Does it exist? NASA/JPL-Caltech

In July, tantalising evidence emerged of the first discovery of a moon around a planet beyond our solar system. Although the exomoon’s existence has yet to be confirmed, new results show that the world may look stranger than anyone thought and may also have been created through some unknown mechanism.

David Kipping at Columbia University, New York, has spearheaded an effort to comb through Kepler spacecraft data in search of hidden moons since 2012. Back in July, he and his graduate student Alex Teachey announced they had found signs of a colossal exomoon that might orbit a gas giant roughly 4000 light years away.

Although the moon is still hypothetical – and Kipping has watched many previous candidates vanish into thin air – the discovery would mark a new chapter in our search for life in the cosmos, causing many to wonder what that world would look like.


René Heller at the Max Planck Institute for Solar System Research in Germany set out to determine just that.

Not that it is an easy question. Given that the exomoon has yet to be confirmed, very little is known about it so far – so its exact size and mass are unknown. Based on calculations from the Kepler data, Heller speculates that the moon could be anything from a small gaseous body the size of Earth to an ocean-covered rocky world as big as Saturn. But he thinks the most likely scenario, sitting squarely in the middle of that range, is a Neptune-like world.

Such a mega-moon doesn’t exist in our solar system, which would make this candidate exomoon an oddball. It also means that its formation mechanism might be quite mysterious.

Three routes to a moon

Indeed, Heller argues that it can’t be explained by what we currently know about how moons form in our solar system. Our moon, for example, was created when a giant impact carved the satellite from our planet. Jupiter’s moons formed around the developing planet from the thick debris of gas and dust that once persisted throughout the solar system. And Neptune’s moon was actually captured by the planet’s gravity.

Those three scenarios are the only ones astronomers understand – and none can explain a moon so large. That means that if the exomoon is validated, it would pose “an exquisite riddle for formation theorists to solve”, Heller says. “It is not clear to me from the Kepler data and from what we know in the solar system, which channel it would have formed through.”

Teachey and Kipping have tried to discourage theorists from spending too much time analysing the possible exomoon. “If it’s there, awesome, have at it,” Teachey says. “But if not, well, we’d feel badly about other people having spent time studying a phantom.” Still, Teachey admits he found Heller’s results interesting.

“I think the conclusion is, ‘this is a strange system,’ and we agree,” Teachey says. Both he and Kipping have argued that the unusual size of the moon is reason to be agnostic about it, but that doesn’t necessarily mean astronomers should rule out its existence. “The universe is very good at surprising us,” Teachey says.

To confirm the exomoon, the pair plan to take another look at the system on 28 October using the Hubble Space Telescope. If they succeed, we may have to devise new theories about how the exomoon formed, which could tell us more about what this world and other exomoons are like.

It is an important question given that exomoons might be the best place to search for life. John Armstrong at Weber State University, Utah, points out that many of the gas giant exoplanets we have discovered exist in a habitable zone not too close and not too far from their star. Should they have moons in tow, those satellites might be ideal worlds for life.

Journal reference: arXiv, 1710.06209

Read more: First exomoon might have been spotted 4000 light years away