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Astronomers May Have Discovered the First Known ‘Exomoon’

A new study suggests that the Jupiter-sized planet Kepler-1625b may host a Neptune-sized moon.
Image: Dan Durda

Astronomers may have detected an exomoon, a moon outside our solar system, for the first time in history.

In a study published Wednesday in Science Advances, Columbia University astronomers David Kipping and Alex Teachey present evidence that a large moon is orbiting Kepler-1625b, an exoplanet the size of Jupiter located 8,000 light years from Earth.

“When we run our models, the moon model emerges as the best explanation for the data,” Teachey said during a Monday media teleconference. “Still we are urging caution here. The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence.”


Exomoon observations would expand our understanding of planetary evolution and shed light on the history of our own solar system. These discoveries could also inform the search for aliens, because exomoons may well be habitable—just as some moons in our solar system are thought to be promising places to look for extraterrestrial life.

Rumors about this hypothetical exomoon have been circulating for more than a year, in the wake of observations of Kepler-1625b’s passage in front of its Sun-like host star. This event is called a transit, and it causes a slight dimming of stellar luminosity like a moth flying in front of a lamp.

A large exomoon could also produce a dimming effect, either shortly before or after its host planet’s transit, depending on where the moon is located in its own orbit. NASA’s Kepler space telescope scanned for this type of pattern, and found that Kepler-1625b had some observational anomalies that suggested it might host an exomoon.

To follow up on this alluring find, Kipping and Teachey were granted 40 hours of time on the Hubble Space Telescope’s Wide Field Camera 3 (WFC3), which is four times as precise as Kepler’s instruments.

In that short period, they observed one 19-hour long transit of Kepler-1625b, followed by a second light-dip 3.5 hours later. That second transit could be a Neptune-sized exomoon orbiting at a distance of about 40 times the planet’s radius, Kipping and Teachey thought. They also observed the exoplanet wobbling and beginning a transit earlier than expected, perhaps due to the gravitational pull of a large exomoon.


To confirm that Kepler-1626b has a moon, astronomers will need to observe at least one more full transit on Hubble. Kipping and Teachey have already applied for an extended time slot in May 2019.

Read More: Exoplanets Are Great, But It's Time to Start Looking for Exomoons, Too

In the meantime, they are looking at other promising systems with similar light patterns. Kipping would not reveal the next exomoon candidate, but gave me some clues by email: “It is another long-period Jupiter-sized planet discovered by Kepler, but around a brighter star. Moreover it’s even further from its star so really has great chances for moons and even rings.”

If any of these worlds are confirmed to host exomoons, it will be a momentous astronomical achievement that reshapes our understanding of alien star systems. Next-generation observatories such as the James Webb Space Telescope (JWST), which will be 100 times more powerful than the Hubble, will also make it easier to detect moons around planets like Earth.

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