Exomoon signal found in archival data hints at possibility of more discoveries to come

Astronomers have reported a second large moon orbiting a Jupiter-sized planet beyond our solar system. If confirmed, the observation could mean that exomoons are as common in the universe as exoplanets, and whether large or small, these moons are a feature of planetary systems. But the wait could be long. The very first sighting of an exomoon four years ago is still awaiting confirmation, and verifying this new candidate could be just as long and controversial.

The discovery, published in natural astronomywas led by David Kipping and his Cool Worlds Lab at Columbia University, which reported the first exomoon candidate in 2017.

“Astronomers have found more than 10,000 candidate exoplanets so far, but exomoons are much more difficult,” said Kipping, who has spent the past decade hunting exomoons. “They are terra incognita.”

The team spotted the giant exomoon candidate orbiting the planet Kepler 1708b, a world 5,500 light-years from Earth toward the constellations Cygna and Lyra. This new candidate is about a third smaller than the Neptune-sized moon that Kipping and his colleagues found earlier orbiting a similar Jupiter-sized planet, Kepler 1625b.

Both supermoon candidates are likely made of gas that has accumulated under the gravitational pull caused by their enormous size, Kipping said. If one astronomer’s hypothesis is correct, the moons may even have started life as planets, only to be dragged into the orbit of an even larger planet like Kepler 1625b or 1708b.

Both moons are located far from their host star, where there is less gravity to pull planets and tear their moons apart. In fact, researchers have been looking for cool gas giant planets in wide orbits in their search for exomoons precisely because our own solar system’s analogue, Jupiter and Saturn, have over a hundred moons between them.

If other moons are out there, they’ll likely be less monstrous, but also harder to spot, Kipping said. “The first detections in any investigation will usually be the bonkers,” he said. “The big ones which are just the easiest to spot with our limited sensitivity.”

Exomoons fascinate astronomers for the same reasons as exoplanets. They have the potential to reveal how and where life may have emerged in the universe. They’re also curiosities in their own right, and astronomers want to know how these exomoons form, whether they can sustain life, and what role, if any, they play in making their host planets habitable.

In the current study, the researchers looked at the sample of the coldest gas giant planets captured by NASA’s planet-hunting spacecraft Kepler. After scanning 70 planets deep, they found a single candidate – Kepler 1708b – with a moon-like signal. “It’s a stubborn signal,” Kipping said. “We threw the kitchen sink at this thing, but it won’t go away.”

Observations from other space telescopes, like Hubble, will be needed to verify the discovery, a process that could take years. Four years later, Kipping’s first exomoon discovery continues to be hotly debated. In a recent paper, he and his colleagues showed how a group of skeptics may have missed Kepler’s moon 1625b in their calculations. Meanwhile, Kipping and his colleagues continue to investigate other sources of evidence.

Eric Agol, a professor of astronomy at the University of Washington, said he doubted this latest signal would turn out to be real. “It could just be a fluctuation in the data, due to the star or instrumental noise,” he said.

Others seemed more optimistic. “It’s science at its best,” said Michael Hippke, a freelance astronomer in Germany. “We find an intriguing object, make a prediction, and either confirm the exomoon candidate or rule it out with future observations.”

“I am very excited to see a second exomoon candidate, although it is unfortunate that only two transits were observed,” he added. “More data would be very cool.”

Spotting a moon or even a planet hundreds or thousands of light-years from Earth is anything but simple. Moons and planets can only be observed indirectly as they pass in front of their host stars, causing the starlight to fade intermittently. Capturing one of these fleeting transit signals with a telescope is tricky, as is interpreting the lightcurve data. Moons are even more difficult to detect because they are smaller and block less light.

But the research is worth it, Kipping said, recalling how the existence of exoplanets was met with the same skepticism as exomoons are today. “These planets are extraterrestrial compared to our home system,” he said. “But they revolutionized our understanding of the formation of planetary systems.”

Other authors are: Steve Bryson, NASA Ames Research Center; Chris Burke, MIT; Jessie Christiansen and Kevin Hardegree-Ullman, Caltech; Billy Quarles, Valdosta State University; Brad Hansen, University of California, Los Angeles; Judit Szulagyi, ETH Zurich; and Alex Teachey, Colombia.

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