Astronomers may have found the first moon outside our solar system, a gas giant the size of Neptune.
Plenty of planets exist beyond our solar system, but a moon around one of those worlds has yet to be confirmed.
Two Columbia University researchers presented their tantalising evidence for a moon on Wednesday.
The potential moon would be considerably larger than Earth, about the size of Neptune or Uranus.
The planet it orbits is as big as mammoth Jupiter.
Researchers Alex Teachey and David Kipping evaluated 284 planets outside our solar system that had already been discovered by Nasa’s Kepler Space Telescope.
Only one planet held promise for hosting a moon, one around the star known as Kepler-1625, which is about the size of our sun but older.
So last October, the pair directed the Hubble Space Telescope at the star in an attempt to verify, or rule out, the possibility of a moon orbiting the planet Kepler-1625b.
They were on the lookout for a second temporary dimming of starlight.
The main dip in stellar brightness would be the planet itself crossing in front of its star.
Another dip could well be a moon, known as an exomoon outside our solar system.
The more powerful and precise Hubble telescope detected a second and smaller decrease in starlight three and a half hours after the planet passed in front of the star, “like a dog following its owner on a leash,” as Mr Kipping put it.
The observation period, however, ended before the moon could complete its transit.
That is why the astronomers need another look with Hubble, hopefully next spring.
Despite the evidence, Mr Teachey stressed “we are urging caution here”.
“The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence,” Mr Teachey said.
He added: “We’re not cracking open Champagne bottles just yet on this one.”
If indeed a moon, it would be about two million miles from its planet and appear twice as big in its sky, as the moon does in ours.
The astronomers are uncertain how this potential moon might have formed, given its size.
“If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of,” Nasa’s science mission chief Thomas Zurbuchen said in a statement.
According to the researchers, another compelling piece of evidence in favour of a moon is that the planet passed in front of its star more than an hour earlier than predicted.
A moon could cause that kind of an uncertain, wobbly path, they noted.
Mr Kipping said that was how the Earth and moon would appear from far away.
This particular planet, or exoplanet, is about the same distance from its star as Earth is to the sun.
Another planet could cause the same gravitational nudge, the researchers noted, although Kepler observations have come up empty in that regard.
Kepler-1625b is the only planet found so far around this star.
For Mr Teachey and Mr Kipping, the best and simplest explanation is that Kepler-1625b has a moon.
“But we were unable to find any other single hypothesis which can explain all of the data we have.”
Their findings were published in the journal Science Advances.
The journal’s deputy editor, Kip Hodges, praised the researchers for their cautious tone, given the difficult and complicated process of identifying an exomoon.
“If this finding stands up to further observational scrutiny, it represents a major milestone in the field of astronomy,” Mr Hodges said.
In the meantime, they are encouraging other scientists to join in and embracing the scrutiny that’s sure to come.
Whether confirmed or not, the subject offers insight into how rare, or how common, our own solar system might be.
Moons are abundant in our own solar system, with close to 200.
Of the eight planets in our solar system, only Mercury and Venus have none.
Given that both the planet and its potential moon are gas giants, no one is suggesting conditions that might support life.
“But going forward, I think we’re opening the doors to finding worlds like that,” Mr Teachey said.