Wednesday, July 8, 2015

Capturing Planetesimals from a Passing Star

A population of far-flung objects orbit the Sun in the outer regions of the Solar System, between the Kuiper belt and the more distant Oort cloud. These objects can be referred to as Sednoids, after 90377 Sedna - the first of its kind to be discovered in 2003. Roughly a dozen or so Sednoids have been detected so far. These objects orbit the Sun at distances of between 150 to 1,500 AU and come no closer than 30 AU to the Sun. They are too far to be Kuiper belt objects, but not far enough to be Oort cloud objects. At present, there is no explanation for how these objects came to be.

Figure 1: Artist’s impression of a planetesimal orbiting far from its host star.

Sednoids are also intriguing because their orbital inclination with respect to the ecliptic range between 10° to 30°, and their argument of perihelion cluster around 340° ± 55°. The terms “orbital inclination” and “argument of perihelion” are parameters used to describe an object’s orbit around the Sun. Basically, the ecliptic is defined as a plane of reference that is coplanar with Earth’s orbit around the Sun, while the argument of perihelion is the angle between an object’s ascending node (i.e. point where the object’s orbit around the Sun crosses the ecliptic from south to north) and its perihelion (i.e. point along the object’s orbit where it is closest to the Sun) in the direction of the object’s orbital motion.

The clustered distribution of the argument of perihelion of the Sednoids is odd because the effect of precession should have scattered the argument of perihelion within a span of several million years. As a consequence, either the clustering happened recently (i.e. within several million years), which is unlikely, or the presence of at least one massive object in the outer Solar System is keeping the Sednoids in the currently observed clustered distribution of their argument of perihelion. In fact, one or more massive objects with several times the mass of Earth could be orbiting the Sun between 200 and 300 AU.

Figure 2: Orbital distributions of planetesimals for the Sun (left) and for the encountering star (right). The top panels give inclination as a function of semi-major axis; the bottom panels give the orbital eccentricity. The red bullets give the orbital distributions of the planetesimals native to the Sun (assuming its disk extended to 90 AU), the light blue bullets are native to the passing star. Both initial planetesimal disks are strongly perturbed beyond about 30 AU, but within this distance they are hardly affected. Jilkova et al. (2015).

A study by Jilkova et al. (2015) propose that the Sednoids are actually a captured population of planetoids from another star that came close to the young Sun when the Sun will still in its crowded birth cluster. Using the orbital parameters of the Sednoids to reconstruct the trajectory of the passing star, it was found that the star had roughly 1.8 times the Sun’s mass and came as close as ~340 AU to the Sun with a relative velocity of roughly 4.3 km/s. During the encounter event, the Sun acquired planetesimals from the passing star and the passing star also acquired planetesimals from the Sun. The region of space where the Sednoids orbit the Sun is estimated to contain ~930 planetesimals.

Since more massive stars have shorter lives, the star that once passed close to the Sun should have already evolved into a carbon-oxygen white dwarf with ~0.6 times the Sun’s mass a few billion years ago. As the star evolved into a white dwarf, it lost a significant fraction of its mass which weakened its gravitational grip. As a result, the planetesimals it acquired from the Sun were expelled into interstellar space as free-floating planetesimals.

Jilkova et al. (2015), “How Sedna and family were captured in a close encounter with a solar sibling”, arXiv:1506.03105 [astro-ph.EP]