Early Solar System May Have Had Two Suns

In a paper published in the Astrophysical Journal Letters, Harvard University astrophysicists Avi Loeb and Amir Siraj show that an equal-mass stellar companion to the Sun in the solar birth cluster — the collection of stars that formed together with our star from the same dense cloud of molecular gas — at a separation of 1,000 AU would have increased the likelihood of forming the observed population of objects in the outer Oort Cloud and of capturing the hypothesized Planet Nine.

An artist’s conception of a potential solar companion, which theorists believe was developed in the Sun’s birth cluster and later lost. If proven, the solar companion theory would provide additional credence to theories that the Oort Cloud formed as we see it today, and that Planet Nine was captured rather than formed in place. Image credit: M. Weiss.

Popular theory associates the formation of the Oort Cloud with debris left over from the formation of the Solar System and its neighbors, where objects were scattered by the planets to great distances and some were exchanged amongst stars.

But a binary model could be the missing piece in the puzzle and shouldn’t come as a surprise to scientists.

“Previous models have had difficulty producing the expected ratio between scattered disk objects and outer Oort Cloud objects,” said Siraj, an undergraduate student in the Department of Astronomy at Harvard University.

“The binary capture model offers significant improvement and refinement, which is seemingly obvious in retrospect: most Sun-like stars are born with binary companions.”

If the Oort Cloud was indeed captured with the help of an early stellar companion, the implications for our understanding of the Solar System’s formation would be significant.

“Binary systems are far more efficient at capturing objects than are single stars,” Professor Loeb said.

“If the Oort Cloud formed as observed, it would imply that the Sun did in fact have a companion of similar mass that was lost before the Sun left its birth cluster.”

More than just redefining the formation of our Solar System, evidence of a captured Oort Cloud could answer questions about the origins of life on Earth.

“Objects in the outer Oort Cloud may have played important roles in Earth’s history, such as possibly delivering water to Earth and causing the extinction of the dinosaurs. Understanding their origins is important,” Siraj said.

The model also has implications for the hypothesized Planet Nine, which the team believes isn’t alone out there.

“The puzzle is not only regarding the Oort clouds, but also extreme trans-Neptunian objects, like the potential Planet Nine,” Professor Loeb said.

“It is unclear where they came from, and our new model predicts that there should be more objects with a similar orbital orientation to Planet Nine.”

If the Sun did have an early companion that contributed to the formation of the outer Solar System, its current absence begs the question: where did it go?

“Passing stars in the birth cluster would have removed the companion from the Sun through their gravitational influence,” Professor Loeb said.

“Before the loss of the binary, however, the Solar System already would have captured its outer envelope of objects, namely the Oort Cloud and the Planet Nine population,” Siraj added.

“The Sun’s long-lost companion could now be anywhere in the Milky Way.”


Amir Siraj & Abraham Loeb. 2020. The Case for an Early Solar Binary Companion. ApJL 899, L24; doi: 10.3847/2041-8213/abac66