'More chance the Moon is made from cheese, than comet is aliens'

Over the past few months, people online have been buzzing with theories that a mysterious object in space - comet 3I/ATLAS - might actually be an alien spacecraft.

Memes, videos and wild guesses have spread fast. The vast majority of experts reject any suggestion that the comet is anything to do with extraterrestrial life, but that doesn't mean scientists aren't excited.

BBC Newsround spoke to Professor Chris Lintott, an astrophysicist from the University of Oxford.

"What's really special about this comet is that it comes from beyond our solar system so it's only the third time we've detected an interstellar wonder like this," says Professor Lintott.

"This particular comet, based on the way that it's moving through the galaxy, we think it's more than 8 billion years old - that makes it older than the solar system and it makes it the oldest thing that we've ever seen."

The comet will come closest to Earth on 19 December, passing within 170 million miles (270 million km) from our planet. However, it requires a fairly strong telescope to be able to spot it.

"It will have travelled for billions of billions of billions of miles to get here, we've just [had] this one window of these few months to look at it before it disappears back into space, and we will never see it again," adds Prof Lintott.

Astronomers can get a lot of information from tracking the way something moves through our Milky Way galaxy.

Professor Lintott says: "So our galaxy is shaped like a pancake, normally stars like the Sun and objects in the galaxy orbit within the pancake and overtime they start to bounce up and down and so what we see with the comet is that it's going in and out of the pancake at quite a speed.

"So, that gives us a clue that it came from from this old part of the galaxy and therefore can tell us about what was happening in the Milky Way way before the solar system was formed."

Despite the rumours, almost every astronomer agrees that 3I/ATLAS behaves exactly like a normal comet.

When we asked Professor Lintott whether it could be an alien craft, he said:

"No, you might as well say the Moon is made of cheese"

Why do people think it might be one?

"It's difficult to say really. This is something that came from beyond the solar system and if we ever find an alien spaceship it will be moving on this sort of trajectory but it looks like a comet, it's behaving like a comet, and therefore that's what it is."

Nasa has detected water ice, carbon dioxide, and even nickel - a metal found in some spacecraft, which sparked even more alien theories.

Professor Lintott explains:

"Every comet is different that's why they're fun to study. There have been big arguments about how much water it has because that would tell us about what was around when it was formed and we've also seen the metal nickel which is not unheard of but is unusual for comets.

3I/ATLAS is what's known as an 'active comet', which means it heats up as it gets close to the Sun.

This is exciting news for comet observers, as it can lead to frozen gases turning into vapour to form a glowing tail.

"The really exciting thing is that it's been nice and bright particularly after it's gone round behind the Sun it's got this lovely tail and it's producing lots of gas."

Professor Lintott says a powerful new telescope called the Rubin Observatory in Chile will spot more of these objects as they enter our solar system.

"We've got this new telescope the Rubin observatory which has got the largest camera ever built and it's gonna scan the whole sky every three nights starting it early next year.

"We think Rubin will find about 100 of these things over the next 10 years - Then we can really get it to these questions about what's happened in the galaxy in the past."