Scientists Theorize That the Sun Once Had a Companion Star

Harvard University scientists have published a new theory suggesting the Sun has a long-lost companion star. The theory, based on a model developed by the scientists, speculates that the formation of the Oort cloud is better explained if the Sun once had a companion. The scientists say that this theory also helps to shed light on the mysterious origins of the hypothetical Planet Nine.

Scientists at Harvard theorize that the Sun once had a stellar binary companion.

M. Weiss

The Harvard scientists responsible for the theory, Dr. Avi Loeb, a Professor of Science, and Amir Siraj, an undergraduate student, recently published their theory in The Astrophysical Journal Letters. Siraj also explained the theory in a tweet thread, generally outlining how he and Loeb came to their conclusions.

With this paper, Siraj says that he and Loeb “make the case for an early solar binary companion,” i.e. a second Sun that was present in the solar birth cluster. Solar birth clusters are groups of stars that have formed from the same cosmic clouds of molecular gas.

The scientists postulate the Sun once had a companion because that theoretical model may explain the observed formation of the Oort cloud. The Oort cloud is a theoretical cloud of mostly icy planetesimals that orbit the Sun at distances of .03 to 3.2 light-years away.

According to the theory, a star with a similar mass to the Sun’s once orbited around our star at a distance of roughly 1,000 astronomical units. (One AU is approximately 93 million miles.) If this were indeed the case, the scientists say this would have increased the likelihood of forming Oort cloud objects as we observe them today.

Scientists at Harvard theorize that the Sun once had a stellar binary companion.

NASA / JPL-Caltech

“The Sun and its companion [would’ve acted] as a fishing net that traps objects gravitationally as they pass near one of the two stars…” Loeb said in a Gizmodo report. He added that the binary model better accounts for our solar system’s population of scattered objects along our planet’s disks. As well as the relatively spherical Oort cloud.

Loeb and Siraj also say that a companion star would help explain the Sun’s capturing of Planet Nine; a hypothetical super-Earth-sized planet in the outer region of the Solar System that could explain unusual clustering of orbits for a group of extreme trans-Neptunian objects.

Scientists at Harvard theorize that the Sun once had a stellar binary companion.


Although Loeb and Siraj only currently have theoretical claims, those claims can be confirmed by observation in the future. The Vera C. Rubin Observatory, which comes online next year, will be able to confirm or deny the existence of Planet Nine. That confirmation would make the binary model favorable to the “the lone stellar history” popularly attributed to our solar system.

Featured image: M. Weiss

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