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It appears our solar system is getting more popular with out-of-towners. 

Astronomers may have found a third interstellar object, something that has origins beyond our own solar system.

The first interstellar object was ‘Oumuamua, discovered in 2017. The second was a comet called 2I Borisov.

This new object, named 3I/ATLAS, or C/2025 N1 (ATLAS) by the International Astronomical Union’s Minor Planet Center, was discovered using a survey telescope called the Asteroid Terrestrial-impact Last Alert System (ATLAS), which serves as an asteroid impact early warning system.

3I/ATLAS is not believed to pose any danger to Earth. 

“We now have observations from over a week or so that indicate that its orbit is pretty clearly interstellar,” said Paul Weigert, a professor at Western University’s department of physics and astronomy in London, Ont. “It’s travelling too fast to be bound to the sun, and so it has presumably come to us from outside our solar system.”

When first discovered, ‘Oumuamua was believed to be an oblong asteroid, but followup observations confirmed that it was a comet, just as 2I Borisov was later. 

Astronomers with new observations have seen some signs of a tail on this new object, meaning it is also likely a comet.

The reason that astronomers believe3I/ATLAS comes from beyond our solar system is due to something called its eccentricity. 

Periodic comets, which orbit the sun repeatedly, tend to have eccentricities between 0.2 and 0.7. Those with numbers greater than one are considered hyperbolic comets and likely originate from the Oort Cloud which surrounds our solar system and contains billions of icy objects. It could also indicate that an object is from beyond our solar system.

Objects with high eccentricities indicate that they come from beyond the solar system. In this case, it’s currently estimated that 3I/ATLAS has an eccentricity of six. As astronomers gather more data over time, this number is likely to change.

So what do we know?

“Right now, it’s beyond the orbit of Mars, so it’s fairly far away,” Weigert said. “It’s almost at Jupiter’s orbit, but it is coming inwards. It won’t get much closer than Mars’ orbit … It’ll be at that closest point in October, so a few months from now, and then it will leave and start heading out of the solar system.”

Good news for Earth.

As for its size, more will be known over time, but right now the indication is that it’s a big one.

“It’s probably about 10 kilometres across,” Weigert said, which would make it the biggest of the three interstellar visitors observed so far. 

So are we sure it’s not an Unidentified Flying Object (UFO) or an Unidentified Anomalous Phenomena (UAP), a term used by scientists to describe things observed in the sky that remain unexplained?

“Well, certain as we can be,” Weigert said. “It has not demonstrated any unusual behaviour. It’s just travelling through the solar system in exactly the way we would expect for an interstellar object … There’s no indication that it’s in any way unusual in that sense.”

Bummer for UAP enthusiasts.

Whatever it is, you can be certain that all the major telescopes now have their sights on 3I/ATLAS. It’s also an exciting time as the new Vera C. Rubin Observatory in Chile has just become operational. Weigert says it’s expected to discover one to 10 of these objects every year.

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An artist’s illustration of the interstellar object ‘Oumuamua, which scientists now suspect is a comet, not an asteroid. (ESA/Hubble, NASA, ESO, M. Kornmesser)

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