Meet TOI-270 d - The 'Rosetta Stone' Exoplanet

By Susha Cheriyedath, M.Sc.Reviewed by Louis CastelMay 9 2025

TOI-270 d is an exoplanet discovered by the Transiting Exoplanet Survey Satellite (TESS) mission of the National Aeronautics and Space Administration (NASA). TESS uses a transit method to discover new planets, detecting changes in a star’s brightness when a planet passes in front of it. TOI-270 d orbits a small, cool star known as an M-dwarf, located about 73 light-years from Earth. Two more planets form a part of this system: TOI-270 b and c. In this multi-planet system, TOI-270 d is the largest and outermost planet that orbits farthest from the star. TOI is an excellent system to study the formation and composition of planets and for exploring the diversity of worlds beyond our solar system.

Image CreditL Nazarii_Neshcherenskyi/Shutterstock.com

Why TOI-270 d Stands Out

TOI-270 d is one of the special and rare exoplanets. It lies in the "radius gap" or "Fulton gap" range, which refers to a size range between that of Earth and Neptune. Planets smaller than the radius gap, like Earth or Mars, are usually rocky, whereas larger planets have a gaseous atmosphere like Neptune.  

A recent study in The Astronomical Journal suggested that this gap may be a result of atmospheric loss in smaller planets, where they lose their gaseous envelopes over time, leaving behind only rocky cores.¹ Planets that reside in the radius gap, like TOI-270 d, are very few and may likely have both a rocky core and a surrounding gaseous layer.

TOI-270 d lies right at the boundary, making it especially interesting. It could be a mini-Neptune with a thick atmosphere or a super-Earth with a rocky core. Therefore, the uncommon size and likely composition of TOI-270 d make it ideal for a detailed understanding of how planets form and evolve.²

A 'Rosetta Stone' for Exoplanet Science

TOI-270 d is often described as a "Rosetta Stone" in the field of exoplanetary science because it offers a rare and valuable opportunity to study the processes behind the transition from small, rocky planets to larger, gas-rich planets. This planet falls within a crucial size range where such transitions are thought to occur, making it a key target for scientific investigation.

One significant advantage of TOI-270 d is that it orbits a quiet and stable M-dwarf star. The star's calm nature results in reduced stellar activity, which minimizes noise in observational data and enhances the precision of spectroscopic measurements. Additionally, TOI-270 d is relatively cool with a moderate temperature, making it more suitable for atmospheric studies compared to extremely irradiated hot Jupiters or Neptunes.²

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Atmospheric Studies and Potential Discoveries

TOI-270 d, being an outer planet, can be best studied using a method called . Because of its relatively large atmospheric scale height and the quiet nature of its host M-dwarf star, this technique can probe starlight as it passes through a planet’s atmosphere during a transit. Various components like , and possibly carbon dioxide can be identified on the basis of the wavelength (or color) of light being absorbed.

A recent study led by the Southwest Research Institute simulated TOI-270 d’s atmosphere and found it to be enriched in carbon dioxide (CO₂) and methane (CH₄) but low in carbon monoxide (CO) and ammonia (NH₃), suggesting a thick, hot atmosphere above a melted rocky surface that traps nitrogen deep inside.³

Future missions such as the ones led by the James Webb Space Telescope (JWST) could detect these atmospheric components with very high precision, including water vapor, methane, carbon dioxide, and other trace gases. This information will allow scientists to understand the processes underlying the formation of planets and changes in their atmospheric properties over time due to stellar radiation and other factors.

These high-resolution observations will also shed light on how planets like TOI-270 d may have migrated from their original orbits and how they gradually evolve to reach a stable state within their systems. Moreover, detailed atmospheric analysis can help researchers test and refine theoretical models such as photo-evaporation, core-loss, and atmospheric escape, which are believed to drive planetary evolution and diversity.³

Key to a New Class of Worlds

TOI-270 d may represent a new class of planets called temperate sub-Neptunes or "water worlds." These types of planets fall in size between Earth and Neptune, and may have thick atmospheres and possibly large amounts of water or other volatile materials beneath.⁴ Although scientists have predicted that such planets exist, they are not well understood because most of them are too far away or orbit bright stars, making it hard to study them in detail.

TOI-270 d is one of the rare planetary systems that is both close to Earth and clear enough for detailed observation and study. Moreover, it exists in a multi-planet system with two smaller inner planets: TOI-270 b and TOI-270 c. This type of “in-system” comparison is rare as it allows researchers to directly compare and study the effects of distance from the star, planet size, and other factors on the atmosphere, temperature, or chemical composition of the planet.

For instance, a recent report in Astronomy & Astrophysics conducted a comparative study of TOI-270 b and d, revealing that TOI-270 d exhibits atmospheric features consistent with a Hycean world: a planet that has a hydrogen-rich atmosphere and a potential subsurface ocean. This contrasts with the smaller, hotter TOI-270 b, emphasizing the diversity within the system.⁵

Broader Implications

TOI-270 d, being a sub-Neptune-sized planet orbiting a relatively quiet and nearby M-dwarf star, is an exciting and valuable system for studying exoplanets. A detailed investigation of this system could enhance our understanding of planetary demographics across the galaxy.^1,2

Close examination and direct comparison of TOI-270 with its inner neighbors, TOI-270 b and c, can provide unique insights into various factors that influence planetary formation, composition, and evolution. Since all three exoplanets orbit the same star and are formed in the same environment, they can provide critical information to refine the predictive models used to describe exoplanet interiors, composition, and atmospheric dynamics.^2,4,5

TOI-270 d is not just another exoplanet. It is an excellent reference point for developing more accurate descriptive and comparative models in exoplanetary science. Its rare size and optimal conditions make it an ideal system for understanding various aspects of exoplanets, advancing our knowledge of potential habitable conditions.

References and Further Reading

1. Fulton, B. J., et al. (2017). The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets. The Astronomical Journal 154(3), 109. doi:10.3847/1538-3881/aa80eb. https://iopscience.iop.org/article/10.3847/1538-3881/aa80eb   
2. Günther, M. N., et al. (2019). A super-Earth and two sub-Neptunes transiting the nearby and quiet M dwarf TOI-270. Nature Astronomy 3, 1099–1108. doi: 10.1038/s41550-019-0845-5 https://doi.org/10.1038/s41550-019-0845-5
3. Glein, C. R., Yu, Y., & Luu, K. (2025). Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d. arXiv preprint. arXiv:2504.09752.
4. L. Zeng et al. (2019) Growth model interpretation of planet size distribution, Proc. Natl. Acad. Sci. U.S.A. 116 (20),9723-9728. doi: 10.1073/pnas.1812905116 https://doi.org/10.1073/pnas.1812905116  
5. Holmberg, M., & Madhusudhan, N. (2024). Possible Hycean conditions in the sub-Neptune TOI-270 d, Astronomy & Astrophysics 683, L2 (1-14). doi: 10.1051/0004-6361/202348238 https://doi.org/10.1051/0004-6361/202348238

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