Neptune-class Planet Discovered 90 L-Yrs from Earth

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Artist’s concept image of TOI-1231-b shows the Neptune-sized world with a deep atmosphere, ideal for further study by HST & JWST. Image via NASA/ JPL-Caltech/ UNM.

With a period of 24 days, TOI-1231-b orbits an M-class (M3V) star of 0.48 solar masses, first discovered by the TESS orbiting exoplanet finder in 2019. Further confirmation came from the Planet Finder Spectrograph on the Magellan Clay telescope at Las Campanas Observatory in Chile.

Led by Jennifer Burt of NASA/JPL, her team at JPL and the University of New Mexico announced the discovery of the 15-earth mass, 3.7-earth-radius gas giant with a thick atmosphere. Orbiting its host star at a distance of 19.3 M Km (0.1288 AU), the exoplanet is 90 light-years distant. Located in the southern constellation Vela, TOI-1231 is one of a handful of red dwarfs within 30 parsecs with high relative proper motions. In the case of TOI-1231, it is receding from us at 73.4 Km/s.

The team’s study was submitted to the Cornell Pre-print server on 17 May of this year with the discovery announcement following in June.

Burt, lead author of the paper, with collaborators including Diana Dragomir, assistant professor in UNM’s Dept. of Physics and Astronomy, measured the radius and mass of the planet, went on to comment:

Working with a group of excellent astronomers spread across the globe, we were able to assemble the data necessary to characterize the host star and measure both the radius and mass of the planet. Those values in turn allowed us to calculate the planet’s bulk density and hypothesize about what the planet is made out of. TOI-1231 b is pretty similar in size and density to Neptune, so we think it has a similarly large, gaseous atmosphere.

Dragomir explains that aspects of M class (red dwarf) systems with larger relative-mass planets lend themselves more readily to more accurate results. She explained that when the host star and planet are closer in mass, when the planet to stellar-mass ratio is greater, the detection methods work better.

It follows from basic orbital mechanics that when the planet of study has a bigger role to play, it stands out more readily in relation to its host star.

With an equatorial radius of 3.7 earth radii, compared to Neptune’s of 3.9 and a mass of 15 earth compared to Neptune’s 17.1, TOI-1231-b is almost Neptune’s twin. Even though the planet is eight times closer to its host star than Earth is to the Sun, its temperature of 330 K (57 C) is similar to the highest recorded temperatures on Earth (134.1° F).


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