Interstellar Objects in the Solar System: 1. Isotropic Kinematics from the Gaia Early Data Release 3

Abstract

1I/'Oumuamua (or 1I) and 2I/Borisov (or 2I), the first InterStellar Objects (ISOs) discovered passing through the solar system, have opened up entirely new areas of exobody research. Finding additional ISOs and planning missions to intercept or rendezvous with these bodies will greatly benefit from knowledge of their likely orbits and arrival rates. Here, we use the local velocity distribution of stars from the Gaia Early Data Release 3 Catalogue of Nearby Stars and a standard gravitational focusing model to predict the velocity dependent flux of ISOs entering the solar system. With an 1I-type ISO number density of ∼0.1 AU −3 , we predict that a total of ∼6.9 such objects per year should pass within 1 AU of the Sun. There will be a fairly large high-velocity tail to this flux, with half of the incoming ISOs predicted to have a velocity at infinity, v ∞ , > 40 km s −1 . Our model predicts that ∼92% of incoming ISOs will be residents of the galactic thin disk, ∼6% (∼4 per decade) will be from the thick disk, ∼1 per decade will be from the halo and at most ∼3 per century will be unbound objects, ejected from our galaxy or entering the Milky Way from another galaxy. The rate of ISOs with very low v ∞ 1.5 km s −1 is so low in our model that any incoming very low velocity ISOs are likely to be previously lost solar system objects. Finally, we estimate a cometary ISO number density of ∼7 × 10 −5 AU −3 for 2I type ISOs, leading to discovery rates for these objects possibly approaching once per decade with future telescopic surveys.

Identifying Interstellar Object Impact Craters

“…As discussed in the following subsection, population studies provide excellent constraints on the dispersion along each principal axis. However, the distribution for speed |v| is not well described by a Gaussian or Boltzmann distribution; a lognormal model provides a reasonable fit (Eubanks et al 2021).…”

“…This step is accomplished by first normalizing the sum over all bins to the ISO number density, n ISO ∼ 0.1 au −3 . This value is half the estimate of Do et al (2018) and used as an upper limit by Eubanks et al (2021), who appealed to the lack of recent detections. Each bin is multiplied by its speed,…”

“…We analyzed the dynamics of ISOs originating within the local stellar neighborhood using Gaia EDR3 data (Gaia Collaboration et al 2020), in a similar fashion as Marchetti (2020) and Eubanks et al (2021). The Sun's peculiar velocity was taken as (11.1, 12.24, 7.25) km s −1 (Schönrich et al 2010) relative to an LSR of (0, 235, 0) km s −1 .…”

“…Motivated by an encouragingly high encounter rate of ISOs, up to ∼seven per year, that pass within 1 au of the Sun (Eubanks et al 2021), we consider an alternative route to characterizing these enigmatic objects: identifying ISO impact craters on terrestrial solar system bodies. For example, molten and vaporized projectile matter may mix with impact-modified target rock (impactites) and impart telltale chemical signatures.…”

Constraints on the Occurrence of 'Oumuamua-Like Objects

“…We do not consider distributions in composition, size, or inbound kinematics, and instead make the simplifying assumption that the reservoir of 'Oumuamua-like ISOs consists of identical objects. Pan-STARRS has operated for a few additional years at higher efficiency without discovering another 'Oumuamua-like ISO since the publication of Do et al (2018), so we set our fiducial value to n iso ∼ 0.1 AU −3 (Eubanks et al 2021). Although the statistics of one object are understandably uncertain, we determine sensible confidence intervals in Table 1 via the classic method of Garwood (1936).…”