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 in one of the Milky Way's spiral arms, it takes us on a a journey around the galaxy roughly once every 230 million years on our lonesome. The nearest star to our Sun, Proxima Centauri, is 4.2 light-years away, so remote that it would take even the fastest spacecraft ever built more than 7,000 years to reach, reports BBC.
Everywhere we look in our galaxy however, the star at the centre of our Solar System seems like more and more of an anomaly. Binary stars - stars that orbit the galaxy inexorably linked together as pairs - appear to be common. Recently astronomers have even spotted a pair orbiting in surprisingly close proximity to the supermassive black hole that sits at the heart of the Milky Way - a location that astrophysicists thought would cause the stars to be ripped apart from each other or squashed together by the intense gravity.
In fact, discoveries of binary star systems are now so common that some scientists believe that perhaps all stars were once in binary relationships - born as pairs, each with a stellar sibling. That has led to an intriguing question: was our own Sun once a binary star too, its companion lost long ago?
It's definitely a possibility, says Gongjie Li, an astronomer at the Georgia Institute of Technology in the US. "And it's very interesting."
Fortunately for us, our Sun does not have a companion today. If it did, the gravitational pull of a solar sibling could have disrupted the orbit of the Earth and the other planets, condemning our home to lurches from extreme heat to terrible cold in a way that may have been too inhospitable for life.
The closest binary stars to Earth, Alpha Centauri A and B, orbit each other at about 24 times the Earth-Sun distance, or 3.6 billion miles. Suggestions that our Sun could also have a faint companion circling our Solar System today - a hypothetical star often called Nemesis - have fallen out of favour since they were first proposed in 1984 after no such star was found in multiple surveys and studies.
But when our Sun first formed 4.6 billion years ago, however, it may have been a different matter.
Stars form when giant clouds of dust and gas tens of light-years across cool and clump together. The material inside these nebulae - as these cocoons of gas and dust are known - collapse together under gravity into ever growing lumps. As it does so, it begins to warm up over millions of years, eventually igniting nuclear fusion to create a protostar with a disk of remnant debris spinning around it, which forms planets.
In 2017, Sarah Sadavoy, 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 might preferentially form protostars in pairs. Indeed, she and her colleagues found it was so likely that they suggested all stars might form in pairs or multi-star systems.
"You get little density spikes within those cocoons, and those are able to collapse and form multiple stars, which we call a fragmentation process," says Sadavoy. "If they're very far away [from each other], they might never interact. But if they're much closer, gravity has a chance to keep them bound together."
