Oumuamua and Borisov are not the only interstellar interlopers. We know of many more. Here’s what we know about them so far.
An interstellar object is an astronomical object in interstellar space that is not gravitationally bound to a star. This term can also be applied to an object that is on an interstellar trajectory but is temporarily passing close to a star, such as certain asteroids and comets.
In November 2018, Harvard astronomers Amir Siraj and Avi Loeb reported that there should be hundreds of ‘Oumuamua-size interstellar objects in the Solar System, based on calculated orbital characteristics, and presented several centaur candidates such as 2017 SV13 and 2018 TL6. These are all orbiting the Sun, but may have been captured in the distant past.
On 8 January 2014, a bolide which has been identified by Loeb and Siraj as a potentially interstellar object originating from an unbound hyperbolic orbit exploded in the atmosphere over northern Papua New Guinea. It had an eccentricity of 2.4, an inclination of 10°, and a speed of 43.8 km/s when outside of the Solar System. This would make it notably faster than ʻOumuamua which was 26.3 km/s when outside the Solar System. The meteor is estimated to have been 0.9 meters in diameter. Other astronomers doubt the interstellar origin because the meteor catalog used does not report uncertainties on the incoming velocity. The validity of any single data point (especially for smaller meteors) remains questionable.
Amir Siraj and Avi Loeb have proposed methods for increasing the discovery rate of interstellar objects that include stellar occultations, optical signatures from impacts with the moon or the Earth’s atmosphere, and radio flares from collisions with neutron stars.