Our Sun may once have had a twin. What happened to this stellar sibling?

Many stars in our galaxy exist in pairs, but our sun is a notable exception. Now scientists are finding clues that it may once have had a companion of its own. The question is, where did it go?

Our sun is a bit of an isolated nomad. Orbiting one of the Milky Way's spiral arms, it takes us on a lonely journey around the galaxy once every 230 million years in our sky. The nearest star to our sun, Proxima Centauri, is 4.2 light-years away, so distant that it would take even the fastest spacecraft more than 7,000 years to reach it.

But wherever we look in our galaxy, the star at the center of our solar system increasingly seems to be an anomaly. Binary stars - stars that orbit the galaxy inseparably linked in pairs - seem common. Astronomers have recently discovered a pair orbiting the supermassive black hole at the center of the Milky Way galaxy — a place where astronomers thought stars would be torn apart or torn together by the intense force of gravity.

In fact, the discovery of binary star systems is now so common that some scientists believe that perhaps all stars were once in binary relationships — born as pairs, each star with a stellar sibling. This raises an intriguing question: Was our own sun also once a binary star, its companion long lost?

It’s certainly a possibility, says Hongjie Li, an astronomer at the Georgia Institute of Technology in the US. “And it’s very interesting.”

Luckily for us, our sun doesn’t have a companion today. If it were, the gravitational pull of a solar sibling could disrupt the orbits of Earth and other planets, trapping our habitat in extreme heat to frigid cold that would be too cold for life.

The closest binary stars to Earth, Alpha Centauri A and B, orbit each other at a distance of about 24 times the Earth-Sun distance, or 3.6 billion miles. The idea that our sun might also have a faint companion — often called Nemesis — has been elusive since it was first proposed in 1984. Multiple surveys and studies have found no such star.

But when our sun first formed 4.6 billion years ago, it may have been a different story.

Stars form when vast clouds of dust and gas across ten light-years cool and coalesce. The material inside these nebulae — as these cocoons of gas and dust are known — is compressed by gravity into growing masses. As it does so, it begins to heat up over millions of years, eventually igniting nuclear fusion to form a protestor around which a disk of remaining debris orbits, forming planets.

In 2017, Sarah Sadako, an astrophysicist at Queen's University in Canada, used data from a radio survey of the Perseus Molecular Cloud—a stellar nursery filled with young binary star systems—to conclude that the process of star formation could likely produce protos tars in pairs. In fact, she and her colleagues thought it was so likely that they suggested that all stars could form in pairs or multiple-star systems. “There are very few density spikes in these cocoons, and they are able to break apart to form multiple stars, which we call a fragmentation process,” Saad boy says. “If they are too far away [from each other], they may never interact. But if they are closer, gravity has a chance to keep them together.”

Saad boy’s work has shown that it is possible that all stars once started out as binaries, and while some are bound together indefinitely, others will quickly break apart within a million years. “They live for billions of years,” he says. “It’s a catastrophe in the grand scheme of things. But a lot happens in that catastrophe.”

That raises the question of whether the same thing happened to our sun. There’s no reason to think it didn’t, Saad boy says. But “if we formed with a companion, we’ve lost it,” he says.

The sea of ​​stars we see in the night sky may now be missing a companion - Sarah Saad boy

Some intriguing clues suggest that our sun was once part of a binary system. In 2020, Amir Siraj, an astrophysicist at Harvard University in the US, suggested that a region of icy comets surrounding our solar system far beyond Pluto, called the Oort Cloud, could contain the imprint of this companion star. This icy shell of ice and rock is so far away that the most distant spacecraft ever launched by mankind - Voyager 1 - won't be able to reach it for at least another 300 years. (Read more about what the Voyager missions are teaching us about the strange places on the outskirts of our solar system.)

If our sun had a companion, Siraj said, there would be more dwarf planets like Pluto in this region. This could also lead to a larger planet ending up here, like the Neptune-sized world Planet Nine that some astronomers believe is still undiscovered on the outer edge of our sun. (Read more about the mystery of Planet Nine in this article by Zaria Govern.)

“As far as we can see, at the far end of the Oort Cloud,” Siraj said, without a companion star, there are billions, even trillions, of objects orbiting in the Oort Cloud. If another planet like Planet Nine is found, it’s “really hard” to explain how a planet could end up so far from the sun, unless we use the disruptive gravitational hand of a companion star. “That could increase the ability to capture comets and the chances of capturing a planet in the solar system,” he said.

Konstantin Butyrin, a planetary scientist at the California Institute of Technology in the US who first proposed the existence of Planet Nine based on a cluster of distant objects in 2016, is not so sure about the idea. “You don’t need a binary companion to explain the Oort Cloud,” Butyrin says. “You can fully explain the existence of the Oort Cloud only by the fact that the Sun formed in a cluster of stars, and as Jupiter and Saturn grew to their current masses, they ejected a bunch of material.” Even Planet Nine could be explained simply by “passing stars in the birth cluster,” he says.