Scott, Madison G., Triaud, Amaury H. M. J., Barkaoui, Khalid, Sebastian, Daniel, Burgasser, Adam J., Collins, Karen A., Dransfield, Georgina, Hellier, Coel, Howell, Steve B., Piette, Anjali A. A., Rackham, Benjamin V., Stassun, Keivan G., Stokholm, Amalie, Timmermans, Mathilde, Watkins, Cristilyn N., Fausnaugh, Michael, Fukui, Akihiko, Jenkins, Jon M., Narita, Norio, Ricker, George, Softich, Emma, Schwarz, Richard P., Seager, Sara, Shporer, Avi, Theissen, Christopher, Twicken, Joseph D., Winn, Joshua N., & Watanabe, David. (2025). TOI-6478 b: A cold underdense Neptune transiting a fully convective M dwarf from the thick disc. *Monthly Notices of the Royal Astronomical Society, 540*(2), 1909-1927. https://doi.org/10.1093/mnras/staf684
As astronomers discover more and more planets beyond our solar system, we continue to learn just how varied planetary systems can be. One area of growing interest is how planets form around small, cool stars known as M dwarfs. These stars are common in our galaxy, and studying them can help us understand how planet formation changes depending on the size of the star, especially when compared to stars like our Sun.
In this study, scientists report the discovery of a new planet named TOI-6478 b, which is a cold, Neptune-like world orbiting a small M5-type star. This star is part of the Milky Way’s “thick disc,” a population of older stars. The planet has an estimated temperature of about 204 K (or -69°C), a size about 4.6 times that of Earth, and orbits its star once every 34 days. While its exact mass isn’t fully known, researchers determined that it is no more than about 9.9 times the mass of Earth, or a little over half the mass of Neptune.
What makes TOI-6478 b especially exciting is that it’s one of the few known planets in a cold region of its solar system—beyond the so-called “ice line”—that also crosses in front of its star from our point of view. This allows astronomers to study its atmosphere. Because the planet has a relatively large atmosphere for its size, it should be easier to analyze, especially using powerful telescopes like the James Webb Space Telescope (JWST). In fact, scientists estimate that the signals from TOI-6478 b’s atmosphere could be more than twice as strong as those from the well-known exoplanet K2-18 b, making it a prime candidate for future atmospheric research.
Figure 1.
Kast spectra (black lines) of the co-moving companions LP 789-76 (top) and TOI-6478 (bottom) compared to the best-fitting M3 and M5 spectral templates from Bochanski et al. (2007; magenta lines). All spectra are normalized at 7500 Å, with the blue and red orders of Kast relatively scaled to match the spectral standard, and the spectrum of LP 789-76 is offset by 0.7 flux units for clarity (zero-points are indicated by dashed lines). Key atomic and molecular spectral features are labelled.
