Constraining atmospheric composition from the outflow: helium observations reveal the fundamental properties of two planets straddling the radius gap

• Main Authors: Zhang, Michael, Bean, Jacob L, Wilson, David, Duvvuri, Girish, Schneider, Christian, Knutson, Heather A, Dai, Fei, Collins, Karen A, Watkins, Cristilyn N, Schwarz, Richard P, Barkaoui, Khalid, Shporer, Avi, Horne, Keith, Sefako, Ramotholo, Murgas, Felipe, Palle, Enric
• Format: Journal Article
• Language: English
• Published: 12.09.2024
• Subjects: Physics - Earth and Planetary Astrophysics
• DOI: 10.48550/arxiv.2409.08318

TOI-836 is a $\sim2-3$ Gyr K dwarf with an inner super Earth ($R=1.7\,R_\oplus$, $P=3.8\,d$) and an outer mini Neptune ($R=2.6\,R_\oplus$, $P=8.6\,d$). Recent JWST/NIRSpec 2.8--5.2 $\mu$m observations have revealed flat transmission spectra for both planets. We present Keck/NIRSPEC observations of escaping helium from this system. While planet b shows no absorption in the 1083 nm line to deep limits ($<0.2$\%), 836c shows strong (0.7\%) absorption in both visits. These results demonstrate that the inner super-Earth has lost its primordial atmosphere while the outer mini-Neptune has not. Self-consistent 1D radiative-hydrodynamic models of c using pyTPCI, an updated version of The PLUTO-CLOUDY Interface, reveal that the helium signal is highly sensitive to metallicity: its equivalent width collapses by a factor of 13 as metallicity increases from 10x to 100x solar, and by a further factor of 12 as it increases to 200x solar. The observed equivalent width is 88\% of the model prediction for 100x metallicity, suggesting that c may have an atmospheric metallicity close to 100x solar. This is similar to K2-18b and TOI-270d, the first two mini-Neptunes with detected absorption features in JWST transmission spectra. We highlight the helium triplet as a potentially powerful probe of atmospheric composition, with complementary strengths and weaknesses to atmospheric retrievals. The main strength is its extreme sensitivity to metallicity in the scientifically significant range of 10--200x solar, and the main weakness is the enormous model uncertainties in outflow suppression and confinement mechanisms, such as magnetic fields and stellar winds.