The search for exoplanets, which orbit distant stars, has opened up a whole galaxy of worlds beyond our own. Over 3,700 have been discovered to date, but they may have companions.
Since the first confirmed discovery of planets beyond our own Solar System over 20 years ago, we have known that our stellar neighbourhood is not unique in the Universe.
But now the frontier of exploration is shifting again; because where there are planets, there should be moons.
And those moons could be surprisingly Earth-like.
Why moons?
Thus far, scientists interested in potential habitable worlds beyond our Solar System have focused on planets that could resemble Earth; currently our only blueprint for life.
But what if these worlds don’t orbit their star, but another planet instead?
A team of researchers at the University of Queensland in Australia set out to explore this possibility; surveying the habitable zone of planetary systems observed by Nasa’s Kepler telescope.
Also known as the Goldilocks zone, this is the region around a star where liquid water can exist at a planet’s surface.
The team found that the predicted number of moons orbiting giant planets in these regions could well exceed that of rocky, Earth-like planets. This could make them the most abundant potential habitats for life.
“Considering the expected number of these moons in the habitable zone of their star, it is quite possible that the first signs of life found outside the Solar System, if it exists, could actually be found on a moon rather than an Earth-like planet,” lead author Michelle Hill told BBC News.
So exomoons could well be some of the most important places in the Universe.
Signal may be from first ‘exomoon’
But they are incredibly difficult to find. In fact, no one has confirmed a discovery yet.
“The habitable zone tends to be just at the very limit of our current detection methods,” Dr Stephen Kane, another author on the study, explains. Spotting planets there is tricky enough, but moons are another matter entirely.
Dr David Kipping, who leads a team at Columbia University, New York, has been interested in the idea of exomoons since he was a student.
“There aren’t very many of us looking for these things, but I’m sure as soon as we start finding them there’ll be more people who join in the hunt,” he told the BBC.
“Trying to understand… how common [they are] will give us some sense of how unique the Solar System is.”
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