New warm exoplanet south of Neptune discovered with TESS

Using NASA’s Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a new warm sub-Neptune exoplanet nearly three times the size of Earth. The discovery was reported in a research paper published on the pre-print server on June 13 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 surveyed about 200,000 of the brightest stars near the Sun with the aim of searching for passing exoplanets ranging from small, rocky worlds to gas giants.

Now a group of astronomers led by Molly Nies of East Stroudsburg University in Pennsylvania reports 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 about 257 light-years away. The planetary nature of this signal was validated by follow-up observations.

“We report the discovery and validation of HD 21520 b, a planet found by TESS 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 believed to be about 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 at 637 K.

Based on the derived parameters, the astronomers calculated that HD 21520 b has a bulk density at a level most likely 2.21 g/cm3.³indicating that there is probably a significant atmosphere present.

When it comes to the planet’s internal structure, scientists hypothesize that it contains a significant fraction of its water mass and radius, as well as a significant fraction of its gas radius. However, more radial velocity measurements are needed to refine the mass of HD 21520 b and confirm 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 the 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 a metallicity of 0.049. The star is estimated to be about 5.3 billion years old.