  Future instruments like NIRCam and MIRI on JWST or METIS at the ELT will be
able to image exoplanets that are too faint for current direct imaging
instruments. 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. We
simulate the cooling of isolated core-dominated and gas giant planets with
masses of 5 Earthmasses to 2 Jupitermasses. 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 use grey,
AMES-Cond, petitCODE, and HELIOS models. We consider solar and non-solar
metallicities as well as cloud-free and cloudy atmospheres. The most important
initial conditions, namely the core-to-envelope ratio and the initial
luminosity are taken from planet formation simulations based on the core
accretion paradigm. We first compare our cooling curves for Uranus, Neptune,
Jupiter, Saturn, GJ 436b, and a 5 Earthmass-planet with a 1% H/He envelope with
other evolutionary models. We then present the temporal evolution of planets
with masses between 5 Earthmasses and 2 Jupitermasses 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. Using black body fluxes and non-grey
spectra, we estimate the detectability of such planets with JWST. It is found
that a 20 (100) Earthmass-planet can be detected with JWST in the background
limit up to an age of about 10 (100) Myr with NIRCam and MIRI, respectively.
