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How to locate a habitable exoplanet by looking for moons

When it comes to the search for habitable exoplanets, there are many factors to consider: A planet’s size and mass, its distance from its star, and its composition. But one factor that might be more important than you imagine is whether or not it has moons.

A new study in The Astronomical Journal suggests finding exomoons could be key to locating a potentially habitable Earth-like world in another solar system.

The PDS 70 system as seen with ALMA.
The PDS 70 system as seen with ALMA. ALMA (ESO/NAOJ/NRAO)/Benisty et al.

Although many astronomers agree that there likely are moons orbiting exoplanets beyond our solar system, there has not yet been a confirmed detection of an exomoon.

“In our solar system, we have an average of 20 moons orbiting around each planet. So, we suspected there are moons around planets in other systems, too. There is really no reason why there shouldn’t be any,” said study author Siegfried Eggl, a professor in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign.

However, there is evidence of moons forming in a system called PDS 70, which has been investigated used the Atacama Large Millimeter Array (ALMA). Previous research into this system found a moon-forming disk of dust and gas called a circumplanetary disk which could potentially form moons similar to those found around Jupiter in our solar system.

As moons are so tiny, it isn’t possible to spot them directly. Instead, astronomers must infer their presence from the effects they have on the system around them. For example, when a planet passes in front of a star and the star’s light dims for a short while, the amount of wobble in that planet can suggest the presence of a moon.

“This is an indirect proof of a moon because there’s nothing else that could tug on the planet in that kind of fashion,” Eggl said.

Finding an exoplanet with moons could even tell us more about whether it could support life. “If we can use this method to show there are other moons out there, then there are probably other systems similar to ours,” Eggl said. “The moon is also likely critical for the evolution of life on our planet, because without the moon the axis tilt of the Earth wouldn’t be as stable, the results of which would be detrimental to climate stability. Other peer-reviewed studies have shown the relationship between moons and the possibility of complex life.”

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