Science & Technology

Astronomers May Have Just Discovered the First Moon Beyond Our Solar System

We have the strongest evidence yet for a moon orbiting a planet outside of our solar system. This moon — if it exists — would be about the size of Neptune, which makes it kind of an oddball. But if there's one thing we've learned about the nearly 4,000 exoplanets we've found outside of our solar system so far, it's that there's a lot of variation out there. So why not expect some strange exomoons in the bunch?

Related Video: Hunting Exoplanets

Playing with Light

This possible exomoon — it still needs to be confirmed — orbits a planet 8,000 light-years away from us. The planet is known as Kepler-1625b, and a paper based on the research was published in the journal Science Advances. How astronomers found its moon is a little complicated.

Often, scientists find distant planets by looking at a pattern of brightening and dimming as the planet passes in front of its star. So how about a moon?

There are two pieces of evidence from the Hubble Space Telescope that suggest a moon is lurking nearby. First is the variation of light. Over 19 long hours, Hubble observed the planet passing across its star. About 3.5 hours after the planet went by, there was a second and smaller dimming. As lead author David Kipping of Columbia University suggested in a statement, this variation suggests "a moon trailing the planet like a dog following its owner on a leash."

The second piece of evidence deals with the timing of the planetary "transit" (the time when the planet passed across the star), which varied wildly from what astronomers expected. The Kepler space telescope had been watching Kepler-1625b for four years, so the astronomers had a good idea of how it should behave.

Hubble discovered that the planet passed across its parent star about 1.25 hours earlier than scheduled. This often happens when a planet has a moon in orbit around it, because it causes the planet to wobble from its predicted location in space. "An extraterrestrial civilization watching the Earth and moon transit the sun would note similar anomalies in the timing of Earth's transit," said Kipping, who is also a well-regarded exoplanet researcher.

The astronomers caution that more evidence is needed to confirm this exomoon. For example, we might be looking not at a moon, but at a second planet in the same star system. But then again, maybe not. After all, the Kepler telescope didn't find any evidence for a second planet in its four years of work, so why should Hubble in 19 hours?

Artists' impression of the exoplanet Kepler-1625b transiting the star with the candidate exomoon in tow.
Artists' impression of the exoplanet Kepler-1625b with its large hypothesized moon. The pair have similar mass and radius ratio to the Earth-Moon system but scaled up by a factor of 11.

Why So Big?

This Neptune-sized moon, if it exists, seems unimaginably large. Neptune is an incredible four times larger in diameter than our own planet. Further, Neptune has 13 confirmed moons of its own! How can a distant solar system have a moon so ginormous that it's the size of a gas-giant planet?

Well, strange planets are nothing new, so we wouldn't expect less of a distant moon, would we? Other solar systems have "hot Jupiters," which are Jupiter-sized gas giants that orbit right next to their star. In our solar system, we only have small, rocky worlds in that zone, so it's a little hard to imagine a planet like Jupiter ending up there. Other solar systems also have exotic exoplanets such as super-Earths, which are rocky worlds bigger than Earth and smaller than Neptune. We don't know of any super-Earths in our own solar system.

And here's the other thing to remember: The moon is probably Neptune-sized, but it's puny compared to the mass of its parent planet. The moon is likely about 1.5 percent the mass of the planet it's orbiting around. Relatively speaking, that's the same mass difference between the Earth and our own moon, even though both this exoplanet and this exomoon are much larger.

Could aliens be living on this moon and planet? Sadly, the answer is "unlikely." The planet and moon do lie inside of the star's "habitable zone," where water could exist on a rocky planet's surface. Unfortunately, the moon is gaseous and way too big to host life as we know it — which means it's likely the same sad situation for life on the planet.

Still, life or not, this possible exomoon might be one of the top scientific discoveries of 2018. All we need to do is get it confirmed. Shall we get started?

Get stories like this one in your inbox each morning. Sign up for our daily email here.

Explore the weird worlds of our universe in vivid illustration with "Exoplanets A to Z," written by Gabriel Recchia and illustrated by Mary Recchia. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Elizabeth Howell October 3, 2018

Curiosity uses cookies to improve site performance, for analytics and for advertising. By continuing to use our site, you accept our use of cookies, our Privacy Policy and Terms of Use.