Tess Finds Mars-Sized Planet That is Dense as Iron

Find May Point to Entire System of Habitable Exoplanets

Researchers using the Transiting Exoplanet Survey Satellite (TESS) have discovered a relatively nearby (at least in astronomical terms) exoplanet that is the size of Mars, but as dense as pure iron. Plus, the team behind the discovery notes, this planet is around the type of star that has often been home to multi-planet systems, including those in the habitable zone, meaning this unique discovery may potentially lead to the discovery of other, more life-friendly planets in the same system.


Known as ultra-short-period exoplanets, or simply USPs, these celestial bodies somehow maintain an orbit that is extremely close to their host star. Only astronomers aren’t sure how they pull off this seemingly impossible feat. Unfortunately, these types of worlds are usually small, often smaller than Earth, so their close orbital period and diminutive size make them particularly hard to identify, much less study.

Now, the team behind this latest discovery shows that existing telescopes can gather enough data to pinpoint the size, mass and orbital mechanics behind at least one of these types of exoplanets. And, they say, this discovery could serve as a signpost pointing to potentially habitable planets within the same system.


“For about a month in 2019, TESS recorded a patch of the southern sky that included the star GJ 376,” explains a press release announcing the exciting discovery. “Scientists at MIT (the Massachusetts Institute of Technology) and elsewhere analyzed the data, and detected a transiting object with an ultra-short, eight-hour orbit.”

According to paper, which is published in the journal Science, the teams ran a number of follow up tests to make sure what they were seeing was a real exoplanet and not a false positive. Such results have been known to happen, they explain, including from a “foreground or background eclipsing binary star.”

Once the various teams were satisfied they had a real USP, which is around a star about 30 light years away, they employed another observation tool to take an even closer look. Known as the High Accuracy Radial Velocity Planet Searcher, or HARPS, it is an instrument already installed on the European Southern Observatory’s telescope in Chile.

Following some careful measurements, HARPS provided data regarding the planet’s radius (72% of Earth) and mass (55% of Earth). These dimensions, the various teams note, indicate the planet has an iron rich core, and may even be the core of a previously rocky planet that has lost its mantle.

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


Although this particular planet is too close to retain an atmosphere or any signs of extraterrestrial life (at least, life as we know it) the research team behind its discovery notes that this type of M dwarf star is known to be home to multi-planet systems. For instance, astronomers awaiting the launch of the James Webb Telescope later this month hope to use the first of its kind space observatory to scan the atmosphere’s of certain planets within a seven planet system, four of which potentially lie in the star’s habitable zone, orbiting the M dwarf star TRAPPIST-1.

For the team behind this latest discovery, this exciting result means that along with improving the methods of finding and analyzing planets this small and this close to their host stars, they are also pointing their fellow extraterrestrial life hunting researchers to other potentially habitable planets around this same star.

“For this class of star, the habitable zone would be somewhere between a two-to-three-week orbit,” says team member George Ricker, senior research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “Since this star is so close by, and so bright, we have a good chance of seeing other planets in this system. It’s like there’s a sign saying, ‘Look here for extra planets!’”

Follow and connect with author Christopher Plain on Twitter: @plain_fiction