According to researchers, scientists on the hunt for habitable alien planets might have better luck searching for moons than for alien planets. A moon that is heated by the pull of its parent planet may be visible even when the planet itself is hidden from view.
The exomoons would remain bright throughout their lifetimes, except not during their youth, thanks to the gravitational pulling from a planet. This means that stars of various ages could be hosting planets with rather photogenic moons.
When a moon travels around its planet, the larger body attempts to circularize the orbit of the smaller, but if the planet hosts more than one moon, a power struggle may ensue as the smaller bodies tug at one another. The heat resulting radiates from the moon, which makes it bright enough to show up in a visual image.
Planets do not emit heat forever, and only emit it for a short time after their formation which limits how long they can be directly imaged. Tidally heated moons, however, would continue to give off heat throughout their lifespans.
The amount of heating a moon undergoes depends on its location. A tight orbit generally results in a stronger gravitational pull and a brighter image, but too close would be quite fatal and it would be torn into a ring, much like the one around Saturn. Too far away of an orbit would mean the moon would be too cool and too dim to image successfully.
The research was presented at the 221st meeting of the American Astronomical Society in Long Beach, California in January.