Since the early 1990s, scientists have cataloged thousands upon thousands of exoplanets drifting beyond our solar system. One rains glass instead of water, another is enveloped in not one but two atmospheres, and there are even a few without anchor stars, traversing the cosmos all alone.
Still, we've yet to confirm an exoplanet with an exomoon companion, a friendship akin to the one between our moon and Earth. "Exomoons are far more challenging," David Kipping, an astronomer at Columbia University and leader of the institution's Cool Worlds Lab, said in a statement. "They are terra incognita."
But having studied the presence of these cosmic objects for the last decade, Kipping has made some headway in the search. In a paper published Thursday in Nature Astronomy, he and his team found evidence of a supersized exomoon orbiting the exoplanet Kepler 1708b, a Jupiter-size giant that floats 5,500 light-years away from Earth.
The discovery, made using NASA's planet-hunting spacecraft, Kepler, comes a few years after Kipping's first exomoon candidate sighting in 2017. He spotted a Neptune-size moon possibly orbiting the planet Kepler 1625b, which is just as massive as Kepler 1708b. That one's awaiting its formal exomoon classification though, so the field of astronomy is technically still exomoon-less.
Regardless of the extensive vetting process required of all possible exoplanet signals, which includes space telescope observations such as those from Hubble, Kipping says the search is worth it. He recalls skepticism that once surrounded the quest to find exoplanets prior to the first one's confirmation.
"Those planets are alien compared to our home system," he said. "But they have revolutionized our understanding of how planetary systems form." In the same vein, finally proving the existence of exomoons could help scientists understand the dynamics of planet-moon systems, and one day, even unveil moons' role in supporting and sustaining life.
Returning to Kipping's newly documented possible exomoon: It's approximately a third smaller than his first contender. However, both are likely made of gas that piled up under the gravitational pull accompanying their heft, he says. They're also each located relatively far from their respective host stars, another reason why Kipping believes they're so big. Such distance means there's less gravity present to strip down their layers.
"The first detections in any survey will generally be the weirdos, the big ones that are simply easiest to detect with our limited sensitivity," Kipping said.
Locating exoplanets, let alone exomoons, is super difficult because their signals only arrive to astronomers when the globes intersect their home star's light. Think of it like Morse code showing up in space from Earth's viewpoint, with each dot representing the planet blocking starlight.
As moons are much smaller than planets, it's naturally harder to see when they block starlight. The Morse code, in a sense, is fainter. In fact, that's why Kipping's finding has earned some scrutiny -- noticing these rare, foreign orbs doesn't happen every day.
"It might just be a fluctuation in the data, either due to the star or instrumental noise," Eric Agol, an astronomer at University of Washington who wasn't involved with the research, said in a statement.
Others, however, are optimistic about Kipping's exomoon data. "This is science at its best," Michael Hippke, an independent astronomer in Germany who also wasn't involved in the research, said in a statement. "We find an intriguing object, make a prediction, and either confirm the exomoon candidate or rule it out with future observations."