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 ∼10³ au 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 Legacy Survey of Space and Time, with similar orbits to Planet Nine, which would result from capture by the stellar binary.