We used to wonder if there were planets outside our solar system, and today the answer to that is a resounding “yes.” Even with our admittedly crude detection methods, scientists have identified thousands of exoplanets. We had every reason to expect those planets would have moons, but we had no way of knowing until now. Astronomers from Columbia University report they are close to confirming the first ever exomoon.
Astronomers David Kipping and Alex Teachey spotted the potential moon while studying a solar system 4,000 light years away. The data gathered by Kepler tracks dips in brightness of the solar system’s sun as objects pass in front of it. If these dips happen with predictable regularity, there’s a strong possibility they’re caused by a planet. They spotted something unusual in the data, though. There was an extra blip that could indicate a large moon.
Kepler has been collecting data for years, and researchers have thoroughly analyzed not all of it. Kipping and Teachey were looking through a collection of 284 exoplanets that had been identified as strong candidates for moon formation. They were all larger planets, which would have pulled in more surrounding material to increase the likelihood of moon formation. They were also far out from their stars because migrating inward could knock moons out of orbit.
Out of all those planets, Kepler-1625b presented the best chance for spotting an exomoon. The Kepler data wasn’t clear enough to be sure, so the pair got time on NASA’s Hubble Space Telescope. As the planet made its 19-hour solar transit, Hubble recorded brightness data for the star. After Kepler-1625b finished its transit, Hubble spotted a second, smaller dip that Kipping and Teachey believe is the planet’s moon. There’s also some evidence the moon affects the planet’s orbit — it started the transit about an hour earlier than expected, possibly thanks to a tug-o-war with the moon.
Kepler has spotted many exoplanets, and now maybe a moon.
It’s more difficult to confirm an exomoon than an exoplanet. The planets have identical transit characteristics every orbit, but an exomoon will vary based on where it is in its orbit of the planet. If Kepler-1625b does have a moon, Kipping and Teachey estimate it would be quite large. Kepler-1625b itself is around the size of Jupiter, and the moon is about the size of Neptune.
We may need to wait for a new generation of planet-hunting instruments to know for sure if Kepler-1625b has the first exomoon. The Transiting Exoplanet Survey Satellite (TESS) isn’t designed to look at objects that far away, and the James Webb Space Telescope is delayed until at least 2020. Some upcoming ground-based observatories might have enough power to verify or refute the claims sooner.
Now read: Astronomers Find Exoplanet In the Same Place as Star Trek’s Vulcan, Kepler Spacecraft Wakes Up to Begin New Observations, and Scientists Say Some Ultrahot Exoplanets Have Star-Like Atmospheres