Physics at the University of Turin.
TESS found Gliese 367 b in 2021 when it detected an extremely weak transit signal from the red dwarf star called Gliss 367. The signal was within TESS’s detection capacity, so astronomers knew it was small, like Earth.
As part of the 2021 effort, researchers used the European Southern Observatory’s High Speed Planet Searcher Spectrometer (HARPS) to determine the mass and density of G 367 b.
They concluded that the planet’s radius is 72% of the Earth’s diameter, and its mass is 55% of the Earth’s mass. This means it is likely an iron planet, the remains of the core of a much larger planet.
Fast forward to now and new research conducted by Goffo and her colleagues.
They also used HARPS to measure the minor planet. This time they used 371 HARPS observations of G 367 b. These results show that the planet is denser than the 2021 study found. Instead of the Earth’s mass being 55% of Earth’s mass, this new research reveals that the planet’s mass is 63% of Earth’s mass. Its radius also decreased from 72% of the Earth’s radius to 70% of the Earth’s radius.
What it boils down to is that the density of G 367 b is twice that of Earth.
How did the planet become this way? It is unlikely that it was formed in the way it is now. Instead, it is likely the core of the planet whose lithosphere has been stripped away.
“You can compare GJ 367 b to an Earth-like planet with its lithosphere removed,” said lead author Jovo.
“This may have important implications for the formation of GJ 367 b. We believe the planet may have formed like Earth, with a dense core made mainly of iron, surrounded by a silicate-rich mantle.”
Something unusual must have happened for the little planet to lose its mantle. “A cataclysmic event could have stripped the planet of its rocky mantle, leaving the planet’s dense core bare,” Jovo explained. Collisions between it and other protoplanets still forming early in their lives could have removed the planet’s outer layer.
Another possibility, according to Jovo, is that the tiny USP was born in an unusually iron-rich region in a protoplanetary disk. But this seems unlikely.
There is a third possibility, and it was first considered when astronomers discovered G 367 b in 2021. It could be the remains of a massive gas giant like Neptune.
For this to be the case, the planet must have formed far from the star and then migrated to it. It is so close to its star now that intense radiation from the red dwarf could boil the atmosphere away.
G 367 b belongs to a very small class of exoplanets called the super-Mercury planet. Its composition is the same as that of Mercury, but it is larger and denser. (Although it is rare, there is one system that does Two of them.)
Mercury may have suffered the same fate as G 367 b. It may have had more mantle and crust at one time, but the impacts removed it.
But even among the supermercuries, G 367 b stands out. It’s the most intense USP we know of.
“Thanks to our precise estimates of mass and radius, we explored the composition and possible internal structure of GJ 367 b and found that it is predicted to have an iron core with a mass fraction of 0.91,” the new paper says.
What happened in this system? How did planet G 367 b find itself in this state, so close to its star?
Researchers also found two other planets in this system: G 367 c and d. Astronomers believe that USP planets are almost always found in systems with multiple planets, so this new research reinforces that. TESS was unable to detect these planets because they do not transit their star. The team found it in HARPS observations, and its presence limits possible formation scenarios.
“Thanks to our extensive observations using the HARPS spectrometer, we discovered the presence of two additional low-mass planets with orbital periods of 11.5 and 34 days, reducing the number of possible scenarios that might have led to the formation of such a dense planet,” said co-author Davide Gandolfi, a professor at the University of Turin.
Companion planets also orbit closer to the star but have less mass. This puts pressure on, but does not eliminate, the idea that any of them formed in an iron-rich environment.
“Although GJ 367 b may have formed in an iron-rich environment, we do not rule out a formation scenario that includes violent events such as giant planetary collisions,” Gandolfi said in a press release.
In the conclusion of their paper, the team delves a little deeper into possible formation scenarios.
In the formation scenario, the protoplanetary disk around Gliese 367 must have contained an iron-rich region. But astronomers don’t know whether this type of iron-rich region exists.
“Possible pathways may include the formation of significantly more iron-rich material than is typically thought to be found in protoplanetary disks. Although it is not clear whether disks with such a large relative iron content near the inner rim specifically (Where most materials can be obtained from) exist,” they write.
In fact, separate Study 2020 Their work on planet formation “failed to reproduce the extreme enrichment in iron needed to form Mercury,” he said. If disk models can’t explain how iron-rich Mercury formed, they can’t explain how G 367 b formed.
Instead, the planet was likely different when it formed and then took on its current shape over time.
Collisional stripping is when a planet’s outer material is removed by one or more collisions. Since the outer matter is less dense than the inner matter Distinct planetsrepeated collisions could have increased the bulk density of G 367 b by removing lighter material.
But there’s at least one problem with that: “Our measurement of the bulk density of GJ 367 b suggests that collisional stripping should be remarkably effective at removing non-ferrous material from the planet if that is the only process going on.” He writes. Remarkably effective, but not impossible.
So there are three possibilities: that the planet formed in an iron-rich environment, that the planet was larger and lost its outer layers through collisions, or that the planet is the remaining core of a massive gas giant that migrated very close to Earth. Its star has been stripped of its gaseous envelope.
Maybe we don’t have to settle on one.
“Of course, it is possible that all of the processes discussed above contributed to the formation of the nearly pure iron sphere, known as GJ 367 b,” the authors wrote.
All we have now are possibilities. The system is like a puzzle, and it’s up to astronomers to solve. Its unusual properties make it an outlier and scientists love outliers because it motivates them to dig deeper. If our current theories are unable to explain these strange things, then our theories need to be improved.
“This unique multi-planetary system hosting such very high density USP Sub-Earth is an exceptional target for further investigation of formation and migration scenarios for USP systems,” the researchers concluded.
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