In the early days of the Solar System, the Sun may have had a companion star, scientists say, making it part of a binary system like many others in the Milky Way galaxy.

But this doesn’t mean the view from the infant Earth would have looked like that from Tatooine, the fictional planet that was home to Luke Skywalker in Star Wars.

This second sun would have been 150 billion kilometres away – so distant it would have merely been a bright pinpoint, casting less light than the full Moon.

Clues to its possible existence lie in two peculiarities of the outer Solar System, says Amir Siraj, from Harvard University, US, first author of a paper in Astrophysical Journal Letters.

One of these is the existence of the Oort Cloud, the farthest part of the Solar System, believed to harbour 100 billion or so icy objects larger than a kilometre in radius.

We know the Oort Cloud is out there because periodically these objects are perturbed from their orbits and fall into the inner Solar System as comets. But how it got there is harder to explain.

One possibility, Siraj says, is that these objects were flung out from the inner Solar System by interactions with the giant planets. Another is that they are cast-offs from other planetary systems, captured by our own Sun’s gravity.

But why there are so many has been difficult to explain. “The capture models have suffered because of the fact it is very difficult to capture enough objects,” Siraj says.

Similar problems apply to models showing them being flung out from the inner system, he says. It’s not that these models are impossible, it’s just that they aren’t super likely to give the right result.

The other puzzle is the mysterious Planet Nine, a Neptune-sized object that many astronomers believe to be lurking out there (though not as far out as the Oort Cloud), based on its apparent gravitational effects on a number of other far outer Solar System objects.

If it indeed exists, Siraj says, it too is difficult to explain. Did it form at that distance? If so, how? Did it form closer in and get thrown out there by the other giant planets? If so, how? Is it a rogue world captured from interstellar space? Same question.

There’ve been many scientific papers looking at these questions, but all of them found each scenario to be “quite unlikely,” Siraj says. “That is very intriguing, because if Planet Nine is verified [to exist], it will be a question we have to reckon with.”

If in its youth the Sun had a companion star, however, the problem is much simpler, because computer simulations show that binary star systems are much more efficient at capturing both rogue worlds like Planet Nine, and smaller objects like those in the Oort Cloud.

Furthermore, Siraj says, we know that a lot of Sun-like stars, “perhaps the majority” are born with binary companions.

As for where the missing companion star went, the simple answer is that’s it’s been lost to interstellar space.

Most likely Siraj says, this happened in the Solar System’s first 100 million years, when it would still have been in its birth cluster – the densely packed star-forming region in which it and other stars condensed from a dense cloud of interstellar gas.  Gravitational interactions with a nearby star could then easily have yanked its companion away and sent it off to wander, alone, through interplanetary space.

“This is a nice idea,” says Mike Brown, an astronomer and planetary scientist at California Institute of Technology, who is one of the leaders in the search for Planet Nine (and not part of the study team).

“The general idea that the Sun might have begun as a binary is not controversial,” he says, “so it reminds us that some very reasonable things (like a binary star) can lead to favourable capture scenarios.” Whether it’s more or less likely than other scenarios for the origin of Plane Nine, he says, is harder to say, “but it’s definitely a reasonable one”.

Siraj suggests that the best hope for verification for his hypothesis will come after the Vera C Rubin Observatory, now being built on a mountaintop in Chile, goes into operation in early 2021.

Not only might that telescope be able to find Planet Nine, but even if it doesn’t, it might find an unexpected number of smaller, dwarf planets at about the same distance from the Sun. If it does, he says, it would be “the real smoking gun” needed to confirm the existence of a now-lost binary companion to the Sun.

And as for the search for Planet Nine?

“We keep plugging away,” says Brown. “We’ve been spending a lot of time during the lockdown examining old data sets in the hope we might be able to find accidental sightings from the past.”