The Case for an Early Solar Binary Companion

We show that an equal-mass, temporary binary companion to the Sun in the solar birth cluster at a separation of $\sim {10}^3\mathrm{A}\mathrm{U}$ 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 LSST, with similar orbits to Planet Nine, which would result from capture by the stellar binary.