How Sedna and family were captured in a close encounter with a solar sibling

Abstract

The discovery of 2012 VP113 initiated the debate on the origin of the Sedna family of planetesimals in orbit around the Sun. Sednitos roam the outer regions of the Solar system between the Egeworth–Kuiper belt and the Oort Cloud, in extraordinary wide (a > 150 au) orbits with a large perihelion distance of q > 30 au compared to the Earth's (a ≡ 1 au and eccentricity e ≡ (1 − q/a) ≃ 0.0167 or q ≃ 1 au). This population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i = 10°–30°. They also have similar argument of perihelion ω = 340° ± 55°. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disc of another star. Assuming that the orbital elements of Sednitos have not changed since they acquired their orbits, we reconstruct the encounter that led to their capture. We conclude that they might have been captured in a near miss with a 1.8 M⊙ star that impacted the Sun at ≃ 340 au at an inclination with respect to the ecliptic of 17°–34° with a relative velocity at infinity of ∼4.3 km s−1. We predict that the Sednitos region is populated by 930 planetesimals and the inner Oort Cloud acquired ∼440 planetesimals through the same encounter.