TOI-1685 b is a hot and rocky super-Earth exoplanet, observations find

An international team of astronomers has performed follow-up observations of a nearby alien world known as TOI-1685 b. Results of the observations, published May 21 on the pre-print server arXiv, indicate that TOI-1685 b is a hot and rocky alien world with an Earth-like density.

The so-called "super-Earths" are planets more massive than Earth but not exceeding the mass of Neptune. Although the term "super-Earth" refers only to the mass of the planet, it is also used by astronomers to describe planets bigger than Earth but smaller than the so-called "mini-Neptunes" (with a radius between two to four Earth radii).

Discovered in 2021, TOI-1685 b is an ultra-short-period (USP) super-Earth orbiting an M-dwarf star about half the size and mass of the sun. The system is located some 122.5 light years away.

Previous observations have found that TOI-1685 b orbits its host every 16.5 hours; however, the physical properties of this planet remain uncertain. Some studies have reported that TOI-1685 b is a water world (with a water fraction of around 50%) about 70% larger than the Earth, while others have suggested that it is a rocky planet with a radius of around 1.46 Earth radii, devoid of any significant water.

To resolve these discrepancies in the data, a group of astronomers led by Jennifer A. Burt of the California Institute of Technology (Caltech) decided to revisit TOI-1685 b using NASA's Transiting Exoplanet Survey Satellite (TESS) and various ground-based observing facilities.

"Here we include a refined stellar classification with a focus on addressing the host star's metallicity, an updated planet radius measurement that includes two sectors of TESS data and multi-color photometry from a variety of ground-based facilities, and a more accurate dynamical mass measurement from a combined CARMENES, IRD, and MAROON-X radial velocity data set," the researchers explained.

New observations found that TOI-1685 b has a radius of approximately 1.47 Earth radii and is about three times more massive than the Earth, which yields a density at a level of 5.3 g/cm³. Therefore, it means that the density of TOI-1685 b is just 4% less than the Earth's bulk density.

The astronomers underline that the similarity to Earth's density suggests that TOI-1685 b is also a rocky planet with minimal volume contributions from any hydrogen or helium atmosphere components. Moreover, TOI-1685 b was found to have a high equilibrium temperature—about 1,062 K.

Summing up the results, the authors of the paper note that although they exclude the possibility that TOI-1685 b may be a water world, it could still have a high mean-molecular weight atmosphere.

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