Our Sun likely had a twin when it came into existence 4.5 billion years ago, say scientists who found that every star in the universe is born with a companion.
Many stars have companions, including our nearest neighbour, Alpha Centauri, a triplet system. Astronomers have long sought an explanation.
Astronomers have even searched for a companion to our Sun, a star dubbed Nemesis because it was supposed to have kicked an asteroid into Earth's orbit that collided with our planet and exterminated the dinosaurs. It has never been found.
They also created a mathematical model that can explain the Perseus observations only if all sunlike stars are born with a companion.
"We are saying, yes, there probably was a Nemesis, a long time ago," said co-author Steven Stahler, researchers at UC Berkeley.
"We ran a series of statistical models to see if we could account for the relative populations of young single stars and binaries of all separations in the Perseus molecular cloud, and the only model that could reproduce the data was one in which all stars form initially as wide binaries," researchers said.
These systems then either shrink or break apart within a million years, Stahler said.
In this study, 'wide' means that the two stars are separated by more than 500 astronomical units (AU) - where one AU is the average distance between the sun and Earth (about 150 million kilometres).
A wide binary companion to our Sun would have been 17 times farther from the Sun than its most distant planet today, Neptune, they said.
Based on this model, the Sun's sibling most likely escaped and mixed with all the other stars in our region of the Milky Way galaxy, never to be seen again, they said.
"Based on our simple model, we say that nearly all stars form with a companion," said Sarah Sadavoy, from the Smithsonian Astrophysical Observatory in the US.
Stars are born inside egg-shaped cocoons called dense cores, which are sprinkled throughout immense clouds of cold, molecular hydrogen that are the nurseries for young stars.
Researchers mathematically modelled various scenarios to explain this distribution of stars, assuming typical formation, breakup and orbital shrinking times.
They concluded that the only way to explain the observations is to assume that all stars of masses around that of the Sun start off as wide Class 0 binaries in egg-shaped dense cores, after which some 60 per cent split up over time. The rest shrink to form tight binaries.