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Exoplanet and exomoon
An artist’s impression shows the Jupiter-sized exoplanet Kepler-1625b transiting its parent star with the Neptune-sized candidate exomoon in tow. (Dan Durda Illustration)

With a lot of help from the Hubble Space Telescope, astronomers now feel confident enough to publish their evidence for the first moon detected in orbit around a planet beyond our solar system.

But they’re still not completely confident.

“At this point, it’s up to us to report what we’re seeing, hand it over to the community and let the community probe it,” said Columbia University astronomer Alex Teachey, one of the authors of a study about the find published in the open-access journal Science Advances. “If they see what we see, I expect some people will be convinced and other people will be skeptical. And that’s all part of the process.”

Reports about the exoplanet and its apparent exomoon have been circulating for more than a year, thanks in part to the Hubble data-gathering campaign. Previous observations from NASA’s Kepler space telescope suggested that the Jupiter-sized planet, known as Kepler-1625b, had a world about the size of Neptune in orbit around it. But there weren’t enough observations to make the case conclusively.

That’s where Hubble was able to help with an extra 40 hours’ worth of observations. Like Kepler, Hubble tracked the faint dimming of light from Kepler-1625, a sunlike star about 8,000 light-years from Earth, as the two worlds passed over the star’s disk. The timing of the dips in the starlight — sometimes with the smaller world in the lead, and sometimes with it trailing — could be analyzed to work out the orbital mechanics behind the pair’s motion.

Hubble’s data, along with fine-tuned data from Kepler, strengthened the case for claiming Kepler-1625b had an exomoon.

“We’ve tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we’re unable to find any other single hypothesis which can explain all of the data we have,” Columbia Professor David Kipping, the study’s other author, told reporters during a teleconference in advance of the study’s release.

The data suggest that the exomoon orbits Kepler-1625b at a distance of about 2 million miles (3 million kilometers), Teachey said. The two worlds together are in an orbit around their parent star that’s similar to Earth’s orbit around the sun. Based on computer modeling, the surface temperature on both the alien planet and the moon would be on the order of 175 degrees Fahrenheit (80 degrees Celsius), Kipping said.

“That’s on the very outer edge of what you’d call habitable,” he said. “To be honest, we never really worried too much about habitability, just because they’re both gas giants.”

Even though gas giants aren’t typically considered hospitable to life, it’s hard to resist comparing the pair to exoplanets and exomoons in the movies, such as Polyphemus and Pandora in “Avatar.”

“I’m partial to Endor myself,” Teachey joked. “I’m more of a ‘Star Wars’ fan.”

The big mystery is how one gas giant could be in orbit around another. Such a scenario is unlike anything that’s seen in our own solar system, of course, and it doesn’t fit any of the standard theories for moon formation. That’s a big reason for the astronomers’ tentativeness about their claims.

“It’s the unknown unknowns which are ultimately uncharacterizable,” Kipping said. “We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis.”

Those observations could come from another go-round with Hubble, or they could come from NASA’s James Webb Space Telescope, which is due for launch in 2021. “We’d be very grateful if we had the means to use James Webb, because then we could really clean up,” Kipping said.

Kipping said there’s already another potential planet-moon combination worth looking at. More may well follow.

“If validated, the planet-moon system, a Jupiter with a Neptune-sized moon, would be a remarkable system with unanticipated properties, in many ways
echoing the unexpected discovery of ‘hot Jupiters’ in the early days of planet hunting,” he said.

The study, titled “Evidence for a Large Exomoon Orbiting Kepler-1625b,” is freely available at the Science Advances website.

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