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New 'super-Neptune' exoplanet discovered with TESS

Target Pixel File (TPF) of TOI-2498 (TIC-263179590) from TESS sector 6. Credit: Frame et al, 2023

Using NASA's Transiting Exoplanet Survey Satellite (TESS), an international team of astronomers has discovered a new hot, bloated "super-Neptune" exoplanet. The newfound alien world, designated TOI-2498 b, is about six times larger and 35 times more massive than the Earth. The finding was published May 16 in Monthly Notices of the Royal Astronomical Society.

TESS is conducting a survey of about 200,000 of the brightest stars near the sun with the aim of searching for transiting exoplanets, ranging from small rocky worlds to gaseous giants. So far, it has identified nearly 6,600 candidate exoplanets (TESS Objects of Interest, or TOI), of which 331 have been confirmed to date.

One of the stars observed by TESS during its primary mission was TOI-2498 (also known as TIC-263179590)—a G-type solar-type star about 26% larger and 12% more massive than the sun. TESS monitored TOI-2498 between December 12, 2018, and January 6, 2019, which resulted in the detection of a transit signal in its light curve. Planetary nature of this signal was confirmed by follow-up spectroscopic and photometric observations conducted by astronomers led by Ginger Frame of the University of Warwick, U.K.

"In this work, we have presented the discovery of a hot, bloated super-Neptune transiting a G type star. Our analysis includes photometry from TESS sector 6 and sector 33, follow-up ground-based photometry from LCOGT and spectroscopy from HARPS," the researchers wrote.

The newfound planet has a radius of about 6.06 Earth radii and its mass is estimated to be 34.62 Earth masses, yielding a density at a level of 0.86 g/cm3. It orbits its every 3.74 days, at a distance of some 0.05 AU from it. The planet's equilibrium temperature was calculated to be about 1,443 K.

Based on the results, Frame's team classified TOI-2498 b as a hot and bloated "super-Neptune." The planet's low density is indicative of the presence of a gaseous envelope that consists of approximately 27% of its mass. However, the astronomers added that their planetary model assumes no or significant atmospheric metallicity of TOI-2498 b.

Summing up the results, the researchers noted that TOI-2498 b is a stable "super-Neptune" that has experienced little evaporation throughout its lifetime, relative to its mass. They suppose that the planet, after formation, might have started out as a puffy Saturn-sized world about 8–10 times larger than Earth and an envelope mass fraction of between 30% and 45%. The age of the planet was estimated to be some 3.6 billion years.

"We find that TOI-2498 b likely started its life as a puffy Saturn-sized world before shrinking to its current size over 3.6 ± 1.1 Gyr," the authors of the paper concluded.

More information: Ginger Frame et al, TOI-2498 b: A hot bloated super-neptune within the neptune desert, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad1452. On arXiv:

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