In the search for life beyond Earth, moons may be the best candidates

We only know of one planet in our universe that harbours life: Earth. So it's logical that humanity has focused its search for life beyond our world on planetary bodies. But a new study suggests that we may be leaving out a larger chunk of the cosmos: moons.

In a new study published in The Astrophysical Journal, researchers suggest that there are 121 giant exoplanets — worlds in orbit around other stars — that could host a potentially habitable moon.

Using data from NASA's Kepler space telescope, which has confirmed more than 2,000 exoplanets, scientists modelled how frequent giant, gas-like planets — like Jupiter and Saturn — might also form in a star's habitable zone, a region where liquid water can exist on a planet's surface.

From there, the researchers determined how many of those planets might have just one moon, arriving at 121.

Squeezing

Life exists on Earth because of several factors, among them:

• Proximity to the sun (which gives us energy).
• Water.
• Our atmosphere.
• A magnetic field that shields us from harmful space radiation.

While we get energy from the sun, scientists believe that exomoons could get energy not only from its star, but also from the giant exoplanet. This, they say make them potentially "super-habitable."

The idea of planets providing additional energy to their moons comes from observations in our solar system: Jupiter and Saturn do just that for their moons.

Some moons have an elliptical orbit (not quite circular) around their planet that causes a gravitational interaction. The moon is squeezed as it goes between being closer and then further away. This, called tidal flexing, causes heating. It's much the same way if you took a stress ball and kept squeezing it: it would generate some heating (not just from the heat of your hand).

Also, as is the case with the moons of Jupiter and Saturn, an exoplanet's magnetism could protect moons from cosmic radiation, thus increasing its potential habitability.

Conservative estimate

The idea of just one potentially habitable moon around an exoplanet is really a conservative number. 

Looking at our solar system, our giant planets have a many moons: Jupiter has 69; Saturn, 62. And there none are identical in composition.

Planetary models suggest that's the case in general planet formation. So this increases the odds that giant exoplanets in habitable zones have many more moons, thus also increasing the odds of one that may be habitable.

"Because there are so … many giant planets in the habitable zone around their star, these giant planets are expected to have multiple moons, just like our giant planets do," said lead author of the paper, Michelle Hill, an undergraduate student at the University of Southern Queensland in Australia. "So, really these moons potentially could be the worlds life is detected on first."

Challenges

While the finding life on exomoons is a possibility, the reality is, it's going to take time. For one, we haven't been able to detect any biosignatures — signs that a something possesses the ingredients for life — on any exoplanets thus far.

In the search for exoplanets, scientists look for small dips in a star's light that indicates a planet is crossing it, called a transit. But a moon would be much smaller and more difficult to detect in this manner.

"Observations of exomoons in the search for life is very challenging, because the radiation coming from the exomoon will be mingled with the planet," said Canadian astrophysicist and planetary scientist Sara Seager, who studies exoplanets at MIT.

"The combined light of the exomoon and planet is already dwarfed by the light of the host star."

Secondly, we haven't even confirmed the existence of an exomoon. Just one exoplanet holds any promise at all, Kepler 1625b, which lies 4,000 light-years from Earth. But in a study published earlier this week, lead author Rene Heller, an astrophysicist at the Max Planck Institute in Germany, concluded that the jury was still out.

"Depending on how we massage the data (i.e. how we try to remove the noise and stellar/instrumental variability), the object is there — or gone," he said. It would involve many more observations to confirm or rule it out, but the planet transits only once every 287 days. It would take years.

Though their paper illustrates the potential habitability of exomoons, Hill knows that it's going to take some time, perhaps a decade or more. But on the plus side, the technology is improving.

"Really, the detections of these moons might coincide with the time that we actually have the capability of detecting any kind of real signatures of life that are indisputable," Hill said.

For now, astronomers and astrobiologists will turn their eyes to the moons of our solar system, such as Jupiter's moon Europa and Saturn's moon Enceladus, with their icy crusts and potential subglacial oceans.

Together with hydrothermal vents — openings on sea floors that contain heated mineral-rich material where life is found on Earth — they are considered two of the most promising candidates.

"I think the prospects of finding life on any of these exotic moons in the solar system are promising," Heller says. "I say that because life on Earth just seems to be everywhere."