Angélique Kahle, Katherine A.; Blecic, Jasmina; Ashtari, Reza; Kreidberg, Laura; Kawashima, Yui; Cubillos, Patricio E.; Deming, L. Drake; Jenkins, James S.; Mollière, Paul; Redfield, Seth L.; Tian, Qiushi Chris; Vines, José I.; Wilson, David J.; Acunã, Lorena; Bitsch, Bertram; Brande, Jonathan; France, Kevin C.; Stevenson, Kevin B.; Crossfield, Ian J.M.; Daylan, Tansu; Dobbs-Dixon, Ian; Evans-Soma, Thomas M.; Gapp, Cyril; García-Muñoz, Antonio; Heng, Kevin; Hu, Renyu; Shkolnik, Evgenya L.; Stassun, Keivan Guadalupe; Teske, Johanna K. (2025). The SPACE Program I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends. Astronomy and Astrophysics, 701, A184. https://doi.org/10.1051/0004-6361/202554916
Sub-Neptune exoplanets—planets smaller than Neptune but larger than Earth—are the most common type of planet discovered so far. Because there are no planets like them in our own Solar System, scientists are still trying to understand what they are made of and how they form. Earlier studies have shown that many of these planets do not have simple, clear atmospheres made mostly of hydrogen and helium, but their exact atmospheric makeup remains uncertain.
In this study, researchers examined HD 86226 c, a sub-Neptune that is about 2.2 times the size of Earth and 7.25 times its mass. It orbits a Sun-like star every four days and has a very high temperature of about 1311 K (around 1900°F). Because it is so hot, this planet should not have methane-based hazes that often make other sub-Neptune atmospheres cloudy. That makes it a good candidate for studying a clear atmosphere.
Using the Hubble Space Telescope, the team observed the planet as it passed in front of its star, analyzing the starlight that filtered through its atmosphere. Surprisingly, the data showed a featureless spectrum, meaning there were no strong signals of specific gases—just a flat line consistent with a constant transit depth of about 418 ± 14 parts per million. This lack of features rules out a clear hydrogen-helium atmosphere and suggests that the planet’s atmosphere is either very rich in heavier elements (with metallicity more than about 200 times that of the Sun) or filled with clouds made of materials such as silicates (rock), iron, or manganese sulfide.
These results show that HD 86226 c behaves differently from other sub-Neptunes studied so far, which often show hazy or aerosol-filled atmospheres. Future observations with the James Webb Space Telescope could reveal whether this planet truly has a metal-rich atmosphere or hosts unusual types of clouds, helping scientists better understand the diversity of sub-Neptune worlds across the galaxy.
Sub-Neptune targets of the SPACE program. HD 86226c is shown with a green square marker, and the other SPACE targets are shown in purple. Top: temperature-radius plane. Gray circles show the known population of planets with radii between 1.8 R⊕ and 4 R⊕ based on the entries of the NASA Exoplanet Archive3 in May 2024. Black crosses mark JWST Cycle 1–4 targets, except for the SPACE targets TOI-431 d and HD 86226 c, which will be observed in Cycle 4. Bottom: mass-radius plane. Color curves show models from Zeng et al. (2019) for various planetary compositions and temperatures.