An international group of astronomers has reported the discovery of Neptune, a new mini exoplanet orbiting a nearby star. The newly discovered alien world, called TOI-4438 b, is approximately 2.5 times the size of Earth. The discovery was detailed in an article published on the preprint server on March 14. arXiv.

NASA’s Transiting Exoplanet Search Satellite (TESS) has discovered more than 7,000 candidate exoplanets (TESS Objects of Interest, or TOIs), of which 417 have been confirmed. Since its launch in April 2018, the satellite has been surveying some 200,000 of the brightest stars near the Sun in search of exoplanets, ranging from small rocky worlds to gas giants.

So-called mini-Neptunes are exoplanets with radii of 1.8 to 4 Earth radii. They are generally common around M-type dwarf stars and are thought to contain hydrogen-dominated atmospheres.

Now a team of astronomers led by Elisa Goffo from the University of Turin in Italy has discovered a new such planet. They reported that the transit signal was identified in the light curve of TOI-4438 (or G 182-34), an M dwarf of spectral type M3.5 V located approximately 98 light-years away. The planetary nature of this signal was confirmed by subsequent spectroscopic and photometric observations.

“The transit signal was detected by NASA’s TESS space mission in sectors 40, 52 and 53. We combined TESS data with high-precision radial velocity measurements to investigate the planet TOI-4438 b and determine the properties of the system. Researchers used the CARMENES spectrograph and terrestrial transition photometry,” he writes.

TOI-4438 b has a radius of 2.52 Earth radii and a mass of approximately 5.4 Earth masses, giving a mass density of 1.85 g/cm ³ . The planet orbits its host at a distance of about 0.05 AU every 7.44 days. The equilibrium temperature of TOI-4438 b is estimated to be 435 K.

Based on the data collected, astronomers concluded that TOI-4438 b is a volatile-rich mini-Neptune with a relatively high minimum water mass fraction of 46%. They add that the planet likely has a hydrogen-helium atmosphere mixed with molecules such as water, carbon dioxide and methane. Given that TOI-4438 b has a high transmission spectroscopy metric, it is one of the most suitable targets for atmospheric characterization with the James Webb Space Telescope (JWST).

As for TOI-4438’s parent star, it is about one-third the size and mass of the Sun. The star is estimated to be 5.1 billion years old, has an effective temperature of about 3422 K and a metallicity of about -0.19 dex.