Astronomers cautiously claim the first 'exomoon'
Data suggests a moon the size of Neptune orbiting a planet 8,000 light years away. Richard A Lovett reports.
Scientists using data from the Hubble Space Telescope have found what appears to be a gigantic moon circling a planet in a solar system 8000 light years away.
That makes it not only the first “exomoon” ever detected, but one vastly larger than any moon in our own solar system.
The planet, called Kepler-1625b, is about the size of Jupiter. The moon, currently unnamed, is approximately the size of Neptune, the solar system’s fourth-largest planet. The two bodies are approximately three million kilometres apart, about eight times more than the distance between the Earth and the moon.
Kepler-1625b first drew attention in 2017, in a survey of 284 transiting exoplanets discovered by the Kepler Space Telescope, when scientists noticed some peculiarities associated with it.
Transiting exoplanets are ones with orbits that cross between the Earth and their suns, causing the stars to dim slightly each time they interfere with the line of sight.
Kepler-1625b is far enough from its sun that it only completes a revolution once every nine-and-a-half months — meaning that Kepler only saw it three times during its three-year initial mission.
That’s a problem for scientists trying to determine if these peculiarities truly resulted from a moon.
“Three transits is tantalising, but not conclusive,” says David Kipping, an astronomer at Columbia University, New York, US.
In order to pin these peculiarities down more conclusively, Kipping and graduate student Alex Teachey turned to the Hubble Space Telescope, with which they watched the star for 40 hours during another transit occurring between October 28 and 29, 2017. They found what Kipping calls “two substantial anomalies”.
One was that the planet began its transit about an hour-and-a-quarter too early.
“That’s indicative of something gravitationally tugging on the planet,” he says, which is possible evidence for the presence of a large moon.
Then, about three-and-a-half hours after the planet completed its transit, the star dimmed again, albeit by a lesser amount. This, Kipping says, was indicative of a moon “trailing the planet like a dog following its owner on a leash”.
These two events don’t categorically prove the existence of a large moon, but they do make it likely.
“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,” Kipping explains.
Teachey adds that the finding was facilitated by the high-quality data available from the Hubble.
“A critical aspect is that we achieved about four times better precision with the Hubble data over the Kepler data,” he says.
Other scientists are excited. Moons are common in the local solar system, so nobody is surprised that they exist elsewhere, says Stephen Kane, a planetary astrophysicist at University of California, Riverside, US – but actually seeing one is a big deal.
“It’s at the very threshold of what we can currently do,” he says.
Also exciting is the moon’s enormous size. Not only was that a surprise, but it raises questions about how it was formed.
Conventional theory holds that there are three ways in which planets can acquire moons. One is as a result of a giant collision, similar to that believed to have formed Earth's moon. Another is by accretion from some of the same materials that helped form the planet itself. A third is by gravitational capture of a once-independent object.
But Kepler-1625b’s moon is on the order of 100 times more massive than any found around here.
“[That is] puzzling and definitely will give a lot of work for us theorists,” says Judit Szulagyi of the University of Zurich, Switzerland. “One possibility is that during a planet-planet encounter these two objects captured each other.”
But, Szulagyi adds, the fact that Kepler-1625b’s moon is huge doesn’t mean that all, or even many, other exomoons are similarly bloated. Rather, it’s a case of big moons being easier to spot. “The larger the moon is in respect to its planet, the easier it is to discover,” she says.
Kipping and Teachey are quick to note that their find isn’t fully confirmed.
“The fist exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” Teachey says.
What’s needed now, Kipping says, is more time on the Hubble to watch the Kepler-1625b’s next transit, in May 2019. If the moon is orbiting as thought, he says, this time it should transit ahead of the planet, rather than behind it, producing anomalies opposite to those seen in 2017.
“If we see that, I think we’re done,” he says.
But the work has to be done with the Hubble, not an Earth-based telescope, Teachey adds, because the transit takes too long for any given ground-based instrument to be able to track the whole thing.
“So you have to go space,” he says. “We have put in a proposal to observe the target again in May 2019, but we’re still waiting on the results of that proposal.”