|That's no planet (Image: Frizaven/3D Space Simulator Celestia)|
Moons, rather than planets, could star in the first images of habitable worlds outside our solar system. Once taken, such images would offer unprecedented clues to the moons' ability to support life by providing the chemical signatures carried in their light.
"If we can direct-image them, we can take their spectra, which means we can determine what sort of molecules are in their atmosphere," says Mary Anne Peters of Princeton University.
So far, more than 800 planets outside our solar system, or exoplanets, have been found using indirect methods, such as picking up the dimming of a star's light when a planet passes in front of it. But spectra from rocky planets similar in size to Earth have been tough to collect with these methods. The planetary photo album is even slimmer: only 4 systems have been imaged.
One challenge is that stars are bright whereas planets are dim, so a planet has to be far enough from its star to avoid being outshined. That means those worlds that have been imaged orbit outside the habitable zone, the region around a star that's warm enough for liquid water. Also, planets shining bright enough to appear in pictures must be glowing from the heat of formation and so are too young to host life.
But if a moon orbits a mature gas giant akin to Jupiter, the planet's gravitational pull might be constantly kneading and stretching the moon, keeping its interior molten. This process, called tidal heating, is known to fuel the furnace of Jupiter's moon Io, the most volcanically active body in our solar system. With tidal heat, an exomoon should shine in pictures.
"In a sense, what we're saying is that there's a way to keep warm other than starlight," says Edwin Turner, also of Princeton. "This will let us directly image moons in planetary systems even when we can't see the planet."
To check this idea, Turner and Peters calculated how hot a moon would have to be for current telescopes to see it. They found that most of today's observatories – such as the Keck telescope in Hawaii or the space-based Hubble and Spitzer telescopes – should be able to take moon shots, but only if the moons are around a searing 700 °C.
Future telescopes will have the sensitivity to pick up moons at a much more life-friendly temperature. The James Webb Space Telescope, for example, should be able to see exomoons with temperatures at a comfortable 27 °C, as long as their host planets are a similar distance from their star as Saturn or Uranus are from the sun.
However, tidal heating may not be a boon for life, warns René Heller of the Leibniz Institute for Astrophysics in Potsdam, Germany. The same squeezing that generates heat might also create unfavourable seismic activity, like the volcanoes on Io that constantly spew lava and sulphurous gases.
"It could mean that tidal heating does not extend your habitable zone, because once you have enough tidal heating to keep surface temperatures above 0 °C, you destroy any life on the surface because of these 'hell' phenomena," Heller says.
Still, even a moon that isn't right for life would be a momentous discovery. "We don't know of a single moon outside the solar system," Turner says. "We have no idea if the moons in the solar system are unusually common, or unusually rare. It's exploration – just finding out what's out there."
The new study does raise the tantalising possibility that we already have a picture of an exomoon. One of the directly imaged planets, Fomalhaut b, is at the centre of a controversy about whether it's really a planet at all, in part because it has an unusual orbit. Turner suggests that instead of a planet, the oddly behaving world could be the first directly imaged exomoon, and its orbit may be due to the moon's path around an unseen world.
Journal reference: arxiv.org/abs/1209.4418