Colossal exoplanet is one of the most massive super-Earths ever discovered

A newly discovered exoplanet just 200 light-years away could shed new light on one of the strangest mysteries in planetary science.

About 1.8 times the radius of Earth, the object named TOI-1075b is among the largest examples of a super-Earth exoplanet we have found to date. It also sits solidly in what we call the small planet radius gap; an apparent deficit of planets between 1.5 and 2 terrestrial radii.

Slightly smaller rocky super-Earths have been found. So slightly larger worlds are full of puffy atmospheres, known as mini-Neptunes. But in the middle, it’s something like a desert.

That added girth isn’t all puff, either. The mass of TOI-1075b is 9.95 times that of Earth. That’s too heavy for a gaseous world; with the inferred density, the exoplanet is likely to be rocky, like Mercury, Earth, Mars, and Venus. This peculiarity makes it an ideal candidate for testing theories of planetary formation and evolution.

The small planet radius gap was only identified a few years ago, in 2017, when we had a large enough catalog of exoplanets (extrasolar planets, or planets outside the Solar System) for scientists to notice a pattern. For exoplanets within some close proximity to their stars, very few worlds have been found to straddle that gap.

There are several possible explanations for this; the main one seems to be that, below a certain size, an exoplanet simply doesn’t have the mass to hold back an atmosphere against evaporative radiation so close to the host star. According to this model, the exoplanets in the gap should have a fairly sizable atmosphere consisting mostly of hydrogen and helium.

Enter TOI-1075b. It was detected in data from NASA’s exoplanet-hunting telescope, TESS. Short for Transiting Exoplanet Survey Satellite, TESS looks for weak, regular dips in light from other stars, suggesting that these stars are being orbited by an exoplanet. Astronomers can also determine the radius of that exoplanet based on how much light from the star is dimming.

The TESS data suggested that the orange dwarf star TOI-1075 was being orbited by an exoplanet about 1.72 times the radius of Earth, in an orbital period of about 14.5 hours. This caught the attention of MIT astronomer Zahra Essack, who is studying hot super-Earths. At that radius and proximity, the candidate world at that time fit the criteria for a radius gap world.

The next step in trying to understand the nature of this exoplanet was to weigh it. This involves taking advantage of a different effect that an exoplanet has on its host star: gravitational. The star provides most of the gravity in a star-planet interaction, but the planet also exerts a small gravitational tug on the star. That means a star wobbles very slightly in place, and astronomers can detect this in tiny changes in the star’s light.

If we know the mass of the star, those changes can be used to measure the mass of the planet that is shaking the star. TOI-1075 has a mass and radius about 60 percent that of our own Sun, so Essack and his colleagues were able to accurately calculate the exoplanet’s mass at 9.95 Earth masses. And his precision measurements of the size returned 1,791 Earth radii.

If you know how big something is and how heavy it is, you can calculate its average density. What about TOI-1075b? It turned out to be an absolute chonk. It has a density of 9.32 grams per cubic centimeter. That’s almost twice the density of Earth at 5.51 grams per cubic centimeter, making it a candidate for the densest super-Earth on the books.

An exoplanet in the mass gap should have a substantial atmosphere of hydrogen and helium. TOI-1075b’s density is inconsistent with a substantial atmosphere. This is very curious. But what the exoplanet might have in its place is potentially even more fascinating.

“Based on TOI-1075b’s predicted composition and ultra-short orbital period, we do not expect the planet to have retained an H/He envelope.” the researchers write in their article.

“But, TOI-1075b could have: no atmosphere (bare rock); a metal/silicate vapor atmosphere with a composition set by the magma ocean vaporizing at the surface, since TOI-1075 b’s equilibrium temperature is hot enough to melt a rock surface; or, especially at the lower end of its allowable medium density range, possibly a thin layer of H/He or CO2 or other atmosphere.”

Yes, you read it right. TOI-1075b is so hot (because it is so close to its star) that its surface could be a magma ocean producing an atmosphere of vaporized rock.

The good news here is that we might find out. As we have seen recently, JWST is very adept at observing the atmospheres of exoplanets. Pointing it at TOI-1075b should reveal whether it has a thin atmosphere, a silicate atmosphere, or no atmosphere at all, and this information could reveal some previously unknown quirk of planet formation and evolution, and how super-Earths lose their gas.

The team’s research has been accepted in The Astronomical Journal and is available on arXiv.