Astronomers Find What May Be First Exomoon—And It’s an Absolute Unit

Astronomers suspect that there’s Neptune-sized celestial body trailing an exoplanet about 8,000 light years

Dan Durda

In what would quite literally be a colossal first, astronomers may have finally found an exomoon, or a moon orbiting a planet outside of our own solar system. The new alien moon is roughly the size of Neptune, which has a diameter four times larger than Earth’s and is 17 times as massive.

The moon was discovered circling Kepler 1625b, a massive gas giant three times the size of Jupiter orbiting a star in the constellation Cygnus about 8,000 light years away, reports Sarah Kaplan at The Washington Post. Unlike some notable space discoveries, this find wasn’t random; Mike Wall at reports that Columbia University astronomers David Kipping and Alex Teachey were doggedly hunting for exomoons—a truly challenging feat—when they found the beast.

Since researchers first began detecting exoplanets, or worlds orbiting stars other than our Sun, in the early 1990s, we’ve gone on to catalogue almost 3,800 alien planets, with thousands more sightings waiting for confirmation. Despite finding all those planets, researchers have never detected a moon.

To detect exoplanets, astronomers usually observe their transit, which shows up when a star's brightness dips, indicating that a planet is passing in front of it. The problem is only large planets that orbit close to stars are detectable, and those types of planets typically don’t have moons.

So Kipping and Teachey pored over data from NASA’s Kepler space telescope, an exoplanet hunting satellite. They decided to look at exoplanets with the widest orbits, or those that take about 30 days to circle their stars. That narrowed their focus to 284 planets. But just one of them, Kepler 1625b, showed the type of anomaly they were looking for.

Later, the team used the powerful lens of the Hubble Space Telescope to observe the planet and found a pair of unexpected abnormalities. First, Kepler 1625b begins its transit of the star over an hour early, an indicator that something with relatively strong gravity is tugging on it, alternating its center of gravity and affecting its orbit.

Second, once the planet finished passing by the face of the star, they observed another decrease in brightness about 3.5 hours later, consistent with “a moon trailing the planet like a dog following its owner on a leash,” says Kipping in a press release.

Both of these data points are best explained if a huge Neptune-sized moon is orbiting Kepler 1625b. If confirmed, this would be the first moon discovered outside our own solar system, according to the new study published in the journal Science Advances.

“We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis,” Kipping tells Wall as “And if validated, the planet-moon system—a Jupiter with a Neptune-sized moon—would be a remarkable system with unanticipated properties, in many ways echoing the unexpected discovery of hot Jupiters in the early days of planet hunting.”

While some may want to define such a massive moon as a planet caught in a binary system with Kepler 1625b, the researchers define it as a moon since its mass is only 1.5 percent that of the planet it orbits, roughly the same as Earth and our moon.

How such a moon would form, however, is the big question. Kaplan reports that there are no easy explanations. It could be rogue planet caught in the gravity of the Kepler 1625b or, like the moons of Jupiter, it coalesced out of gas, dust and other space debris. Because it’s a gaseous moon circling a gas giant, it’s possible—but unlikely—that it was formed like our moon when a cosmic collision broke a chunk off its host planet.

“It’s raising new questions about sort of the dynamical processes that go on to create the planets and moons,” Teachey tells Kaplan.

But the astronomers are careful to emphasize that this is only a candidate moon. It will take more observation with the Hubble to confirm it. And finding future moons will require looking at planets much further out from their stars, something that is difficult now, but should possible once the powerful but long-delayed James Webb Space Telescope finally begins scanning the skies.

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