On the Direct Imaging of Tidally Heated Exomoons

We demonstrate the ability of existing and planned future telescopes, on the ground and in space, to directly image tidally heated exomoons orbiting gas-giant exoplanets. Tidally heated exomoons can plausibly be far more luminous than their host exoplanet and as much as 0.1% as bright as the system’s stellar primary if it is a low mass star. Because emission from exomoons can be powered by tidal forces, they can shine brightly at arbitrarily large separations from the system’s stellar primary with temperatures of several hundreds degrees Kelvin or even higher in extreme cases.

Furthermore, these high temperatures can occur in systems that are billions of years old. Tidally heated exomoons may thus be far easier targets for direct imaging studies than giant exoplanets which must be both young and at a large projected separation (typically at least tens of AU) from their primary to be accessible to current generation direct imaging studies. Specifically, current instruments are capable of detecting exomoons with brightness temperature >600K and R>1Re in K-band. Future mid-infrared space telescopes, such as JWST and SPICA, will be capable of directly imaging tidally heated exomoons around the nearest two dozen stars with brightness temperature >300K and R>1Re orbiting at >12AU around stars within 4 parsecs of Earth at a 5 sigma confidence level in a 10000 second integration. In addition it is possible that some of the exoplanets which have already been directly imaged are actually tidally heated exomoons or blends of such objects with hot young planets; we speculate that Fomalhaut b could be such a case.

If such exomoons exist and are sufficiently common (i.e., nearby), it may well be far easier to directly image an exomoon with surface conditions that allow the existence of liquid water than it will be to resolve an Earth-like planet in the classical Habitable Zone of its primary.

Mary Anne Peters, Edwin L. Turner
(Submitted on 20 Sep 2012)

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1209.4418v1 [astro-ph.EP]
Submission history
From: Mary Anne Peters [view email]
[v1] Thu, 20 Sep 2012 04:11:15 GMT (903kb,D)