Exomoons May Offer a Better Chance at Finding Extraterrestrial Life Than the Planets Beyond Our Solar System

One of the most important and interesting questions science has to answer is: Are we alone? Trying to find other worlds that are habitable to life is not just an interesting topic—but serves as future planning.

Although it may not be a problem for this generation, our own Sun has a finite life and at some point, we need to consider that we may need to go out into space to look for a new home.

Earth-like planets outside our solar system are difficult to find. Because of their distance and relatively small size, they're often difficult to observe directly. Most exoplanets (any planet beyond our solar system) are discovered using the transit method. As a planet passes in front of the star it orbits, it obscures some of the light that reaches Earth, allowing us to detect and analyze it. This allows us to acquire a surprising amount of detail about a planet, such as its size and orbit.

Nearly 4,000 exoplanets have been discovered, but only a small proportion of these could potentially sustain life. Unfortunately, many of the potentially habitable planets we have discovered so far orbit small red dwarf stars, which are renowned for their large life-extinguishing stellar flares.

One of the properties a planet needs to be considered as a candidate for life is that the temperature falls within a range that allows liquid water to exist. This usually means that planets need to be located in what is known as the habitable zone. Close enough to its star that water isn't permanently frozen, but not so close that it boils.

Jupiter's icy moon Europa. NASA/JPL-Caltech/SETI Institute

But some planets, especially large gas giants, may harbor moons which contain liquid water. These moons can be internally heated by the gravitational pull of the planet they orbit, which can sustain liquid water oceans outside the habitable zone for planets. In our own solar system one of Jupiter's moons, Europa (pictured above) and its Saturnian twin Enceladus, contain vast liquid oceans beneath their frozen surfaces, far outside the habitable zone.

I believe that if we can find them, exomoons (moons orbiting planets around stars other than our own Sun) offer a more promising avenue to finding extraterrestrial life.

So far only one exomoon has been discovered. This is a Neptune-sized moon orbiting a planet ten times the mass of Jupiter (named Kepler-1625b). While still in the early days of exomoon science, this recent discovery shows that large gas giants can support a large moon which may also have an atmosphere. The problem we face is that exomoons are even harder to find than exoplanets, as they block out much less light than planets due to their smaller size.

To detect exomoons, we often need to look indirectly at the effect they have on objects nearby to reveal their presence. J1407b is an exoplanet located 433.8 light years from Earth. It has a huge ring system which is 200 times larger than the rings around Saturn. The ring system appears to contain large gaps, analogous to the gaps found in Saturn's rings which are caused by interactions by many of the nearby moons.

Daphnis inside Saturn's rings. NASA/JPL-Caltech/Space Science Institute

The gaps are formed by one of two main processes. The first is by an embedded moon within the ring itself, like Pan or Daphnis observed inside Saturn's rings (pictured above). The second is by a type of resonance between the motion of the moon and the ring which is seen by the Cassini Division in Saturn's rings created by the moon Mimas (pictured below) orbiting outside the ring.

Saturn's moon Mimas NASA/JPL/Space Science Institute

We hypothesized that the gaps in the rings of J1407b may be caused by an orbiting moon, like how Mimas forms the Cassini Division in Saturn's rings. If confirmed, this would indirectly discover a new moon orbiting an exoplanet. Computer simulations were run to analyze the effect of a moon placed outside of the ring system, to see whether a gap formed where expected.

Findings revealed that while the orbiting moon did have an effect on the scattering of particles along the ring edge, the expected gaps in the ring structure were unlikely to be caused by the gravitational forces of a currently unseen moon orbiting outside the rings.

This could mean the gaps were caused by a moon within the ring system clearing a gap, but this requires further research to confirm.

As technology moves forward, we will get better at detecting exoplanets and their moons. The James Webb Space Telescope is due to launch in 2021 and one of its mission goals is to directly observe exoplanets. It will be powerful enough to look back more than 13.5 billion years to observe the first stars and galaxies forming and will open up an entirely new avenue for searching for extraterrestrial life.

This is just the start for exomoon science and over the coming years we expect to uncover a wealth of potential new worlds—although there is no need to be packing our suitcases just yet.

Dr Phil Sutton is a lecturer in Astrophysics at the University of Lincoln, U.K. He specializes in the computer modelling of planetary rings to help our understanding of moon and planet formation.