Although stars look like they’ve been sitting in the darkness of space in the same positions for an eternity of nights, closer inspections show that they do move about relative to one another. Astrometry examines the movements up, down, left and right of stars in the night sky. The movements can be due to the star’s actual motion (proper motion), which looks to be small because of the distance between us and it, as well as the motions induced by the Earth’s orbital dynamics.
In order to look at the motion of the star towards or away from us, examinations are made of the Doppler shift of light from a star. If a star exhibits little Doppler shifting (correcting for Earth’s orbit) and discernible astrometric proper motion, we know the star is mainly moving orthogonal to our solar system’s own motion. If, on the other hand, there is no astrometric motion, but a lot of blue shift in the spectrum, we know there’s something coming over to pay a visit.
One such object is the star Gliese 710, presently 63 light years away and closing. Calculations show an eighty-six percent chance that in a million and a half years or so, this star will plough into the outskirts of the Oort cloud, a vast cloud of icy debris surrounding the solar system and believed to be the source of many long period comets. There is a remote chance that the star will intrude further into the solar system, skimming the region within which its influence will disrupt objects in the Kuiper belt. There is also the chance that the star will bring its own Oort cloud with it, which may also bring a shower of comets.
The research was conducted by the ESA space telescope Hipparcos, which spends its time doing precise astrometry. It has identified nine stars expected to pass within three light years of the Sun, of which Gliese is likely to produce the closest encounter.
Some scientists hypothesis that an object already exists within the void between the Kuiper belt and the Oort cloud. This object, known as Nemesis, is thought to be a brown dwarf orbiting the Sun in an elliptical motion. Once every 26,000 years, it approaches close enough to disrupt the Kuiper belt and cast comets into the inner solar system. Furthermore, it may be responsible for tugging Sedna into an orbit that is unusually far from the Sun compared to other objects in the belt.
If it exists.
The problem with Brown dwarfs is they are relatively small (more dense than planets of a similar mass according to recent research on the most massive exoplanets) and do most their glowing in the infrared. But there is a big infrared camera out there quite capable of picking up the region of light where an object such as this glows brightest – WISE – and if there is a Nemesis out there, it should be able to pick it up.
Still, if we do find ourselves deluged by material from the Kuiper belt, these guys at the Secure World Foundation aim to help with establishing an early warning system for Near Earth Objects.