TESS telescope discovers super-hot exoplanet the size of Neptune

An international group of astronomers, using the NASA TESS (Transiting Exoplanet Survey Satellite), has discovered a new exoplanet, TOI-3261 b. This exoplanet is nearly the size of Neptune and has a temperature exceeding 1700 Kelvin.

TOI-3261 b was detected in the brightness curve of the star TOI-3261, which is a K-type main sequence star located approximately 972 light-years away. This transit signal was confirmed as a planet using multiple independent detection methods, including ground-based observatories that reproduced the transit signals initially detected by TESS.

The radius of TOI-3261 b is approximately 3.82 times that of Earth, making the planet only 2% smaller than Neptune. However, this newly discovered exoplanet has an unusually large mass for its size—about 30.3 Earth masses—resulting in a high density of 3.0 g/cm³.

TOI-3261 b orbits its star every 0.88 days at a distance of 0.017 astronomical units (AU). Due to these characteristics, the exoplanet has been classified as an ultra-short period planet (USP), and it is currently only the fourth known Neptune-sized USP. Moreover, the planet's proximity to its parent star suggests a surface temperature around 1722 Kelvin.

Based on the collected data, astronomers suggest that TOI-3261 b retains a gaseous envelope with a core potentially enriched in water. They estimate that this gaseous envelope constitutes about 5% of the planet's total mass, which is the second largest fraction among known Neptune-sized USPs.

Regarding the star TOI-3261, it has a spectral type of K1.5 V, a temperature of about 5070 Kelvin, and is approximately 13% smaller and less massive than the Sun. The star is estimated to be 6.5 billion years old, with a metallicity level of 0.11 dex—indicating it has 1.29 times more metals (elements heavier than helium) than the Sun.

In conclusion, the authors note that TOI-3261 b is an excellent target for future atmospheric observations, which can be conducted using space observatories such as the James Webb Space Telescope (JWST).