For the entirety of human history, we’ve looked up and seen just a single sun, but a new study claims we almost had a second. This scenario, put forward by astronomers at the Center for Astrophysics at Harvard & Smithsonian, features our solar system forming as a binary with another star that eventually drifted away. This might explain some of the more perplexing things about the solar system like the nature of the Oort Cloud and even the murky Planet Nine.
Astronomers know multi-star solar systems are common–the Centauri system exists right next door, and it consists of three stars. The new paper, authored by Avi Loeb and Amir Siraj, suggests that a now-missing binary companion for our sun could shed light on the confounding happenings in the outer solar system. Specifically, it could explain why we have such a bulky Oort Cloud.
The Oort cloud is at the very edge of the solar system–it’s out past Neptune, past Pluto, and even beyond the most distant Kuiper Belt Objects (KBOs). Unlike the Kuiper Belt, which is donut-shaped, the Oort Cloud is a spherical area of space filled with icy chunks of primordial material, like comets that never approach the inner solar system. Thus far, no model for solar system formation has been able to explain the volume of the Oort Cloud. If you add a second sun, however, things make more sense.
Alpha Centauri AB is on the left, Beta Centauri on the right, and Proxima Centauri is at the center of the red circle. Image by Wikipedia.
The study posits that our sun remained gravitationally locked to another star after both formed from the same molecular cloud, sometimes called a stellar nursery. If the solar system had a second star with about the same mass as the sun, it could have helped to bulk up the Oort Cloud. Astronomers believe it’s common for stars to spread out from a cloud after formation, often due to gravitational interactions with other stars in the “birth cluster.” So, this extra star helped rope in Oort Cloud material, and then floated away billions of years ago, becoming just another sun-sized star in our little corner of the galaxy like Epsilon Indi or Tau Ceti.
A missing sun could also explain the Planet Nine conundrum. Astronomers have identified unusual perturbations in the orbits of some KBOs, so there’s something out there impacting their orbits. If it is indeed a planet, the leading explanation is that it formed closer to the sun and migrated outward. However, Loeb and Siraj say that a star passing close enough to pull away the sun’s twin could also have left something behind: a planet. Planet Nine might be an alien planet that we captured as its original star glided past. This is, of course, all hypothetical, but it would explain some things.
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