Nature’s strange exceptions put pressure on our models and push scientists to dig deeper. Gliese 367 b (or Tahay) is definitely a weirdo. This is an ultra-short period planet (USP) that orbits its star in just 7.7 hours. There are nearly 200 other USP planets in our catalog of more than 5,000 exoplanets, so Gliese 367 b is not unique in this regard. But there’s another exception: it’s also an incredibly dense planet – almost twice as dense as Earth. This means it must be almost pure iron.
Astronomers found Tahay in 2021 Transiting Exoplanet Survey Satellite (TESS) data. But new research published in the Astrophysical Journal has refined the mass and radius of this strange planet using improved measurements. He also found two siblings for the planet. The study is titled “Society for Ultra-High-Density, Ultra-Short-Duration Sub-Earth GJ 367 b: Discovery of Two More Low-Mass Planets at 11.5 and 34 Days.” » The main author is Elisa Goffo, PhD. student in the Department of Physics at the University of Turin.
TESS found Gliese 367 b in 2021 when it detected an extremely faint transit signal from a red dwarf star called Gliese 367. The signal was at the limit of TESS’s detectability, so astronomers knew it was small, like Earth. As part of the 2021 effort, researchers used the European Southern Observatory’s High Precision Radial Velocity Planet Searcher (HARPS) spectrometer to determine the mass and density degree 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 could be an iron planet, the remaining core of a once much larger planet.
Fast forward to the present and new research from Goffo and his colleagues. They also used HARPS to measure the asteroid. This time, they used 371 HARPS observations of G 367 b. These results show that the planet is even denser than the 2021 study. Instead of 55% of Earth’s mass, this new study reveals that the planet is 63% of Earth’s mass. Its radius also decreased, from 72% of Earth to 70% of Earth. In short, G 367 b is twice as dense as Earth. How did this planet get here? It is unlikely that it was formed as it is today. Instead, it could be the core of a planet that has had its rocky crust stripped away. “You can compare GJ 367 b to an Earth-like planet when the rocky crust is removed,” said lead author Goffo.
“This could have important implications for the formation of GJ 367 b. We think the planet may have formed like Earth, with a dense core made mostly of iron, surrounded by a silicate-rich mantle. » Something extraordinary must have happened to cause this small planet to lose its shell. “A catastrophic event could have stripped away its rocky shell, leaving the planet’s dense core bare,” Goffo explains. Collisions between it and other protoplanets still forming early in its life could cause the planet’s outer layer to disappear. According to Goffo, another possibility is that the small USP was born in an unusually iron-rich region of a protoplanetary disk. But that seems unlikely. A third possibility exists, and it was first considered when astronomers discovered G 367 b in 2021. It could be the remnants of a once gas giant like Neptune.
For this to happen, the planet would form further from the star, then move toward it. It is now so close to its star that intense radiation from the red dwarf could cause the atmosphere to boil. G 367 b is part of a group of very small exoplanets known as superMercuries. Their composition is the same as Mercury, but they are larger and denser. (Even though they are rare, there is one system that has two of them.) Mercury may suffer the same fate as G 367 b. There may have once been more mantles and crusts, but impacts have removed them. But even among Mercury’s supergiants, G 367 b stands out. This is the densest USP we know.
“Through accurate mass and radius estimates, we explored the composition and potential internal structure of GJ 367 b and found that it must have an iron core with a mass fraction,” the new document says. is 0.91″. So what happened in this system? How did G 367 b end up in this state and 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 study reinforces that. TESS cannot detect these planets because they do not transit their star. The team found them in HARPS observations, and their presence limits possible training scenarios. “Through in-depth observations with 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. could lead to the formation of such a dense planet.” said co-author Davide Gandolfi, professor at the University of Turin.
Companion planets also orbit close to the star but have lower masses. This puts pressure on the idea that any of them formed in iron-rich environments, but does not rule out the possibility. “While GJ 367 b could have formed in an iron-rich environment, we do not rule out a scenario of formation involving violent events such as a planetary collision,” Gandolfi said in a press release. giant”. In the conclusion of the article, the team delves a little deeper into possible training scenarios. In the formation scenario, the protoplanetary disk around Gliese 367 is thought to have an iron-rich region. But astronomers don’t know whether this type of iron-rich region exists. “Possible pathways could include the formation of material much richer in iron than is generally believed to be present in protoplanetary disks. Although it is unclear whether there were discs with such relatively high iron content, especially near the inner edge (where most of the material could have been obtained from), they wrote.
In fact, a separate 2020 study said their work on planet formation “failed to reproduce the extreme levels of Fe enrichment needed to explain the formation of Mercury.” If disk models cannot explain how iron-rich Mercury formed, they cannot explain how G 367 b formed. Instead, it’s more likely that the planet was different when it formed and then took its current shape over time. Collision stripping occurs when a planet’s outer material is removed by one or more collisions. Because the outer material is less dense than the inner material of the distinct planets, repeated collisions would increase the mass density of G 367 b by removing lighter material. Artist’s impression of a collision between a protoplanetary object and a Mercury-sized planet. Impact stripping may have removed the outer layers of G 367 b, leaving only the iron core. (NASA/JPL-Caltech) But there’s at least one problem with this: “Our measurements of the mass density of GJ 367 b show that impact stripping should be remarkably effective in removing ferrous material from the planet’s surface.” “it is the only process at work,” the authors write. The effect is obvious but not impossible.
This article was originally published by Universe Today.