We cannot understand nature without understanding its distribution. This is evident in exoplanet science and our theories of planet formation. Nature’s outliers and oddities put pressure on our models and encourage scientists to dig deeper. Gliese 367 b (or Tahay) is certainly an oddity. It is an Ultra Short Period Planet (USP) that orbits its star in just 7.7 hours. There are almost 200 other USP planets in our catalog of more than 5,000 exoplanets; thus Gliese 367 b is not unique in this respect. But it is different on the other hand: It is also a super-dense planet; It is almost twice as dense as Earth.

This means it must be almost pure iron.

Astronomers found Tahai in 2021 TESS (Transiting Exoplanet Survey Satellite) data. But a new study Astrophysics Journal Letters refines the mass and radius of this strange planet with improved measurements. He also found two brothers for the planet. The study is titled “Ultra-high-density, ultra-short-period sub-Earth GJ 367 b Enterprise: Discovery of two additional low-mass planets in 11.5 and 34 days.” Lead author Elisa Goffo, Ph.D. Student at the Faculty of Physics of the University of Turin.

TESS discovered Gliese 367 b in 2021 when it detected an extremely weak transit signal from a red dwarf star called Gliese 367. The signal was at the limit of TESS’ detection capabilities, so astronomers knew it was as small as Earth.

As part of the 2021 study, researchers used the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph at the European Southern Observatory to determine the mass and density of G 367 b. They determined that the planet’s radius is 72% of Earth’s and its mass is 55% of Earth’s. This means it was probably once an iron planet, the remnant core of a much larger planet.

Let’s go back to the new study by Goffo and colleagues.

They also used HARPS to measure the minor planet. This time they used 371 HARPS observations of G 367 b. These results suggest that the planet is even denser than the 2021 study suggests. This new study shows that instead of 55% of Earth’s mass, the planet is 63% of Earth’s mass. Its radius also decreased from 72% Earth to 70% Earth. This comes down to the fact that G 367 b is twice as dense as Earth. How did the planet get like this? It is unlikely that it will develop as it is now. Instead, it is likely the planet’s core from which the rocky mantle has been stripped away.

“You can compare GJ 367 b to an Earth-like planet with its rocky mantle removed,” said Goffo, the study’s lead author. “This could have significant implications for the formation of GJ 367 b. We believe the planet may have formed like Earth, with a dense core composed mostly of iron and surrounded by a silicate-rich mantle.”

Something extraordinary must have happened for the small planet to lose its mantle. “A catastrophic event would have stripped the planet of its rocky mantle, leaving the planet’s dense core exposed,” Goffo explained. Early collisions between it and other still-forming protoplanets may have removed the planet’s outer layer. Another possibility, according to Goffo, is that the little USP was born in a region of the proto-planetary disk that is extremely iron-rich. But this seems unlikely.

There is a third possibility, and it was first considered when astronomers discovered G 367 b in 2021. It may be the remnant of a once huge gas giant like Neptune. For this to happen, the planet would have to form farther from the star and then migrate. It is now so close to its star that the intense radiation from the red dwarf could disperse the atmosphere.

G 367 b belongs to a class of very small exoplanets called super Mercury. Their composition is the same as that of Mercury, but they are larger and denser. (Although rare, there is a system with two.) Mercury may have suffered the same fate as G 367 b. It may once have had more mantle and crust, but impacts removed it.

But even among the super Mercurys, G 367 b stands out. This is the most intense USP we know. “With our precise estimates of mass and radius, we investigated the potential internal composition and structure of GJ 367 b and found that it is expected to have an iron core with a mass ratio of 0.91,” the new paper says.

So what happened in this system? How did G 367 b get to this point and get so close to its star?

Researchers also found two more planets in this system: G 367 c and d. Astronomers believe that USP planets are almost always found in multi-planet systems, so this new study confirms that. TESS could not detect these planets because they did not pass near their stars. The team found these in HARPS observations, and their presence limits possible formation scenarios.

“Thanks to our intensive observations with the HARPS spectrograph, we discovered the existence of two additional low-mass planets with orbital periods of 11.5 and 34 days, which reduces the number of possible scenarios that could lead to the formation of such a dense planet,” said the partner, a professor at the University of Turin. Author: Davide Gandolfi

Satellite planets also orbit close to the star but have less mass. This puts pressure on the idea that any of these formed in an iron-rich environment, but does not eliminate it. “Although GJ 367 b formed in an iron-rich environment, we do not rule out a formation scenario involving violent events such as the collision of giant planets,” Gandolfi said in a press release. said.

At the end of their work, the team investigates possible formation scenarios.

In the formation scenario, the protoplanetary disk around Gliese 367 should have an iron-rich region. But astronomers don’t even know if such an iron-rich region exists.

“Possible pathways may include the formation of much more iron-rich material than is normally thought to be present in protoplanetary disks. Although it is not clear whether disks with such high relative iron content exist in the immediate vicinity of the inner edge, where most of the material can be obtained,” they write. .

In fact, a separate 2020 study says planet-forming studies “failed to reproduce the extreme Fe enrichment required to form Mercury.” If disk models cannot explain how iron-rich Mercury formed, they cannot explain how G 367 b formed.

Instead, it is more likely that the planet was different when it formed and took its current shape over time. Collision denudation is the removal of a planet’s outer material by one or more collisions. Because the outer material is less dense than the inner material of differentiated planets, repeated collisions would increase the mass density of G 367 b by removing lighter material.

But there is at least one problem with this. “Our measurement of the mass density of GJ 367 b suggests that collisional desorption, if this is the only process involved, must be extremely effective in removing non-ferrous material from the planet,” the authors write. Extremely effective, but not impossible. So there are three possibilities: The planet formed in an iron-rich environment, the planet was once larger and lost its outer layers as a result of collisions, or the planet is the remnant core of a once massive gas giant that migrated too close to its star. and stripped from the gas mantle.

Maybe we shouldn’t stop at one point. “Of course, all of the processes described above may have contributed to the formation of the nearly pure iron globule known as GJ 367 b,” the authors write. All we have now are opportunities. The system is like a puzzle and astronomers must solve it. Its unusual features make it extraordinary, and scientists enjoy these floods because it encourages them to dig deeper. If our current theories cannot explain these oddities, then our theories need to be improved.

“This unique multi-planetary system containing an ultra-high density USP subsurface is an outstanding target for further investigation of the formation and migration scenarios of USP systems,” the researchers conclude.