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New warm sub-Neptune exoplanet discovered with TESS

Using NASA's Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a new warm sub-Neptune exoplanet, which is nearly three times larger than the Earth. The finding was reported in a research paper June 13 on the pre-print server arXiv.

To date, TESS has identified more than 7,200 candidate exoplanets (TESS Objects of Interest, or TOI), of which 475 have been confirmed. Since its launch in April 2018, the satellite has been conducting a survey of about 200,000 of the brightest stars near the sun with the aim of searching for transiting exoplanets—ranging from small, rocky worlds to gaseous giants.

Now, a group of astronomers led by Molly Nies of East Stroudsburg University in Pennsylvania, reports the confirmation of another TOI monitored by TESS. They identified a transit signal in the light curve of HD 21520, or TOI-4320—a sun-like G star at a distance of about 257 light years. Planetary nature of this signal was validated by follow-up observations.

"We report the discovery and validation of HD 21520 b, a transiting planet found with TESS and orbiting a bright G dwarf," the researchers wrote in the paper.

HD 21520 b has a radius of 2.7 Earth radii and its mass is assumed to be around 7.9 Earth masses (with an upper limit of 17.7 Earth masses. The planet orbits its host every 25.13 days, at a distance of 0.17 AU from it. The equilibrium temperature of HD 21520 b was estimated to be 637 K.

Based on the derived parameters, the astronomers calculated that HD 21520 b has a bulk density at a level of most likely 2.21 g/cm³, which suggests that it likely contains a significant atmosphere.

When it comes to the planet's interior structure, the scientists suppose that it contains a significant water mass and radius fraction, as well as a significant gas radius fraction. However, more radial velocity measurements are required in order to refine the mass of HD 21520 b and to confirm the assumptions regarding its composition.

The authors of the paper noted that HD 21520 b is a promising candidate for atmospheric characterization. This could be achieved through transmission spectroscopy—due to the planet's size, likely low mass, and bright host star.

"Due to the bright nature of its host and likely significant gas envelope of the planet, HD 21520 b is a promising candidate for further mass measurements and for atmospheric characterization," the researcher concluded.

The host star HD 21520 b is about a few percent larger and more massive than the sun. It has an effective temperature of 5,871 K and metallicity at a level of 0.049. It is estimated that the star is approximately 5.3 billion years old.