Abstract
Context.Future instruments like the Near Infrared Camera (NIRCam) and the Mid Infrared Instrument (MIRI) on theJames WebbSpace Telescope (JWST) or the Mid-Infrared E-ELT Imager and Spectrograph (METIS) at the European Extremely Large Telescope (E-ELT) will be able to image exoplanets that are too faint (because they have a low mass, and hence a small size or low effective temperature) for current direct imaging instruments. On the theoretical side, core accretion formation models predict a significant population of low-mass and/or cool planets at orbital distances of ~10–100 au.Aims.Evolutionary models predicting the planetary intrinsic luminosity as a function of time have traditionally concentrated on gas-dominated giant planets. We extend these cooling curves to Saturnian and Neptunian planets.Methods.We simulated the cooling of isolated core-dominated and gas giant planets with masses of 5M⊕–2M♃. The planets 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 and of the H/He envelope, as well as radiogenic decay. For the atmosphere we used grey,AMES-Cond,petitCODE, andHELIOSmodels. We considered solar and non-solar metallicities as well as cloud-free and cloudy atmospheres. The most important initial conditions, namely the core-to-envelope-mass ratio and the initial (i.e. post formation) luminosity are taken from planet formation simulations based on the core accretion paradigm.Results.We first compare our cooling curves for Uranus, Neptune, Jupiter, Saturn, GJ 436b, and a 5M⊕planet with a 1% H/He envelope with other evolutionary models. We then present the temporal evolution of planets with masses between 5M⊕and 2M♃in terms of their luminosity, effective temperature, radius, and entropy. We discuss the impact of different post formation entropies. For the different atmosphere types and initial conditions, magnitudes in various filter bands between 0.9 and 30 micrometer wavelength are provided.Conclusions.Using blackbody fluxes and non-grey spectra, we estimate the detectability of such planets with JWST. We found that a 20 (100)M⊕planet can be detected with JWST in the background limit up to an age of about 10 (100) Myr with NIRCam and MIRI, respectively.
Highlights
During the last few years, the Kepler satellite has detected numerous exoplanets of which many are in the sub-Neptunian or super-Earth mass range (e.g. Batalha et al 2012; Burke et al 2014; Fressin et al 2013; Petigura et al 2013)
The planets 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
We found that a 20 (100) M⊕ planet can be detected with James Webb Space Telescope (JWST) in the background limit up to an age of about 10 (100) Myr with Near Infrared Camera (NIRCam) and Mid Infrared Instrument (MIRI), respectively
Summary
During the last few years, the Kepler satellite has detected numerous exoplanets of which many are in the sub-Neptunian or super-Earth mass range (e.g. Batalha et al 2012; Burke et al 2014; Fressin et al 2013; Petigura et al 2013). Bodenheimer & Lissauer (2014) studied the origin and evolution of low-density planets in the mass range from 1–10 M⊕, which are within 0.5 au from their star. Additional tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http:// cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/623/A85 planets formed in situ or further out and moved inwards. Another analysis conducted by Chen & Rogers (2016) was dedicated to computing mass-radius-composition-age relations for low-mass planets and how these depend on the evolution history of the planets. Jin & Mordasini (2018) studied whether through photoevaporation a certain planetary composition is revealed
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