While the number of confirmed extrasolar planets is now approaching 300, the tally of extrasolar moons so far identified is still a rather disappointing zero.
Planets beyond our solar system are incredibly challenging to find. Moons are nearly impossible with today's technology, given that they are generally expected to be quite small compared to their parent worlds.
Even Earth's moon is invisible on the famous "pale blue dot" image obtained by Voyager 1 from the comparatively small distance of 3.7 billion miles — a photograph taken from well within our solar system.
But the search is not impossible, says Darren Williams, associate professor of physics and astronomy at Penn State Erie, the Behrend College. Williams believes a moon in orbit around a known extrasolar planet will also be detectable if we look hard enough with the right techniques.
"It will add a periodic component to the combined infrared signal" of the planet-moon system, he said.
Why it matters
Finding moons is more than just an academic quest to count them up. Planetary satellites can be highly interesting in their own right.
It's possible, for example, that life could exist on extrasolar moons, researchers say.
And it has been suggested that the ocean tides induced by Earth's moon may have been necessary to create the conditions for life on our planet to begin. At the least, the evolution of life has been affected by our moon's constant tugging.
"We certainly owe our present climate stability to the Moon and its stabilizing influence on the spin axis, but I'm not convinced that big moons are a requirement for simple or advanced life," Williams said. "I do think that Earth would have evolved advance life even with greater seasonal extremes, but it may have taken a different evolutionary path."
Williams has modelled an Earth-like planet with moons of varying sizes and concluded that satellites as small as Earth's moon could be detectable in the infrared data, owing to their large surface temperature variations. By studying an extrasolar planet and building up a picture of that world's infrared output, any sizable moons present should be detectable in this way.
So far, however, no planet as small as Earth has been detected around another star. But astronomers expect that barrier to be broken soon. Future missions, such as NASA's Terrestrial Planet Finder and The European Space Agency's Darwin, will have the ability to return the valuable data required both for finding other Earths and, Williams figures, some moons.
"The present goal is to build instruments capable of seeing something as large as the Earth or possibly Mars. Smaller Mercury- or Titan-sized objects fall below that first-order threshold," Williams said.
So could these missions cut to the chase and spot an extrasolar moon directly?
"They might, if the light collectors are big enough and if the moons are big enough. It will be easier to see moons that happen to transit the face of a star, such as what the space telescope Kepler will attempt to do starting next year," Williams explained. The space-based Kepler observatory will note dips in starlight caused by planets crossing in front of stars. If the planets are aligned in such a favourable manner, then thinking goes, moons ought to transit the stars too.
A similar conclusion is reached by Szabó, Szatmáry, Diveki and Simon in a paper published in Astronomy and Astrophysics in 2005. They conclude that the Kepler mission should identify a few extrasolar moons using this method of detection.
Yet even if we are not lucky enough to catch an extrasolar moon in transit, these future space-based planet hunters will be able to do the observational groundwork, in visible light and in the infrared, needed to search for satellites.
These planet finders will even be capable of detecting the glint of starlight reflecting off any oceans of liquid water an extrasolar planet may harbor.
"Water is extremely dark in the infrared except when the light reflects from the surface at a glancing angle," Williams said.
This glint will be most apparent when the planet is in a crescent phase, when the starlight hits the reflective surface at an oblique angle. (Mercury and Venus, as seen from Earth, go through phases similar to our moon. Observations of other planets around distant stars will undergo phasing, too.) Observing such reflections can help map the planet's thermal output and infer the distribution of oceans and continents.
Indeed the Mars Express spacecraft is set to observe crescent Earth's ocean reflection this summer and in fall of 2009 to help understand the phenomenon.