Harvard University Professor Avi Loeb has challenged theories about the early formation of the Solar System, proposing it was originally a two-star system. Even though the Sun's companion is long gone, Loeb thinks its legacy can be read in the comets of the Oort cloud. Loeb and undergraduate student Amir Siraj set out their case in the Astrophysical Journal Letters. However, Loeb may find it harder to convince colleagues of his ideas after the reception of some of his other recent theories.
Most stars exist in pairs. Although the presence of a second star was once expected to disrupt the formation or orbit of planets, we now know many binary systems contain planets. Some are so widely spaced anything orbiting one star experiences little influence from the other, while so-called Tatooine systems have the planet in orbit around both members of a close pair.
Loeb and Siraj claim the former presence of a star too distant to affect Earth or another known planets would explain the relative numbers we see of two categories of comets; those from the scattered disk and visitors from the more distant Oort Cloud.
"Previous models have had difficulty producing the expected ratio between scattered disk objects and outer Oort cloud objects,” Siraj said in a statement.
Siraj proposes the long-lost companion was no puny red dwarf, but of approximately similar mass to the Sun, making the pair like a more widely spaced version of our near neighbor Alpha Centauri, whose components differ in mass by 20 percent.
The gravity of such a large object would have helped capture some of the comets that make up the Oort cloud, Loeb and Siraj argue, rather than them forming from the same cloud of gas as the Sun and planets. “Binary systems are far more efficient at capturing objects than single stars,” Loeb said.
When the two stars parted, the Sun got custody not only of many of these comets, the pair claim, but also the proposed Planet Nine, which they think is still out there, shaping cometary orbits. “Our new model predicts there should be more objects with a similar orbital orientation to Planet Nine,” Loeb said. These are expected to be dwarf planets, more distant versions of Pluto.
Today the Sun is so socially distanced it experiences little external disruption. However, like most stars, it is thought the Sun was born in a tightly packed cluster that gradually drifted apart. In such an environment there would have been many early gravitational encounters with nearby objects, including some stars large enough to break the bonds between the Sun and a distant companion. If our fellow traveler was 1,500 times as far from the Sun as Earth is, the chance of such a separation was around 50 percent, the paper claims, and higher still at greater distances.
If the authors are right, the other star, being of similar size and life-expectancy to the Sun, would still be roaming the galaxy, accompanied by some planets and the comets it got to keep. Were it still here, it would probably appear a little brighter than the full Moon to us.
The eventual response to Loeb and Siraj's work will stand or fall on whether other scientists can replicate their modeling. However, it may meet with a more skeptical initial response than if they had released the same idea several years ago. Loeb has had a distinguished career, publishing almost 700 papers and pioneering several important subfields of astrophysics. However, he frustrated his colleagues by proclaiming interstellar object 'Oumuamua to be a likely alien spaceship, and holding to that claim after many other scientists had discredited it.