Evolutionary models of low-mass planets: Cooling curves, magnitudes, and detectability by JWST

Coming instruments like NIRCam and MIRI on the JWST will be able to image low-mass planets that are too faint for current direct imaging instruments. On the theoretical side, core accretion formation models predict a significant population of distant low-mass planets at orbital distances of 10-1000 au. So far, evolutionary models predicting the planetary intrinsic luminosity as a function of time have been concentrated on gas-dominated giant planets, going down to e.g. 0.5 Jupiter masses in Baraffe et al. 2008.

We extend these cooling curves to isolated lower mass planets such as Saturnian, Neptunian, and super-Earth planets (down to 5 Earth masses). The planets in our model consist of a core made of iron, silicates, and ices, surrounded by a H/He envelope, similar to the ice giants in the solar system. The luminosity includes the contribution from the cooling and contraction of the core, the H/He envelope, and radiogenic decay. Different atmospheric models (AMES-Cond, petitCODE, HELIOS) are used. We validate our model by simulating the solar system gas giants as well as a 2 Jupiter mass planet and find good agreement with the literature for the luminosity. Considering a wide range of metallicities and cloud parameters magnitudes in JWST filter bands are calculated. Comparison with the JWST sensitivity limits lets us estimate the detectability of low-mass planets over time. For example, a 20 Earth mass planet in the frame of our model is visible until 10 Myr after its formation.

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