Two Super-Earths Found Orbiting a Red Dwarf Star at the Edge of the Habitable Zone

NASA’s Transiting Exoplanet Survey Satellite, or TESS, was designed to find other worlds. Following in the tradition of the Kepler spacecraft, TESS has a hundred thousand stars looking for small but regular dips in their brightness. These dips are typically caused by planets as they pass in front of the star. TESS has been quite effective, logging nearly 6,000 candidate exoplanets. Confirming or rejecting these candidates takes time, but it has led to some interesting discoveries.

Recently a team has been observing a red dwarf star known as TOI-2095. Its name comes from the fact that it’s a TESS Object of Interest, with candidate exoplanets. TOI-2095 is about half the mass of the Sun, but it’s only about 140 light-years away. So the team used ground observations to gather further data. Their data confirmed the transits of two planets, TOI-2095b and TOI-2095c.

The transit method of finding exoplanets is very effective, but it only tells part of an exoplanet’s story. From the pattern of transits, you can determine the period of an exoplanet’s orbit, and from the amount the star dims, you can gauge the apparent size of the planet relative to its star. But it can’t tell you things about its mass and density.

So the team also gathered Doppler data about the starlight. This information tells you how the star moves relative to Earth, and by measuring the wobble of the star the team could calculate how strongly the two planets tug on the star. This told them their masses, and knowing their size they could calculate their densities and estimate their surface temperatures.

It turns out both worlds are super-Earths. TOI-2095b is about 25% larger than Earth with about four times our planet’s mass, while TOI-2095c is about a third larger than Earth with a mass of about 7 Earths. Their surface temperatures are estimated to be about 300 K to 350 K, compared to about 290 K for Earth. The two worlds are just on the inner range of their star’s habitable zone.

It’s rare for a star to have more than one potentially habitable world, but there is also reason to be cautious. To begin with, the habitable zone of a red dwarf star is well within the orbit of Mercury, and red dwarfs are known to have some pretty intense flares, so it might not be particularly friendly. It should also be noted that the term “super-Earth” is a bit of a misnomer. You might imagine a large terrestrial world similar to Earth, but they are likely to be small gas-like planets. For example, Uranus has a mass of about 14 Earths, so these worlds could also be called sub-uranus exoplanets.

Where this study really shines is in showing how we are able to confirm ever smaller exoplanets. Most of the exoplanets we’ve discovered have been large worlds closely orbiting small stars. These planets are roughly similar to Earth’s size, showing that in time we may find a truly Earth-like world.

Reference: Murgas, F., et al. “Two super-Earths at the edge of the habitable zone of the nearby M dwarf TOI-2095.” arXiv preprint arXiv:2304.09220 (2023).