Astronomers say early Solar System may have been 'binary' with two suns

A new study by astronomers out of Harvard University proposes that the Solar System originally had two stars instead of one – a theorem that could also help to shed light on the origins a possible ninth planet that scientists suspect may lie beyond the reaches of Neptune. 

Harvard's Amir Siraj and Abraham Loeb – a graduate student in, and a professor of, astronomy, respectively – suggest in their recently published study in the Astrophysical Journal Lettters that "an equal-mass, temporary binary companion to the Sun" existed at the beginning of the Solar System rough 4 billion years ago. 

A dual-star system, the researchers argue, would help explain the origins of "Planet Nine," a hypothetical planet that astronomers are increasingly confident exists in a massive orbit far from the sun but that has never been directly observed. 

Many scientists thinks Planet Nine may have been "captured" by the gravity generated by the sun. "[T]he discovery of a captured origin for Planet Nine would favor our binary model by an order of magnitude relative to a lone stellar history," the Harvard scientists write. 

They argue that the Sun's twin may have eventually been pulled out of the Solar System by the gravity of a passing star, leaving the remaining planets with just the Sun.

Binary systems are regularly observed throughout the galaxy, with one of the Solar System's nearest neighbors, the Sirius system, consisting of two stars. The concept of a two-star system was made particularly famous by the original 1977 "Star Wars," which depicted the desert world Tatooine with two suns.