New ultra-hot exoplanet the size of Neptune discovered

Using NASA’s Transiting Exoplanet Satellite (TESS), an international team of astronomers has discovered a new exoplanet. The newly discovered alien world, named TOI-3261b, is nearly the size of Neptune and its equilibrium temperature exceeds 1700 K. The discovery was reported in a research paper published July 5 on the preprint server. arXiv.

To date, TESS has identified more than 7,200 candidate exoplanets (TESS Objects of Interest, or TOIs), of which 482 have been confirmed. Since its launch in April 2018, the satellite has conducted a survey of around 200,000 of the brightest stars near the sun in order to search for transiting exoplanets – ranging from small rocky worlds to gas giants.

Another TOI monitored by TESS has recently been confirmed by a group of astronomers led by Emma Nabbie of the University of Southern Queensland in Australia. They report that a transit signal identified in the light curve of TOI-3261, a K-type main sequence star about 972 light-years away, is caused by a planet.

“We confirm TOI-3261 b as a planet through multiple, independent detection methods. From ground-based observatories, we find transit signals on the target, reproducing the first signals seen by TESS,” the researchers explained.

TOI-3261 b has a radius of approximately 3.82 Earth radii, so it is only about 2% smaller than Neptune. However, the newly discovered exoplanet has an unusually high mass for its size – about 30.3 Earth masses. These results give a high bulk density of 3.0 g/cm³.

TOI-3261 b orbits its host every 0.88 days at a distance of 0.017 AU from it. Because of this, it was classified as an ultra-short-period planet (USP), and so far, it is only the fourth known Neptune-sized USP. Furthermore, the planet’s proximity to its parent star, which has an effective temperature of about 5,070 K, suggests that this extrasolar world is ultra-hot—with an estimated equilibrium temperature of 1,722 K.

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

When it comes to the star TOI-3261, it has a spectral type K1.5 V and is about 13% smaller and less massive than the sun. The star is estimated to be 6.5 billion years old and its metallicity is at the level of 0.11 dex.

Summarizing the results, the authors of the paper note that TOI-3261 b is an excellent target for follow-up atmospheric observations. Such studies can be conducted using space observatories such as the James Webb Space Telescope (JWST).

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