Astronomers haven’t found a single one—yet.
But those elusive exomoons—satellites orbiting planets outside our solar system—may be everywhere in the Milky Way.
“It would be amazing to have examples in the solar system and zero examples in the galaxy,” says David Kipping, astronomy professor at Columbia University.
In our solar system, 185 moons orbit six of the eight planets. Which means moons outnumber planets by more than 20 to 1.
Extrapolate that: If hundreds of billions of planets reside in the galaxy—and many scientists think so—then trillions of exomoons might exist.
“That’s certainly possible,” says Kipping. “It’s not unreasonable. Satellite formation may be a natural by-product of planet formation.”
There are stipulations, of course. For instance: The solar system isn’t necessarily a template for the rest of the galaxy.
“It has a particular architecture,” Kipping says. “We don’t know how common it is.”
But play with the math a moment. Suppose 200 billion planets are in the galaxy, a typical estimate. And suppose the 20-1 ratio is about right.
That’s four trillion exomoons.
If anywhere near true, that’s an exponential leap in the number of places where extraterrestrial life could launch.
“Habitable moons could be par for the course,” acknowledges Kipping, the principal investigator for the HEK project.
Both satellites might be an archetype for the galaxy—a large moon with a subsurface ocean, orbiting a large planet far from its star.
Says Kipping: “Those moons are clearly common in the solar system. There are very good chances they are all over the place.”
Most satellites in our solar system are little more than big rocks. Only four of Jupiter’s 79 moons are large enough to hold an atmosphere. Without a blanket of air surrounding it, a world is inhabitable.
Best bet for life: A moon the mass of Earth orbiting a Jupiter-size planet within the system’s habitable zone.
Satellites like that, suggests Kipping, could be “similar in climate to Earth and capable of supporting life.”
Which leads to “one of the biggest questions you can ask,” he says.
“How common are Earth-like bodies in the galaxy?”
No one knows. Maybe there are billions. Maybe there’s none.
But one thing is certain. “You can’t answer that question until you think about moons as well,” Kipping says.
“The frequency of Earth-like moons may be a significant or even a dominant component of that question.”
And this too is certain: Should astronomers ever discover even one, they’ll surely be dancing in the moonlight.