Hidden Planets: Implications from ’Oumuamua and DSHARP

Abstract

The discovery of 'Oumuamua (1I/2017 U1), the first interstellar interloper, suggests an abundance of free-floating small bodies whose ejection into galactic space cannot be explained by the current population of confirmed exoplanets. Shortly after 'Oumuamua's discovery, observational results from the DSHARP survey illustrated the near-ubiquity of ring/gap substructures within protoplanetary disks, strongly suggesting the existence of a vast population of as-yet undetected wide-separation planets that are capable of efficiently ejecting debris from their environments. These planets have $a \gtrsim 5$ au and masses of order Neptune's or larger, and they may accompany $\sim$50% of newly formed stars (Zhang et al. 2018). We combine the DSHARP results with statistical constraints from current time-domain surveys to quantify the population of detectable icy planetesimals ejected by disk-embedded giant planets through gravity assists. Assessment of the expected statistical distribution of interstellar objects is critical to accurately plan for and interpret future detections. We show that the number density of interstellar objects implied by 'Oumuamua is consistent with 'Oumuamua itself having originated as an icy planetesimal ejected from a DSHARP-type system via gravity assists, with the caveat that 'Oumuamua's lack of observed outgassing remains in strong tension with a cometary origin. Under this interpretation, 'Oumuamua's detection points towards a large number of long-period giant planets in extrasolar systems, supporting the hypothesis that the observed gaps in protoplanetary disks are carved by planets. In the case that 'Oumuamua is an ejected cometary planetesimal, we conclude that LSST should detect up to a few interstellar objects per year of 'Oumuamua's size or larger and over 100 yr$^{-1}$ for objects with $r > 1\,{\rm m}$.