Astronomers Discover Ultrahot Neptune around TOI-3261

The newly-discovered exoplanet orbits TOI-3261 (also known as TIC 358070912), an inactive main-sequence star located 300 parsecs (978.5 light-years) away in the constellation of Hydrus.

Named TOI-3261b (TIC 358070912b), the newfound exoplanet has a radius of 3.82 Earth radii.

It has a mass of 30.3 Earth masses — more than twice the median mass of Neptune-sized planets on longer orbits.

To determine these properties, University of Southern Queensland astronomer Emma Nabbie and her colleagues used data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the Las Cumbres Observatory Global Telescope, and the ESPRESSO and HARPS instruments.

“Our measurements placed TOI-3261b squarely in the hot Neptune desert — a category of planets with so few members that their scarcity evokes a deserted landscape,” the astronomers said in a statement.

“This variety of exoplanet is similar to our own Neptune in size and composition, but orbits extremely closely to its star.”

The planet orbits its parent star, TOI-3261, once every 21 hours.

Such a tight orbit earns this planet its place in the exclusive group with, so far, only three other ultra-short-period hot Neptunes: LTT-9779b, TOI-849b and TOI-332b.

“TOI-3261b proves to be an ideal candidate to test new computer models of planet formation,” the researchers said.

“Part of the reason hot Neptunes are so rare is that it is difficult to retain a thick gaseous atmosphere so close to a star.”

“Stars are massive, and so exert a large gravitational force on the things around them, which can strip the layers of gas surrounding a nearby planet.”

“They also emit huge amounts of energy, which blow the gas layers away.”

“Both of these factors mean that hot Neptunes such as TOI-3261b might have started out as much larger, Jupiter-sized planets, and have since lost a large portion of their mass.”

According to the team, the TOI-3261 system is about 6.5 billion years old, and that the planet started out as a much larger gas giant.

“It likely lost mass, however, in two ways: photoevaporation, when energy from the star causes gas particles to dissipate, and tidal stripping, when the gravitational force from the star strips layers of gas from the planet,” the scientists said.

“The planet also might have formed farther away from its star, where both of these effects would be less intense, allowing it to retain its atmosphere.”

“The remaining atmosphere of the planet, one of its most interesting features, will likely invite further atmospheric analysis, perhaps helping to unravel the formation history of this denizen of the hot Neptune desert.”

“TOI-3261b is about twice as dense as Neptune, indicating that the lighter parts of its atmosphere have been stripped away over time, leaving only the heavier components,” they added.

“This shows that the planet must have started out with a variety of different elements in its atmosphere, but at this stage, it is hard to tell exactly what.”

The discovery is reported in a paper published in the Astronomical Journal.

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Emma Nabbie et al. 2024. Surviving in the Hot-Neptune Desert: The Discovery of the Ultrahot Neptune TOI-3261b. AJ 168, 132; doi: 10.3847/1538-3881/ad60be