In the “Hot Neptune Desert,” a Rare Planet Offers Clues to Cosmic Mysteries

By Grace Jacobs Corban, NASA November 30, 2024

Collected at: https://scitechdaily.com/in-the-hot-neptune-desert-a-rare-planet-offers-clues-to-cosmic-mysteries/

Researchers have discovered a Neptune-sized planet, TOI-3261 b, which orbits its star every 21 hours, placing it in the rare category of “hot Neptunes.”

This planet provides critical insights into the formation and atmospheric evolution of planets close to their stars, suggesting a larger initial mass and subsequent atmospheric stripping.

Discovering TOI-3261 b: A Rare Hot Neptune

TOI-3261 b, a Neptune-sized planet, orbits its host star at an exceptionally close distance. It is only the fourth known planet of its kind, offering valuable insights into how such rare planets form.

The discovery was made by an international team of scientists using NASA’s Transiting Exoplanet Survey Satellite (TESS). Follow-up observations with ground-based telescopes in Australia, Chile, and South Africa confirmed the finding. TOI-3261 b resides in what astronomers call the “hot Neptune desert” — a region of space where planets of this size and composition are unusually scarce.

Similar to Neptune in size and makeup, TOI-3261 b stands out for its extremely tight orbit, completing a “year” in just 21 hours. This rapid orbit places the exoplanet in a highly exclusive category: ultra-short-period hot Neptunes with precisely measured masses, of which there are only three others discovered so far.

The Challenges of Hot Neptune Atmospheres

Exoplanet TOI-3261 b proves to be an ideal candidate to test new computer models of planet formation. 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-3261 b might have started out as much larger, Jupiter-sized planets, and have since lost a large portion of their mass.

By modeling different starting points and development scenarios, the science team determined that the star and planet 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 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.

Atmospheric Mysteries and Future Research

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.” Planet TOI-3261 b 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.

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. This mystery could be solved by observing the planet in infrared light, perhaps using NASA’s James Webb Space Telescope – an ideal way to see the identifying fingerprints of the different molecules in the planet’s atmosphere. This will not just help astronomers understand the past of TOI-3261 b, but also begin to uncover the physical processes behind all hot, giant planets.

Expanding the Ultra-Short-Period Hot Neptune Club

The first-ever discovery of an ultra-short-period hot Neptune, LTT-9779 b, came in 2020. Since then, TESS discoveries TOI-849 b and TOI-332 b have also joined the elite ultra-short-period hot-Neptune club (with masses that have been precisely measured). Both LTT-9779 b and TOI-849 b are in the queue for infrared observations with the James Webb Space Telescope, potentially broadening our understanding of these planets’ atmospheres in the coming years.

An international science team led by astronomer Emma Nabbie of the University of Southern Queensland published their paper on the discovery, “Surviving in the Hot Neptune Desert: The Discovery of the Ultrahot Neptune TOI-3261 b,” in The Astronomical Journal in August 2024.

Reference: “Surviving in the Hot-Neptune Desert: The Discovery of the Ultrahot Neptune TOI-3261b” by Emma Nabbie, Chelsea X. Huang, Jennifer A. Burt, David J. Armstrong, Eric E. Mamajek, Vardan Adibekyan, Sérgio G. Sousa, Eric D. Lopez, Daniel Thorngren, Jorge Fernández Fernández, Gongjie Li, James S. Jenkins, Jose I. Vines, João Gomes da Silva, Robert A. Wittenmyer, Daniel Bayliss, César Briceño, Karen A. Collins, Xavier Dumusque, Keith Horne, Marcelo Aron F. Keniger, Nicholas Law, Jorge Lillo-Box, Shang-Fei Liu, Andrew W. Mann, Louise D. Nielsen, Ares Osborn, Howard M. Relles, José J. Rodrigues, Juan Serrano Bell, Gregor Srdoc, Chris Stockdale, Paul A. Strøm, Cristilyn N. Watkins, Peter J. Wheatley, Duncan J. Wright, George Zhou, Carl Ziegler, George Ricker, Sara Seager, Roland Vanderspek, Joshua N. Winn, Jon M. Jenkins, Michael Fausnaugh, Michelle Kunimoto, Hugh P. Osborn, Samuel N. Quinn and Bill Wohler, 27 August 2024, The Astronomical Journal.
DOI: 10.3847/1538-3881/ad60be