Some planets orbit their stars so closely that they have extremely high surface temperatures and extremely rapid orbits. Most of the ones astronomers have found are Hot Jupiters— planets in the size range of Jupiter and with similar compositions as Jupiter. Their size and proximity to their star make them easier to spot using the transit method.

But there’s another type of planet that also orbits very close to their stars and has extremely high surface temperatures. They’re small, rocky, and they orbit their star in less than 24 hours. They’re called ultra-short-period (USP) planets and TESS found one that orbits its star in only eight hours.

And the planet’s density is almost equivalent to pure iron.

The planet orbits a red (M-type) dwarf star named GJ 367 about 31 light-years away. It’s named GJ 367b and it’s about 70% as large as Earth and about 55% as massive. Astronomers call it a sub-Earth.

The discovery is detailed in a new paper published in Science. Its title is “GJ 367b: A dense ultra-short period sub-Earth planet transiting a nearby red dwarf star.” The first author is Kristine Lam, currently a Post-Doctoral Researcher at the German Aerospace Center (DLR.)

“We’re finding a Mars-sized planet that has the composition of Mercury,” said study co-author Roland Vanderspek, a principal research scientist at MIT. “It’s among the smallest planets detected to date, and it’s spinning around an M dwarf on a very tight orbit.”

Sub-Earth’s can be very difficult to detect around other stars because they’re so small. Their small size makes their transit signals extremely weak, and their low masses mean they barely tug on their host stars. In this case, the detection was a little easier because the star it orbits is also a small M-dwarf.

Sub-Earths usually have no atmosphere because their stars strip it away. They have neither enough mass nor a strong enough magnetic field to retain their atmospheres.

Why is this planet missing its outer atmosphere? How did it move close in? Was this process peaceful or violent?”

Natalia Guerrero, TESS Team Member

GJ 367b is no different.

Its surface is showered with about 576 times more radiation than Earth is and there’s no way an atmosphere can withstand that intensity. All that solar radiation means the surface temperature is around 1500 C (2700 F; 1775 K.) Any atmosphere would have been stripped away by all that energy, and of course, no living thing could withstand it either.

It has no atmosphere and no chance of supporting life, but it’s an extremely interesting exoplanet for another reason: its density.

Because the planet is so close to its star, astronomers were able to measure some of the planet’s other properties, something difficult to do with other USPs. Though TESS found the planet initially, follow-up observations with the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument at the ESO’s La Silla Observatory determined that the planet is rocky and likely has a core of solid iron and nickel, similar to Mercury. Those observations also helped determine the planet’s size and mass.

From there they determined that the iron core makes up 86% of GJ 367b’s interior.

The discoverers of GJ 367b say that the exoplanet's structure is similar to Mercury's. This image shows the internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: NASA/JPL
The discoverers of GJ 367b say that the exoplanet’s structure is similar to Mercury’s. This image shows the internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: NASA/JPL

While the interior structure of the exoplanet is similar to Mercury, the planet and its situation are like nothing in our own Solar System. And its discovery begs a bunch of questions.

“Understanding how these planets get so close to their host stars is a bit of a detective story,” said TESS team member Natalia Guerrero. “Why is this planet missing its outer atmosphere? How did it move close in? Was this process peaceful or violent? Hopefully, this system will give us a little more insight.”

We know that planets can migrate from the original position they formed in. Jupiter did so. The working theory of planet formation is the nebular hypothesis. Briefly, the nebular hypothesis states that after a star

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