Kavli Affiliate: Sara Seager
| First 5 Authors: Angelica Psaridi, Hugh Osborn, François Bouchy, Monika Lendl, Léna Parc
| Summary:
We present the discovery and characterization of two warm mini-Neptunes
transiting the K3V star TOI-815 in a K-M binary system. Analysis of the spectra
and rotation period reveal it to be a young star with an age of
$200^{+400}_{-200}$Myr. TOI-815b has a 11.2-day period and a radius of
2.94$pm$0.05$it{R_{rmmathrm{oplus}}}$ with transits observed by TESS,
CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of
2.62$pm$0.10$it{R_{rmmathrm{oplus}}}$, based on observations of three
non-consecutive transits with TESS, while targeted CHEOPS photometry and radial
velocity follow-up with ESPRESSO were required to confirm the 35-day period.
ESPRESSO confirmed the planetary nature of both planets and measured masses of
7.6$pm$1.5 $it{M_{rm mathrm{oplus}}}$
($rho_mathrm{P}$=1.64$^{+0.33}_{-0.31}$gcm$^{-3}$) and
23.5$pm$2.4$it{M_{rmmathrm{oplus}}}$
($rho_mathrm{P}$=7.2$^{+1.1}_{-1.0}$gcm$^{-3}$) respectively. Thus, the
planets have very different masses, unlike the usual similarity of masses in
compact multi-planet systems. Moreover, our statistical analysis of
mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend
to have higher bulk densities compared to those suffering strong irradiation.
This could be ascribed to their cooler atmospheres, which are more compressed
and denser. Internal structure modeling of TOI-815b suggests it likely has a
H-He atmosphere constituting a few percent of the total planet mass, or higher
if the planet is assumed to have no water. In contrast, the measured mass and
radius of TOI-815c can be explained without invoking any atmosphere,
challenging planetary formation theories. Finally, we infer from our
measurements that the star is viewed close to pole-on, which implies a
spin-orbit misalignment at the 3$sigma$ level.
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