Astronomers using the NASA/ESA Hubble Space Telescope and NASA’s Kepler Space Telescope have assembled compelling evidence for the existence of a large exomoon orbiting Kepler 1625b, a gas giant located approximately 8,000 light-years away from Earth.
An artist’s impression of the gas giant Kepler 1625b with its large moon, Kepler 1625b-i; the pair has a similar mass and radius ratio to the Earth-Moon system but scaled up by a factor of 11. Image credit: Sci-News.com.
The hunt for exoplanets provided its first results only 30 years ago. While astronomers now find these planets on a regular basis, the search for moons orbiting exoplanets wasn’t successful — until today.
In 2017, Kepler detected hints of an exomoon orbiting the planet Kepler 1625b.
Now, Columbia University astronomers Alex Teachey and David Kipping have used the incomparable capabilities of Hubble to study the planet and its host star, Kepler 1625.
The Hubble observations show compelling evidence for a large exomoon orbiting the planet.
Named Kepler 1625b-i, the moon is unusual because it has a mass and radius similar to Solar System’s Neptune.
“This may yield new insights into the development of planetary systems and may cause astronomers to revisit theories of how moons form,” Teachey said.
Like its moon, Kepler 1625b is also bigger than its counterparts in the Solar System. The exoplanet is a gas giant, several times more massive than Jupiter.
It orbits its parent star at a distance similar to the distance between the Sun and Earth, which puts it — and its moon — at the inner edge of the habitable zone of the planetary system.
“This would be the first case of detecting a moon outside our Solar System,” Dr. Kipping said.
“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.”
In looking for exomoons, the team analyzed data from 284 Kepler-discovered exoplanets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit.
The astronomers found one instance, in Kepler 1625b, that had intriguing anomalies.
“We saw little deviations and wobbles in the light curve that caught our attention,” Dr. Kipping explained.
The Kepler results were enough for the team to get 40 hours of time with Hubble to intensively study the planet, obtaining data four times more precise than that of Kepler.
The researchers monitored Kepler 1625b before and during its 19-hour-long transit across the face of the star.
“After it ended, Hubble detected a second and much smaller decrease in the star’s brightness 3.5 hours later, consistent with a moon trailing the planet like a dog following its owner on a leash,” Dr. Kipping said.
“Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured.”
In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.
“An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth’s transit,” Dr. Kipping said.
The findings are published in the journal Science Advances.
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Alex Teachey & David M. Kipping. 2018. Evidence for a large exomoon orbiting Kepler-1625b. Science Advances 4 (10): eaav1784; doi: 10.1126/sciadv.aav1784
