An article published in “The Astronomical Journal” reports the identification of the exoplanet TOI-3261 b, an ultrahot Neptune very close to its star, one of the very few planets of that type known. A team of researchers used observations conducted with NASA’s TESS space telescope and detections obtained with the ESPRESSO and HARPS instruments to confirm its existence and obtain information on its characteristics. TOI-3261 b orbits in what is known as the Neptune desert precisely because it’s in an area where it’s very rare to find Neptune-type planets. For this reason, it will help to understand the evolution of certain types of planets.
The existence of the Neptune desert was hypothesized in 2011 by examining the data about the exoplanets known at the time, a still limited amount. In the following years, the discovery of many more exoplanets confirmed that there were no exoplanets with a mass comparable to Neptune’s with a year shorter than a few Earth days.
Over time, the Neptunians NGTS-4b and LTT 9779 b were found in the Neptune desert. In 2021, the candidate Vega b was reported but as of today, it seems that it doesn’t exist. In short, these are extremely rare events even among the thousands of exoplanets known today. For this reason, the discovery of TOI-3261 b, almost 1,000 light-years from Earth, is very interesting for astronomers.
The exoplanet candidate later identified as TOI-3261 b was discovered thanks to observations conducted by NASA’s TESS space telescope. Other telescopes offered confirmation of its existence. Mass estimations using the radial velocity method were conducted with the ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) instrument on the Very Large Telescope (VLT) and the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument on the 3.6-metre telescope at La Silla Observatory, both ESO observatories in Chile.
The data indicates that the exoplanet TOI-3261 b has a mass of about 30 times the Earth’s for a radius with a peak probability of 3.82 times the Earth’s. Its year is extremely short since it orbits its star in just 21 hours. The star TOI-3261 is slightly less massive than the Sun, so this ultrahot Neptune exoplanet is hit by strong stellar winds and radiation.
The extreme proximity of the exoplanet TOI-3261 b to its star suggests that stellar winds are slowly blowing away its atmosphere, and even its own gravity could be stealing gas from it. This means that TOI-3261 b may have once had a much larger mass, of the Jupiter type.
TOI-3261’s planetary system is estimated to be around 6.5 billion years old. This suggests that TOI-3261 b formed much further away from its star and then moved closer to it, and only then started losing its atmosphere. Planetary migration is being studied, and astronomers believe it’s the result of gravitational interactions between planets. For now, TOI-3261 b is the only planet detected in its system, but detecting a planet passing in front of its star if its year is very long requires luck.
The current atmosphere of exoplanet TOI-3261 b may be very different from the original one since the lightest parts are the ones that are lost first. Nevertheless, its examination could provide interesting information to better understand the evolution of hot Neptunes in the Neptune desert. The James Webb Space Telescope is ideal for this type of in-depth study.
Understanding whether exoplanets like TOI-3261 b formed like other planets very close to their stars or if it was originally a gas giant of the Jupiter type will help to improve models of planetary formation and evolution.
