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鈥� ability to support life by providing the chemical signatures carried in their light.
鈥淚f we can direct-image them, we can take their spectra, which means we can determine what sort of molecules are in their atmosphere,鈥� 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鈥檚 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.
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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鈥檚 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.
Tidal heating
But if a moon orbits a mature gas giant akin to Jupiter, the planet鈥檚 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鈥檚 moon Io, the most volcanically active body in our solar system. With tidal heat, an exomoon should shine in pictures.
鈥淚n a sense, what we鈥檙e saying is that there鈥檚 a way to keep warm other than starlight,鈥� says , also of Princeton. 鈥淭his will let us directly image moons in planetary systems even when we can鈥檛 see the planet.鈥�
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鈥檚 observatories 鈥� such as the Keck telescope in Hawaii or the space-based Hubble and Spitzer telescopes 鈥� should be able to take moon shots, but only if the moons are around a searing 700鈥壜癈.
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鈥壜癈, as long as their host planets are a similar distance from their star as Saturn or Uranus are from the sun.
鈥淗ell鈥� phenomena
However, tidal heating may not be a boon for life, warns 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.
鈥淚t could mean that tidal heating does not extend your habitable zone, because once you have enough tidal heating to keep surface temperatures above 0鈥壜癈, you destroy any life on the surface because of these 鈥榟ell鈥� phenomena,鈥� Heller says.
Still, even a moon that isn鈥檛 right for life would be a momentous discovery. 鈥淲e don鈥檛 know of a single moon outside the solar system,鈥� Turner says. 鈥淲e have no idea if the moons in the solar system are unusually common, or unusually rare. It鈥檚 exploration 鈥� just finding out what鈥檚 out there.鈥�
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鈥檚 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鈥檚 path around an unseen world.
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