Looking for extra-Solar objects in the Solar System.

Whilst astronomers have known for a long while that our Solar System was likely to contain objects of extra-Solar origin (i.e. from outside the system), but since the passage of interstellar object `Oumuamua (or 1I/2017 U1, or A/2017 U1) through the Inner Solar System in 2017, interest in the subject has grown. This has led to a search for other extra-Solar objects within the Solar System, with one more object, asteroid (514107) 2015 BZ509, having been suggested as to be a possible candidate object, on the basis of an orbit unlikely to have originated within the Solar System.

In a paper published on the  arXiv database at Cornell University Library on 23 November 2018, and submitted for publication in the Monthly Nottices of the Royal Astronomical Society, Amir Siraj and Abraham Loeb of the Department of Astronomy, Harvard University, describe a method for looking for new extra-Solar objects within our Solar System, and suggest four more candidate objects.

Siraj and Loeb estimate that about twelve extra-Solar objects enter the Solar System each century, and that there are about 6000 such objects in the Solar System at any given time. In order to determine where to look for such objects they used a computer model of the Solar System to plot the course of simulated extra-Solar objects through the system, finding that 2.571% of such objects were captured due to gravitational interactions with the Sun and Jupiter.

These objects became captured in orbits with average distances from the Sun of between 0.49 AU (i.e. 49% of the distance of the distance at which the Earth orbits the Sun) and 2000 AU, with 25%  of objects orbiting at an average distance of less than 6.9 AU, 50% at less than 13 AU, and 75% at less than 28 AU. Furthermore, the objects had aphelion distances (furthest points from the Sun on their orbits) of up to 4000 AU, with an average of 21 AU from the Sun, with 25% of objects having aphelions of less than 9.6 AU and 75% at distances of less than 50 AU. The objects had an average perihelion (closest point on their orbit to the Sun) of 4.0 AU, with 25% having perihelions of less than 2.6 AU and 75% having a perihelion of less than 4.9 AU.

The objects had an avarage orbital eccentricity of 0.74 (where 0.00 is a circular orbit and 1.00 is a straight line), with 50% of objects having an eccentricity of between 0.54 and 0.89. The objects had an average inclination to the plain of the Solar System of 33 °, and 50% of objects orbiting at inclinations of between 24 ° and 47 °, and a maximum inclination of 160°. Finally these objects had an average orbital period of 42 years, with  25% of objects having orbital periods of less than 17 years and 75% of objects having orbital periods of less than 140 years.

In order to look for candidate extra-Solar objects, Siraj and Loeb concentrated on the Centaurs, a population of Solar System bodies on paths that cross the orbits of the major planets of the Outer Solar System (a body is defined as a Centaur if its orbit brings it no closer to the Sun than the planet Jupiter, but its average distance from the Sun is less than that of Neptune). They searched previously published records for Centaurs with average distances from the Sun of between 6.9 AU and 28 AU, and eccentricities of between 0.54 and 0.89, and concentrated on objects with inclinations to the Solar plain of greater than 77° (the majority of extra-Solar objects are predicted to have inclinations lower than this, however the majority of non-extra-Solar objects also have lower inclinations, and objects with such high inclinations are thought to be more likely to have an extra-Solar origin, since the the majority of Centaurs are thought to have a common origin, having been pulled inwards from the Scattered Disk by the gravitational pull of the planet Neptune, and it is very difficult for such objects to reach highly inclined orbital paths).

They found four objects that matched these criteria, 2018 TL6, which has an orbit that takes it from 1.72 to 14.8 AU from the Sun, with an average of 8.26, an inclination of 170.9° (or an inclination of 9.1° and a retrograde orbit, an orbit in the opposite direction to almost everything else in the Solar System), and an eccentricity of 0.79, 2017 SV13, which has an orbit that takes it from 2.01 to 17.3 AU from the Sun, with an average of 9.65, an inclination of 113.2°, and an eccentricity of 0.79, 2011 SP75, which has an orbit that takes it from 2.27 to 36.8 AU from the Sun, with an average of 19.54, an inclination of 109.1°, and an eccentricity of 0.88, and 2017 RR2, which has an orbit that takes it from 2.63 to 44.7 AU from the Sun, with an average of 23.65, an inclination of 89.1°, and an eccentricity of 0.89.

The orbit and current position of asteroid 2018 TL6, a Centaur and potential extra-Solar object. JPL Small Body Database Browser.

Siraj and Loeb further note that the previously nominated candidate extra-Solar object, (514107) 2015 BZ509, falls within the possible candidate extra-Solar object range established by their model, but does not fall within the most likely part of the distribution range, having an average distance from the Sun and an average eccentricity lower than 75% of the simulated objects (though they do note that these parameters would do make it much easier to spot, and therefore what might be expected of one of the first such objects detected).

The orbit and current position of asteroid 2017 RR2, a Centaur and potential extra-Solar object. JPL Small Body Database.

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Looking for the origin of Asteroid (514107) 2015 BZ509.

The Centaurs are a population of asteroids with orbits that cross those of the Solar System’s giant planets (Jupiter, Saturn, Uranus, and Neptune). The majority of these are on somewhat chaotic pathways; they are presumed to have originated in the outer reaches of the Solar System and been knocked inwards by interaction with some other body and are now buffeted from orbit to orbit by close encounters with the giant planets. Such asteroids typically survive as Centaurs for less than a million years, and almost never survive for more than about 10 million years. Other Centaurs manage to achieve resonant orbits with one or more giant planets, finding a pathway on which they are held steady by periodic encounters with the resonant planet or planets. Such asteroids may remain in stable orbits for hundreds of millions of years, and may be in prograde or retrograde orbits (i.e. may orbit in the same direction as the planets, or in the opposite), with those in steeply inclined retrograde orbits often the most stable.

Asteroid (514107) 2015 BZ509 was discovered in January 2015 by the University of Hawaii's PANSTARRS telescope (technocally it was first captured in an image in November 2014, but not actually noticed until January 2015); the name 2015 implies that it was the 12 750th asteroid (asteroid Z509) discovered in the second half of January 2015 (period 2015 B), while the designation (514107) implies that it was the 514 107th asteroid ever discovered. (514107) 2015 BZ509 is a Jupiter-orbit crossing Centaur Asteroid with an orbit similar to those of the Jupiter Trojans (asteroids on the same orbital path as Jupiter, but within its Trojan Points, 60° ahead and behind the planet), but on a retrograde orbit inclined at 163° to the plane of the Solar System (or 17° from the plane of the Solar System, but travelling in the wrong direction).It is the first asteroid ever discovered in a 1:1 resonance with a planet, completing one orbit for every one orbit of Jupiter, and has a diameter of about 3 km.

The calculated orbit of (514107) 2015 BZ509 Minor Planet Center.

In a paper published in the journal Monthly Notices of the Royal Astronomical Society on 26 May, Fathi Namouni of the Observatoire de la Côte d’Azur at the Université Côte d’Azur and Maria Helena Morais of the Instituto de Geociências e Ciências Exatas at the Universidade Estadual Paulista, present the results of a study of (514107) 2015 BZ509, in which they attempted to use a computer model to backtrack the orbit of the asteroid, and determine its origin.

To Namouni and Morais’s surprise, the current orbit of (514107) 2015 BZ509 appears to be stable for a period of around 4.5 billion years – roughly the age of the Solar System. Based upon this they conclude that the asteroid originated outside the Solar System, and was captured during the system’s formation. This is not completely outrageous; the Solar System is thought likely to contain a number of bodies originating outside the system that have been captured, though these are by nature difficult to detect.

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