Finding exoplanets—planets in other star systems—is difficult. Finding moons is even harder, right? Yes—which is why astronomers are yet to confirm any “exomoons”—but what about big moons?

 

A paper published today in Nature Astronomy reveals the discovery of just that. Identified around a cool giant Jupiter-sized planet called Kepler-1708 b that orbits a long way from its host star, the exomoon discovered is 2.6 times larger than Earth.

It’s an incredible find.

 

Moons are ubiquitous in our Solar System, which contains some some pretty big examples, relatively speaking. Both Ganymede at Jupiter—most massive satellite in our Solar System—and Titan at Saturn are larger than the planet Mercury. However, neither get anywhere near the size of Kepler-1708 b-i.

 

If this new candidate for an exomoon is confirmed it will be a missing piece in the puzzle in attempts to understand the formation and evolution of extrasolar planetary systems.

 

The hunt for Kepler-1708 b-i

 

This is not the first detection of a candidate exomoon. Back in 2018 the discovery of a Neptune-sized exomoon, Kepler-1625 b-i, was announced, but it’s yet to be confirmed despite theories of other exmoons around the same star. That too was orbiting a cool giant planet orbiting its star from a long distance. This new research followed suit by focusing on 70 cool gas giant planets that take more than a year to orbit their star.

 

They found only one signal, around Kepler-1708 b, that is best explained by the existence of an exomoon. However, they admit that there’s a 1% chance that the signal is an artefact.

 

The only real way to confirm its existence, says the authors, would be to watch several transits of Kepler-1708 b across its host star. The next one will occur on March 24, 2023.

 

Early last year astronomers also announced evidence for an icy exomoon that had been consumed by a white dwarf star called GALEX J2339-0424.

Why exomoons are so hard to find

The lack of confirmed exomoons is down to technology limitations. Exoplanets are mostly detected via the transit method, which is when a small dip in the brightness a star is measured.

 

It’s much harder to detect a moon simply because it’s more difficult to separate that signal into distinct objects.

 

Moreover, the transit method doesn’t detect cool giants like Kepler-1708 b very often. That’s doubly frustrating for astronomers because moons are thought to be a natural outcome of how cool giant planets form.

 

It’s thought that the transit method can only be used to detect moons with masses higher than 0.1 − 0.5 Earth masses. That means only moons one order of magnitude more massive than Ganymede.

 

Why astronomers care about exomoons

Moons have been very important in the formation and evolution of the planets in our Solar System—and may either host life themselves or be a deciding factor on whether there’s life on a planet.

 

Moons are everywhere in our own Solar System. There are thought to be over 200 and only Mercury and Venus do not have moons.

 

Moreover, the speed at which Earth rotates is thought to be a result of a giant impact 4.5 billion years ago with a Mars-sized object, of which the Moon may be a leftover. It’s also thought that the Moon’s gravitational pull—which causes tides in Earth’s oceans—may have at least sped-up the evolution of life by transporting heat around the planet, thus encouraging migrations.

 

So the discovery of moons around extrasolar planets is crucial if astronomers are to understand how these systems formed and evolved—and increase the chances of finding life out there, somewhere.