Artist’s rendering of exomoon candidate Kepler-1625b-i and its planet
Artist’s rendering of the Jupiter-sized exoplanet Kepler-1625b with its hypothesized Neptune-sized moon, Kepler-1625b-i. Credit: Dan Durda

A team of astronomers has announced new evidence supporting the existence of an exomoon in orbit around a distant exoplanet.

“When it comes to exomoons, the question is not if they exist but ‘what are the physical properties of such a population.’”

“Within our solar system, satellites are abundant,” said David Kipping, assistant professor of astronomy at Columbia University in New York City. “We have long assumed that, when it comes to exomoons, the question is not if they exist but ‘what are the physical properties of such a population.’” Kipping, who has been hunting exomoons for nearly a decade, coauthored a Science Advances research paper that announced the discovery today.

Using the Hubble Space Telescope (HST), the team observed a Jupiter-sized planet as it transited its host star and blocked a fraction of the star’s light. They found that the timing, shape, and strength of the planet’s transit showed peculiarities that were highly consistent with it hosting a Neptune-sized moon.

“We are looking forward to the scrutiny of the scientific community on this work,” said lead author Alex Teachey, “and we hope that we will have an opportunity to observe the target again before too long.” Teachey is a graduate student in the Department of Astronomy at Columbia University.

If future observations validate this hypothesis, the exomoon candidate, tentatively dubbed Kepler-1625b-i, would be the first moon detected around a planet outside of our solar system.

Two Telescopes, One Possible Target

The planet, called Kepler-1625b, first came to the researchers’ attention as a possible exomoon host in 2017 after they analyzed data from the Kepler Space Telescope. Kepler observed three transits of the Jupiter-sized world during its primary mission. The planet, about 8,000 light-years from Earth, takes about a year to orbit an old star that is slightly larger and more massive than the Sun.

Kepler-1625b was the only system out of nearly 300 that showed any hint of a moon.

The Kepler data contained hints—subtle blips in the host star’s emitted light that were slightly offset from the planet’s signal—that an exomoon might orbit the planet. Kepler-1625b was the only planetary system out of nearly 300 viable targets that showed any hint of a moon.

The preliminary results from Kepler were tantalizing enough for the team to observe a fourth transit in October 2017 using HST, which provided a fourfold improvement in precision over Kepler for this star and also made observations at infrared wavelengths. They obtained about 40 hours of observing time on Hubble, then got to work meticulously scrutinizing the data.

Anomalies in Timing and Brightness

Teachey and Kipping found that two aspects of the transit data from Hubble were consistent with their exomoon hypothesis. First, the planet transited the star 1.25 hours earlier than expected on the basis of the orbital period measured by Kepler. “That is indicative of something gravitationally tugging on the planet” during this particular transit, Kipping explained.

Artist’s impression of Kepler-1625b and its potential moon transiting their host star
Artist’s impression of the exoplanet Kepler-1625b (larger black dot) and its potential exomoon (smaller black dot) transiting their host star. In this rendering, the planet and moon would orbit the star from left to right. New data from HST first showed a dip in the star’s brightness as the planet transited across the face of the star. This dip was followed by a second, smaller dip, which the team attributed to a smaller exomoon. Credit: Dan Durda

If a moon did exist, he continued, the position of the moon in its orbit about the planet could help explain why the timing of the Hubble transit differed from that of the Kepler data. Imagine the influence of the moon like pushing someone on a swing: Depending on when you push, the direction of your push, and where you’re standing when you do push, you (the moon) could make the swing (the planet) move faster or slower or not change speed at all.

Second, the Hubble observations of the host star’s brightness showed two dips in brightness instead of just the one from the planet. “The location, shape, and depth of this event appear consistent with a Neptune-sized moon [also] transiting in front of the star,” Kipping said.  The team also saw this secondary dip in the star’s light in some of the Kepler transits.

“We are unable to find any other single hypothesis which can explain all of the data that we have.”

The team compared their data with outputs from a variety of transit models—some that included or excluded exomoons and some that included or excluded other exoplanets. “We have tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system, or stellar activity,” Kipping said, “but we are unable to find any other single hypothesis which can explain all of the data that we have.”

“The combination of Hubble data with the Kepler data is really an essential part of the moon search,” Teachey said.

Moon of a Surprising Size

The exoplanet-exomoon system suggested by the new observations has mass and radius ratios similar to those of the Earth-Moon system, but scaled up by a factor of 11. If you were on a spaceship flying through the planet’s atmosphere, the exomoon would appear to be around twice as large in the sky as our Moon does, Teachey explained.

“A moon like this is not necessarily readily explainable right off the bat.”

The team was surprised that a Neptune-sized moon seemed to orbit a Jupiter-sized planet. Moons in our solar system formed as by-products of a collision (like our Moon), by asteroids or other objects being captured (like the moons of Mars and Neptune), or as leftovers of planet formation (like Jupiter’s Galilean moons). A Neptune-sized moon around a Jupiter-sized planet does not fit any of these formation scenarios, the team said.

“A moon like this is not necessarily readily explainable right off the bat,” said Teachey. However, “you certainly can’t rule it out on those grounds because nature makes all sorts of things that we are still struggling to explain,” he said.

Cautious Optimism

The exomoon-hunting team received media attention last year when their planned observations appeared on Hubble’s public schedule. This sparked unsupported claims that the team was more confident in a Kepler-1625b moon than they had said.

With this paper, “we are urging caution here,” Teachey said. “The first exomoon is obviously an extraordinary claim, and it requires extraordinary evidence.” The researchers hope to observe the May 2019 transit of Kepler-1625b with HST to further test their exomoon hypothesis.

For more, see the video below, in which Teachey details how they first learned of Kepler-1625b, their HST observations, why they believe that an exomoon might orbit that planet, and what else it might be.

YouTube video

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer


Cartier, K. M. S. (2018), Large exomoon likely orbits a faraway world, Eos, 99, Published on 03 October 2018.

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