“The closest analog would be picking up Neptune and putting it around Jupiter,” says David Kipping, the astronomer who discovered the object with Alex Teachey, his colleague at Columbia University. Their findings were published Wednesday in Science Advances.
Kipping and Teachey first came across the object in data from the Kepler space telescope, a NASA mission that has discovered more than 5,000 exoplanets and confirmed about half of them since it launched in 2009. The telescope detects planets through a technique called the transit method. When a planet passes, or transits, across the face of its star, it blocks a tiny fraction of the star’s light. Kepler stares at stars for years and watches for this dimming in action.
Kepler recently surveyed 284 exoplanets in its repertoire that astronomers suspected would make good hosts for moons. They’re large, and “the bigger the planet, the more leftover material there is to form moons,” Kipping explained. (The biggest planet in our solar system, Jupiter, also has the most moons and may add more; just this summer, astronomers discovered 10 new ones around the gas giant.)
A flurry of tiny moons around Jupiter
The 284 planets also have wide orbits around their star, which means they may not have experienced a migration that can be risky for moons. Scientists believe planets across the universe form farther out from their star, as ours did, and then move closer. Planets can survive this journey, but their moons might not. “It really can strip off all of the moons and eject them, or lead them to collide with the planets,” Kipping says.
Of the candidate planets, one called Kepler-1625b stood out. Kipping and Teachey detected some strange features in the light coming from its direction.
The astronomers decided they needed to get a better look, so they applied for observation time on NASA’s Hubble Space Telescope, which is four times more precise than Kepler. They monitored 1625b as it made its 19-hour trek across its star, dimming its light. About three and a half hours after 1625b finished its transit and disappeared from view, the astronomers detected a second, much smaller dimming in the star’s brightness. They believe the signal indicates the presence of a moon trailing behind its planet.
Kipping and Teachey also detected a shift in 1625b’s orbit. During the Hubble observations, the planet began its transit about an hour earlier than expected, which suggests that an interaction between the planet and a nearby moon caused the planet to wobble slightly from its usual course. An alien astronomer watching the Earth and moon move around the sun would see similar effects, Kipping says.
But when the target of observations is thousands of light-years away, how can you be sure you’re looking at a moon and not, say, another planet? The explanation lies in the transit signals of the two objects, Kipping says. “An exoplanet is normally pretty easy [to detect], to be honest,” he says. “It repeats like clockwork, or very close to clockwork. Every orbital period, you should see the planet in the same place.”