In their hunt for elusive exomoon candidates, astronomers have looked at 70 cool gas giants found by NASA’s Kepler space telescope. They’ve found only one signal that could indicate an exomoon, about 2.6 times larger than Earth, orbiting a Jupiter-sized exoplanet called Kepler-1708b. Named Kepler-1708b-i, it joins Kepler-1625b-i as another example of an unexpectedly large exomoon candidate — echoing the surprise that hot-Jupiter discoveries elicited in the mid-1990s.

An artist’s impression of the gas giant Kepler-1708b, its moon Kepler-1708b-i, and their parent star. Image credit: Helena Valenzuela Widerström.

An artist’s impression of the gas giant Kepler-1708b, its moon Kepler-1708b-i, and their parent star. Image credit: Helena Valenzuela Widerström.

In the last three decades, more than 4,000 planets around stars other than the Sun have been discovered.

These alien worlds display remarkable diversity, from highly eccentric Jupiters to compact, coplanar systems of rocky planets.

In an effort to understand the formation and evolution of such systems, more detailed knowledge about their environment and properties is sought — such as the existence and nature of potential.

Given the abundance of moons in our Solar System, it is reasonable to presume that exomoons will reside around some exoplanets — which has motivated efforts to detect them.

One of the most promising strategies for seeking exomoons focuses on transiting planets.

“Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are far more challenging. They are terra incognita,” said lead author Professor David Kipping, an astronomer in the Department of Astronomy at Columbia University.

In the current study, Professor Kipping and his colleagues looked at the sample of the coldest gas giant planets captured by NASA’s planet-hunting spacecraft, Kepler.

After scanning 70 planets in depth, they spotted the new exomoon candidate orbiting Kepler-1708b, a giant planet about 5,500 light-years away in the direction of the Cygnus and Lyra constellations.

“It’s a stubborn signal. We threw the kitchen sink at this thing but it just won’t go away,” Professor Kipping said.

The new exomoon candidate, Kepler-1708b-i, is about a third smaller than the Neptune-sized moon that the team earlier found orbiting a similar Jupiter-sized planet, Kepler-1625b.

“Both supermoon candidates are likely made of gas that has piled up under the gravitational pull caused by their enormous size,” Professor Kipping said.

“If one astronomer’s hypothesis is correct, the moons may have even started life as planets, only to be pulled into the orbit of an even bigger planet like Kepler-1625b or Kepler-1708b.”

Both moons are located far from their host star, where there’s less gravity to tug at planets and strip off their moons.

In fact, the astronomers sought out cold, giant gas planets on wide orbits in their search for exomoons precisely because the analog in our own Solar System, Jupiter and Saturn, have more than a hundred moons between them.

“If other moons are out there, they will likely be less monstrous, but also harder to spot,” Professor Kipping said.

“The first detections in any survey will generally be the weirdos. The big ones that are simply easiest to detect with our limited sensitivity.”

“Observations from other space telescopes, like Hubble, will be needed to verify the discovery, a process that could take years,” he added.

The discovery is described in a paper published in the journal Nature Astronomy.

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D. Kipping et al. An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i. Nat Astron, published online January 13, 2022; doi: 10.1038/s41550-021-01539-1




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