Astronomers believe they have discovered the first known moon beyond our solar system, in orbit around a gigantic planet 8,000 light years away. “The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” said David Kipping, an astronomer at Columbia University in New York who has spent more than a decade hunting for far-flung moons. The moon’s Neptune size was on a scale that had “hardly been anticipated” and “defies easy explanation” based on current theories of moon formation.
The so-called exomoon, which is estimated to be the size of Neptune, would also be the biggest known moon, far exceeding anything known to exist in our own solar system. The team behind the apparent discovery said it needed further confirmation but that they had failed to find any other convincing explanation for their data.
Kipping and his colleague Alex Teachey made the discovery after analyzing data from nearly 300 distant planets discovered using the Kepler space telescope. The planets are revealed by a momentary dimming of starlight as they pass in front of their host star, in what astronomers call a transit.
Kipping and Teachey noticed strange anomalies in the transit data of a gas planet, Kepler 1625b, which is several times the size of Jupiter. Artist’s impression of the planet Kepler-1625b transiting its star with the suspected exomoon in tow is shoown above. (Dan Durda/Science)
“We saw little deviations and wobbles in the light curve that caught our attention,” Kipping said.
Artist’s impression of the exoplanet Kepler-1625b with its suspected moon. The pair would have a similar mass and radius ratio to the Earth and its moon but scaled up by a factor of 11. (Dan Durda/Science)
The findings were intriguing enough to win them 40 hours of observation time on the Hubble space telescope,
detecting a second, much smaller dip in the star’s brightness 3.5 hours after the planet’s transit, hinting at “a moon trailing the planet like a dog following its owner on a leash”. On another transit, the planet appeared 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common centre of gravity, causing the planet to wobble from its predicted location.
The moon is estimated to be only 1.5% the mass of its companion planet, which is close to the mass-ratio between the Earth and its moon. Our moon is thought to have formed through an early collision with a larger body that blasted off material that later coalesced into a ball. However, Kepler 1625b and its moon are gaseous, not rocky, raising questions about how such a moon could have formed in the first place.
Based on the moon’s distance from its star, its surface temperature was estimated to be 80C, which is considered just on the upper end of conditions that could support life. However, astronomers said they had not dwelled on the question of habitability, since the moon is thought to be composed of gas. It would not resemble the fictional exomoon Pandora in the film Avatar, Teachey said.
“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” said Stephen Kane, an associate professor of planetary astrophysics and a member of the University of California Riverside’s Alternative Earths Astrobiology Center. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”
In a paper in The Astrophysical Journal, researchers at the University of California, Riverside and the University of Southern Queensland identified more than 100 giant planets that potentially host moons capable of supporting life. Their work will guide the design of future telescopes that can detect these potential moons and look for tell-tale signs of life, biosignatures, in their atmospheres.
Since the 2009 launch of NASA’s Kepler telescope, scientists have identified thousands of planets outside our solar system, which are called exoplanets. A primary goal of the Kepler mission is to identify planets that are in the habitable zones of their stars, meaning it’s neither too hot nor too cold for liquid water—and potentially life—to exist.
Terrestrial (rocky) planets are prime targets in the quest to find life because some of them might be geologically and atmospherically similar to Earth. Another place to look is the many gas giants identified during the Kepler mission. While not a candidate for life themselves, Jupiter-like planets in the habitable zone may harbor rocky moons, called exomoons, that could sustain life.
The researchers identified 121 giant planets that have orbits within the habitable zones of their stars. At more than three times the radii of the Earth, these gaseous planets are less common than terrestrial planets, but each is expected to host several large moons.
Scientists have speculated that exomoons might provide a favorable environment for life, perhaps even better than Earth. That’s because they receive energy not only from their star, but also from radiation reflected from their planet.
Until now, no exomoons have been confirmed.
The Daily Galaxy via The Guardian, Science Advances, and UC Riverside
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