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Astronomy News: Astronomers Found the First Object Coming from Outside the Solar System

On October 19, 2017, a researcher in Hawaii, USA reported to the Minor Planet Center that a suspected asteroid was detected. As usual, astronomers continued to observe the object in order to calculate its orbit. However, the observation was a bit different this time. After calculation, astronomers found that this object should originate from outside the Solar System.

This object, now designated as A/2017 U1, came from a region near the star Vega in the constellation Lyra. It was moving at a speed of 25.5 kilometres per second through interstellar space and entered our Solar System almost at a vertical angle to the ecliptic (the plane of the Earth's orbit). Astronomers have reconstructed its path, showing that it had crossed the ecliptic inside the Mercury's orbit on 2nd September 2017, and reached the perihelion on 9th September. The direction and speed of its motion were affected by the Sun's gravity. On 14th October, it went past the Earth at a distance of about 24 million kilometres. Until the end of October, it was still moving at about 44 kilometres per second towards the constellation Pegasus. It is now leaving the Solar System and will never come back.

Orbit of the object A/2017 U1
Credits: NASA/JPL-Caltech
Orbit of the object A/2017 U1. It came from "above" the Solar System and went past the Sun and the Earth at a close distance. Astronomers use the term "Eccentricity" to describe the orbit of celestial bodies. A/2017 U1 has an eccentricity of about 1.195. It is the highest value that has ever been observed in the Solar System. For any object with an eccentricity above 1, its orbit is a hyperbola, meaning that the orbit is not a closed loop and extends infinitely. This object is not gravitationally bound to the Sun. Therefore, it is believed that this object originates from the interstellar space.

Astronomers estimated that the diameter of this object does not exceed 400 metres. When it was found in October, it had already gone past the perihelion and perigee. It only had a star magnitude of about 20 at that time. Since it is leaving the Sun and the Earth, its brightness will continue to decrease. Having a very peculiar orbit, this object has attracted many astronomers using their telescopes to observe it. Since it was once very close to the Sun, astronomers at first thought that it could be a comet. However, no features of a comet could be found, and it is now classified as an asteroid.

object A/2017 U1
Credit: Alan Fitzsimmons (ARC, Queen's University Belfast), Isaac Newton Group
This image was taken on 29th October by a 4.2 metre aperture telescope in Spain. It was tracking the object A/2017 U1, which was moving at a high speed leaving many tracks of the stars in the background. Astronomers did not find any features of a comet for this object.

參考:
https://www.nasa.gov/feature/jpl/small-asteroid-or-comet-visits-from-beyond-the-solar-system
https://apod.nasa.gov/apod/ap171103.html