We first saw it as a little blob on a picture of a distant moon. And now it might be one of our best hopes of finding alien life in our own solar system.

Scientists announced this week that there is a water plume coming out of Europa, a moon of Jupiter and one of the prime candidates for life in our solar system.

This comes in the wake of discovering plumes on Saturn’s moon Enceladus, during the Cassini mission, which suggested the strange moon has the building blocks of life.

While the discovery about Europa is brand new, the data used to deduce it was old. The plume itself might be ancient, and could help us to finally solve the mystery of whether are alone in the universe – or even alone in our little patch of it: the Solar System.

That’s because, while the plume themselves might be thrilling, they’re more exciting for what they might mean. They are spewing out water that would otherwise be hidden beneath the moon’s thick icy crust, and which would therefore be very difficult to access.

“If Europa has plumes erupting into space, that really provides a new window for future exploration,” says Xianzhe Jia, a space physicist at the University of Michigan in Ann Arbor and the lead author on the new study. “We might be able to find out what’s going on in the ocean.”

The awareness that there might be a plume arrived gradually: first in pictures taken from the Hubble Space Telescope that seemed to suggest something strange was happening, and then through further, clearer Hubble pictures. One of those later sets of images sent Jia and his team back to the data – which already been collected all the way back in 1997, from a spacecraft that set off in 1989.

“We already had the data; we had a flyby over this region,” he says, and the scientists resolved to “look more carefully at the data we have”.

It worked. “In fact, when we look at the data more carefully – at the closest approach to the plume, we did detect strange changes which had not been explained before in the past,” he says.

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“So we looked at another data set, from around the time, which seem to indicate that during this interval the spacecraft is in a very different plasma environment. These two pieces of information immediately stood up – seeming to tell us that this minimal signal is being produced by a localised source, of particles.”

In scientific terms, the team had a very good hunch that something was going on. But they needed to prove it, so they used extensive computer modelling to work out whether the data lined up with what would be be expected if there were a plume – and it did, with what Jia calls “stunning” precision.

That’s why, thought the data is fairly old, the discovery could never have been made at the time. Only with a hunch, and brand new modelling techniques, could scientists make sense of it.

“Twenty years ago, you might have had the idea that this was related to the plume,” he says. “But the models of the time wouldn’t allow you to draw that conclusion – we’d have to wait for better images from future missions.”

The Cassini mission to Enceladus (which Jia was not involved in) has already given scientists an understanding that they can bring to other plumes.

“The knowledge that has been developed on the Enceladus missions and the studies gives us a very good idea about, if you do have water plumes in space, how would that change the magnetic and the plasma environment?”

One of the central disappointments of the Cassini mission was that much of what it learnt about Enceladus suggested this strange moon was worthy of much more study – but its mission was imminently coming to a close.

That is not the case with Europa. Both Nasa and the European Space Agency are gearing up to send missions to Jupiter and the system that orbits around it.

The ESA hopes to send its Juice mission to Jupiter’s moons in 2022 – and it will look at the other moons that are thought to have large oceans hiding beneath their surfaces, Ganymede and Callisto, as well as Europa. Nasa’s Europa Clipper mission is being prepared for launch the same year.

“If plumes exist, and we can directly sample what’s coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life,” said Robert Pappalardo, Europa Clipper project scientist at NASA’s Jet Propulsion Laboratory.

“That’s what the mission is after. That’s the big picture.”

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The two spacecrafts will be very well prepared for finding the plume if they do: they will be carrying a whole host of instruments that will be perfect for coming to understand whether the strange world would have the ability to host alien life. They will also be carrying radars that can see down beneath the ice to help us learn more about where the plume is coming from – and also about the mysterious watery world that it is erupting up and out of.

“Habitable conditions require a certain balance of chemical elements: on Earth, life doesn’t want it to be too acidic, too alkaline, though there are a range of conditions in which life could potentially exist,” says Leigh Fletcher, senior research fellow in planetary science at the University of Leicester.

“The more favourable it is, the more likely it is they would be there.”

We’ve already had a go at some of this, on Enceladus, where we were able to detect the so-called building blocks of life. Such a discovery doesn’t prove there is aliens hiding in our own solar system, but it does suggest they could be.

The time around, on Europa, we’re going to get an even better look.

“The Cassini mission was able to characterise in exquisite detail what is happening on Enceladus,” says Fletcher. “I would say that has been our testing ground, with a spacecraft that was based on 1980s, 1990s technology.

“Now we know what we’re looking for in the Europa environment. It’s been a fantastically useful thing.”

He adds: “One wonders if we ever went out to Saturn and Neptune, with their icy moons, would we be able to see these plumes? These places are barely explored.