First Ever Spotted Exomoon May be Just Outside Our Solar System

NASA's Kepler and Hubble space telescopes have provided compelling evidence of the first ever spotted exomoon in a distant solar system in what may be a historic celestial sighting.
Shelby Rogers
Artist's rendering of the new moon.Dan Durda/Columbia University

Using a pair of NASA's most powerful telescopes, a team of Columbia University astronomer's have evidence of a new moon some 8,000 light years away. 

Columbia researchers Alex Teachey and David Kipping used the Hubble Space Telescope and the Kepler Space Telescope to make the discovery. The pair said they found a candidate for an exomoon, or a moon orbiting planets in other star systems. 

First signs of the exomoon

This particular exomoon captured the attention of the researchers not only because of its novelty but because of its large size. The exomoon candidate is comparable to the diameter of Neptune. In our solar system, that massive size of a moon doesn't exist in any one of the 200 cataloged natural satellites.  

"This would be the first case of detecting a moon outside our solar system," said Kipping, an assistant professor of astronomy at Columbia. "If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets."

The pair of researchers looked at data from 284 planets discovered by the Kepler telescope. They narrowed the scope to those with wide orbits lasting longer than 30 days around the host star. Those observations helped capture the momentary dimming of starlight as a planet passed in front of its star (or the transit). 

The particular exomoon candidate -- Kepler 1625b -- had very interesting anomalies.  "We saw little deviations and wobbles in the light curve that caught our attention," Kipping said.

Bringing in Hubble Space Telescope for more data

NASA allowed the team 40 hours of time with the Hubble telescope as well. In that time, the researchers gathered data four times more precise than what Kepler showed them. They could monitor the planet before and during the 19-hour transit across the face of the star. 

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After the transit, Hubble spotted a second and smaller decrease in the star's brightness. According to Kipping, it was consistent with "a moon trailing the planet like a dog following its owner on a leash."

"Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured."

Hubble also gave the astronomers evidence for it being a moon by measuring that the planet started its transit over an hour earlier than predicted. This data was consistent with a planet and moon orbiting a common center of gravity, the researchers noted, that would make a planet to 'wobble' from its anticipated location.

"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit," Kipping said.

The researchers think this activity would be caused by the gravitational pull of a second planet in the system. However, Kepler didn't find evidence of additional planets during its four-year mission around the star. 

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation."

Exomoons aren't easy to spot because they're smaller than their companion planets. This gives them a weak transit signal. What makes this particular exomoon intriguing, however, is that it exists in the solar mass star's habitable zone -- where temperatures would allow for liquid water to exist on a planet's surface. 

The findings have been published in a recent edition of the journal Science Advances.

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