Spaceflight Insider

Has NASA’s Kepler Space Telescope detected first exomoon?

Kepler-1625b-i is the first exomoon candidate. If it is verified to be the first moon discovered outside of our Solar System. Image Credit: NASA/ESA/L. Hustak

Kepler-1625b-i is the first exomoon candidate. If it is verified to be the first moon discovered outside of our Solar System. Image Credit: NASA/ESA/L. Hustak

Two scientists have found evidence that an exoplanet found by NASA’s Kepler Space Telescope may have a Neptune-sized moon orbiting it.

Close to 4,000 exoplanets have been discovered since the 1990s. If confirmed, this discovery would be the first known exomoon.

Columbia University astronomers Alex Teachey and David Kipping first suspected the presence of a moon around the planet Kepler-1625b when studying Kepler data on 284 exoplanets with orbits of 30 days or longer in a search for exomoons. Kepler uses the transit method, which involves measuring the regular dimming of a star’s light caused by an orbiting planet passing in front of it, to indirectly detect exoplanets. When analyzing transit data on Kepler-1625b, they saw anomalies suggesting the possible presence of a moon.

“We saw little deviations and wobbles in the light curve that caught our attention,” Kipping stated.

To further search for this natural satellite, Teachey and Kipping observed the planet, located an estimated 8,000 light years away in the constellation Cygnus, for approximately 40 hours, which included its 19-hour transit in front of its star as well as the periods before and after the transit, using the Hubble Space Telescope (HST).

Two unusual phenomena surprised the astronomers. First, the planet passed in front of the star approximately an hour earlier than predicted. Second, three-and-a-half hours after the transit, a much smaller second transit occurred, which involved a very slight dimming of the star’s light.

Both anomalies could indicate the presence of a moon in orbit around the planet. When planets have large moons, the two objects orbit a common center of gravity. This could cause a wobble in the planet’s orbit, causing it to deviate from its predicted location and transit its star earlier than expected.

The second, much smaller transit could have been caused by a moon that follows or trails after its parent planet.

Unfortunately, the scientists’ time on Hubble ended before they could completely observe the second transit.

Because their transit signals are weaker than those of planets and their positions change as they orbit their parent planets. This makes exomoons much harder to detect than the worlds that they orbit.

While Kepler 1625b’s wobble could be caused by the gravitational pull of another planet orbiting the parent star rather than by an orbiting moon, Teachey and Kipping believe a moon is the most likely explanation.

“A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation. It was definitely a shocking moment to see that light curve–my heart started beating a little faster, and I just kept looking at that signature,” Kipping said.

Kepler-1625b and its possible moon are both located in the habitable zone of the parent star, Kepler-1625, a Sun-like star about eight percent more massive than the Sun. Estimated to have a mass several times of Jupiter, the planet is almost certainly gaseous and not likely to support life as we know it.

Teachey and Kipping believe the moon is approximately the size of Neptune, making it larger than any moon in our solar system. They estimate it to be 1.5 percent the mass of its planet and the mass ratio between planet and moon to be close to that of the Earth-Moon system. At this size, the moon, tentatively designated Kepler-1625b-i, is likely to be gaseous as well.

Further efforts to find exomoons will likely focus on Jupiter-sized planets that orbit their stars at distances greater than the Earth orbits the Sun. Planets in wide orbits take longer to transit their stars and are likely to have big moons, which should make them easier to find than small ones.

The James Webb Space Telescope (JWST), scheduled for launch in 2021, should make it possible for scientists to find even the smallest exomoons, Teachey said.

“This intriguing finding shows how NASA’s missions work together to uncover incredible mysteries in our cosmos. If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of,” emphasized Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate.

A paper on the finding has been published in the journal Science Advances.

Video courtesy of NASA Goddard





Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

⚠ Commenting Rules

Post Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.