Did the Sun have a twin? New study rewrites the star's early history

Before astronomers first pointed their telescopes at the Sun in the early 17th century, the star had already existed for around 4.5 billion years. We've only ever seen a brief glimpse of our Sun's long and extensive lifecycle.

According to a new study, we missed a spectacular view: Long ago, the Sun varied in brightness and activity. It may have also had a twin.

The study, published this week in The Astrophysical Journal Letters, suggests a model whereby the Sun was part of a binary system. The authors theorize that the Sun's companion was kicked out by another star that got too close.

The model has implications on the mysterious Planet Nine, increasing its chances of having once been captured by the stellar duo.

The idea may seem wild to most, but Amir Siraj, the paper's lead author and a student at Harvard University, finds it rather unsurprising.

"A large portion of Sun-like stars are born with binary companions," Siraj tells Inverse. "And so in retrospect, it’s quite surprising that it was never seriously considered that the Sun had an early binary companion that was lost."

The link between the Sun and Planet 9

The idea of this study emerged while Siraji sought answers for the formation of the Oort Cloud and the mystery behind an alleged Planet Nine.

The Oort Cloud is a theoretical cloud of small icy objects that surround the Sun at a distance that reaches up until 3.2 light-years away. Meanwhile, Planet Nine is a hypothetical giant ninth planet that orbits the Sun in a highly elongated orbit that lies far beyond Pluto.

This potential planet, also called "Planet X," may have a mass that's about 10 times that of Earth and orbit about 20 times farther from the Sun than Neptune, the eighth and farthest known planet from the Sun. However, it has never been observed directly.

"I was interested in two independent problems, the first being the formation of the Oort Cloud," Siraj says.

While there's been extensive modeling done of this cloud, none of the models quite add up with the ratio of objects that exist in the Oort Cloud.

"Separately, there's Planet Nine which personally fascinates me," Siraj adds.

Astronomers from the California Institute for Technology (CALTECH) announced the discovery of Planet Nine in 2015 based on mathematical evidence. Although its existence is still theoretical till today, direct detection of a ninth planet would be the first discovery of a new planet orbiting the Solar System in two centuries.

However, astronomers have not been able to come up with a model to explain how Planet Nine could have emerged and travel so far from the Sun in the first place. The three leading theories are either:

• It was captured and pulled
• It formed in a distant location
• It formed amongst the giant planets of the solar system, then scattered

"What surprised me was that all of these formation scenarios had low likelihoods," Siraj says. "I had these two sorts of puzzles in the solar system that I had been thinking about for a while."

Through his research on binary star systems, Siraj knew that these systems were much more efficient at capturing objects than lone stars. Based on that, he created a model of the Sun as part of a binary system, along with a companion star.

"This greatly increases the probability that Planet Nine exists."

The stars would have been approximately 1,500 Astronomical Units apart and orbited around their shared center of mass until a passing star sort of split the binary duo and kicked out the companion, according to the study.

As a binary system, the two stars would be able to capture the number of objects in the Oort cloud, as well as a ninth, distant planet. On its own, a lone Sun would not be capable of the same.

The model increases the chances that Planet Nine was captured by a factor of 20, according to the researchers.

"This is significant because none of the origin scenarios for Planet Nine has a particularly high probability," Siraj says. "This greatly increases the probability that Planet Nine exists."

Siraj and his colleagues will be able to verify their model through observations of Planet Nine by the upcoming Rubin Observatory's Large Synoptic Survey Telescope, which is expected to launch in the year 2021.

"It’s sort of poetic that perhaps we were once part of a binary system ourselves, which might have shaped the Oort Cloud," Siraj says.

Abstract: We show that an equal-mass, temporary binary companion to the Sun in the solar birth cluster at a separation of ~10³ au would have increased the likelihood of forming the observed population of outer Oort Cloud objects and of capturing Planet Nine. In particular, the discovery of a captured origin for Planet Nine would favor our binary model by an order of magnitude relative to a lone stellar history. Our model predicts an overabundance of dwarf planets, discoverable by Legacy Survey of Space and Time, with similar orbits to Planet Nine, which would result from capture by the stellar binary.