TESS Finds a New Mars-Sized Planet (With the Density of Mercury) – Universe Today

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

And the planets density is almost equivalent to pure iron.

The planet orbits a red (M-type) dwarf star named GJ 367 about 31 light-years away. Its named GJ 367b and its 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.)

Were finding a Mars-sized planet that has the composition of Mercury, said study co-author Roland Vanderspek, a principal research scientist at MIT. Its among the smallest planets detected to date, and its spinning around an M dwarf on a very tight orbit.

Sub-Earths can be very difficult to detect around other stars because theyre 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?

GJ 367b is no different.

Its surface is showered with about 576 times more radiation than Earth is and theres 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 its 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 planets 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 ESOs 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 planets size and mass.

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

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 forms, the leftover material forms a protoplanetary disk that planets form from. But the nebular hypothesis places some limitations on where planets can form.

Planets cant form as close to their star as GJ 367b is. Theres less material available for planetary formation that close to the star, and the temperatures are too high for material to condense into solids. Rocky planets like GJ 367b cant form there. According to the paper, the surface temperature is high enough to begin to melt and vaporize any silicates or metallic iron. They must form further out and migrate in somehow.

Planetary migration is a complex topic and there are different types of migration. Basically, planetary migration takes place when theres still a significant disk of gas around a young star. As the planets move through the gas, they exert a gravitational influence on the gas itself. That triggers an equal and opposite force on the planet, which can change its angular momentum. Depending on the particular circumstances, there can be either an inward or an outward migration.

What happened in GJ 367bs solar system is unclear, but astronomers will likely study it further.

Its like theres a sign saying, Look here for extra planets!

GJ 367 is an M-type star or red dwarf. Theyre known to often host multiple small planets, with some estimates saying they host an average of 2.5 planets of less than four Earth radii, with orbital periods less than 100 days. And since M-dwarfs are low mass themselves, its somewhat easier to detect planets around them. Due to the small stellar radius, the transit signal produced by a planet orbiting an M dwarf is larger than a planet of the same size orbiting a solar-type star (G dwarf), the authors write in their paper.

A similar principle holds true for radial velocity measurements. The radial velocity (RV) signal induced by a planet is also larger for an M dwarf host than for a G dwarf, due to the lower stellar mass, the authors write. M dwarfs, therefore, provide an opportunity to search for exoplanets with small radii and low masses. However, M dwarfs also have higher stellar activity, which adds some difficulty to accurate RV measurements.

Since this star is so close by, and so bright, we have a good chance of seeing other planets in this system. Its like theres a sign saying, Look here for extra planets! said team member George Ricker, a senior research scientist in MITs Kavli Institute for Astrophysics and Space Research.

M dwarfs may be unlikely to support life on nearby planets because they can flare so powerfully. That flaring would sterilize any planet that wasnt already too close to being too hot for life, like GJ 367b. But if the star hosts other planets further out, could they be habitable?

For this class of star, the habitable zone would be somewhere near a month-long orbit, said Ricker.

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TESS Finds a New Mars-Sized Planet (With the Density of Mercury) - Universe Today