The discovery of the first potential exomoon has astronomers scratching their heads. It’s the size of Neptune, and its planet is the same size as Jupiter but ten times the mass. It’s also made of gas. Simply put: it’s unlike any moon we’ve ever seen before.
Astronomers David Kipping and Alex Teachey published evidence for the moon in the journal Science Advances. Although while some have called this the first ever discovery of an exomoon, it is not that clear-cut.
It is, however, looking like the most probable explanation for the data that has been collected so far. And if it does turn out to be an exomoon, this would be a huge step for astronomy, and the hunt for alien life.
Finding an exomoon is an incredibly difficult task. We know of more than 3,500 exoplanets, yet no exomoons have been found. If these exoplanets are anything like our own Solar System, the moons will be there too. It’s just they’re hard to find.
Exoplanets are usually found by watching light from the host star, the exoplanet’s sun, dim as the planets pass between them and us. This is known as the transit method.
Yet finding an exomoon is another thing altogether. By studying the dip in brightness very closely, astronomers can search for hints of moons orbiting exoplanets. For example, if the planet takes longer to pass the star or if the brightness goes down more than usual, this could potentially be evidence for a moon orbiting the planet.
This is what Kipping and Teachey found when they monitored Kepler 1625b, a gas giant 8,000 light-years from Earth.
About 3.5 hours after the planet’s transit ended, the Hubble telescope recorded another, this time smaller, dimming of the star’s brightness.
This is consistent with “a moon trailing the planet like a dog following its owner on a leash,” Kipping said. “Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured.”
Hubble also found the planet began its transit 1.25 hours earlier than predicted. “An extraterrestrial civilisation watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth’s transit,” Kipping said.
If and when astronomers are able to confirm it is an exomoon, then such findings will provide an exciting new place to hunt for life beyond Earth. The moons in our own Solar System are some of the most exciting places for the potential of life; Saturn’s Enceladus has everything you need for life to exist, Titan has lakes of methane and Jupiter’s Io has water ice on its surface.
While super-Jovian planets – those bigger than our own biggest planet, Jupiter – might not be right for life, if each has multiple moons then it increases our chances of finding a tiny world with just the right conditions. But first we’ve got to find them.
“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 we have,” said Kipping.
“It is exciting because it shows we’re getting close to what would be an incredible ‘first’ in astronomy – the first detected extrasolar moon” says Hugh Osborn, an astronomer hunting exoplanet at Marseille University.
Osborn is wary of calling it a discovery just yet. “To me, for any discovery but especially the first of its kind, the evidence needs to be compelling,” he says. “It needs to convincingly rule out all other possibilities. I don’t think we are there yet with Kepler-1625.”
Hints of an exomoon were first seen with the Kepler space telescope, which is why Kipping and Teachey wanted to re-examine it with the Hubble space telescope. “We saw little deviations and wobbles in the light curve that caught our attention,” Kipping said.
“But since their first paper, the data have actually got better and the initial signal of an exomoon has been all but washed away” says Osborn. “Their hypothesis of there being an exomoon rests almost exclusively on the new dip seen in Hubble, but it is a very small signal and there are many other reasons a star may dim by a tiny amount – changes in the telescope, variability in the star itself, or a moon.”
Osborn says there are still other potential explanations; the shift in the time of transit could point to a moon but could also point to another planet in the system, for example.
For now, astronomers around the world will keep searching in the hope of finding a habitable exomoon, and more data on Kepler-1625b will let us work out whether or not the dips in brightness mean there is a moon there too.
“We’re hoping that one day we might really find something like a true Earth analogue in a moon,” Teachey said during a conference call on Monday. “That’s something more akin to what people are thinking of when they think of an ‘Avatar’ Pandor.”
Another chance is there may be no moons. Just because our Solar System has a lot of moons does not mean they are this common elsewhere.
Our Solar System could be an outlier. In some ways it already is. Earth-like planets have turned out to be surprisingly rare elsewhere, while ‘hot Neptunes’ – planets the size of Neptune but close to their star – are very common and we have none.
To find out, we had better keep looking.
