They’re not certain yet.
But a pair of scientists at Columbia University in New York City say they may have discovered a moon outside our solar system.
If so, the finding is a first.
The researchers published their results at 2 PM ET today in the journal Science Advances.
Says Alex Teachey, the paper’s lead author: “The moon model emerges as the best explanation for the data.”
The exomoon candidate orbits Kepler-1625b, a planet the size of Jupiter.
The moon’s size—enormous, comparable to Neptune—is “really surprising,” says Teachey. Among the 194 satellites in our solar system, there’s not a one anywhere near as large.
Both planet and moon are unimaginably distant—approximately 8,000 light-years from Earth, more than 47 quadrillion miles.
“Some of the weirdest stuff happens to live quite far away,” says David Kipping, Columbia astronomy professor and a co-author of the paper. “It makes it difficult to pursue this object.”
Indeed, confirming this exomoon candidate—so faint, so remote—remains astoundingly complex. The researchers, although characterizing their findings as “compelling,” are cautious.
“The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” says Teachey. “We’re not cracking open champagne bottles just yet on this one.”
“We want to see a little more before we come out and say, ‘yes, this thing is definitely there.’”
The scientists suspected a moon after analyzing data from the Kepler Space Telescope. Amidst a media blitz, they released those preliminary results last year.
The latest findings combine the Kepler research with 40 hours of follow-up observation from the Hubble Space Telescope.
“We achieve about four times better precision with the Hubble data over the Kepler data,” says Teachey.
What they found was enough to prompt today’s guarded announcement.
But little else is known about Kepler-1625b or its supposed moon.
Although within their system’s habitable zone, both worlds are gas giants with no rocky surface. Life on either place is problematic at best.
The star they orbit—in the constellation Cygnus—is slightly larger than our Sun and about 10 billion years old, more than twice the age of our solar system.
The worlds are roughly 80 million miles from the star; a year on Kepler-1625b is 287 Earth days.
The scientists discovered the exomoon candidate while looking for eclipse-like events called “transits.” That’s when an object—whether planet or moon—passes between the parent star and the space telescope.
As that happens, the world blocks a tiny fraction of the star’s light. That dip in the light, minuscule but measurable, tells scientists something might be there.
Finding a moon transit is especially tricky.
“Imagine the moon going around the planet,” says Teachey. “The moon will show up in a different place every time the planet transits—sometimes showing up before the planet, sometimes after.”
Stellar noise can get in the way too, causing mysterious flickering in the data, enough to masquerade as a world.
But the parent star is “an unusually quiet object,” says Kipping. “We don’t see any evidence for any kind of stellar activity.”
The Columbia team observed three transits with the Kepler data; one more with Hubble. Still, that’s not enough to confirm the exomoon.
“We are hoping to observe the target again with Hubble,” says Teachey.
That proposal is already in; approval is likely. The next transit occurs in May 2019, with months of analysis to follow.
Should the researchers discover a single “separate, clean, moonlike event,” says Kipping, that could be the clincher.
“If we see that, I think we’re done,” he says.
