Stars identified that could disrupt the Solar System
Sen—Close encounters with passing stars in the distant past may have sent comets from the Oort Cloud towards the Earth. The same thing is likely to happen again in the future.
But before you start to worry, the latest study, by Coryn Bailer-Jones of the Max Planck Institute of Astronomy, in Germany, comes with some reassuring caveats, or conditions.
For a start the paper identifies four potential candidates for future close encounters; the earliest of which will only pass by perihelion in 240-470,000 years. This object, labelled Hip85605, is a cool, K or M-class dwarf star that is expected to pass between 0.04 to 0.2 parsecs (0.13 to 0.65 light-years) from our Sun.
The gravitational influence of such a close encounter could potentially dislodge objects from the Oort Cloud—a spheroidal swarm of icy, rocky cometary bodies about two light years in diameter that surrounds our Solar System. Past encounters are theorized to be responsible for the K-T impact that wiped out the dinosaurs. Such direct causal links are very difficult to determine, however.
Although the idea of stellar close encounters is by no means a new one, Bailer-Jones has performed a more comprehensive study than work that has happened previously. Using improved astrometry performed by Floor van Leeuwen of the Cambridge Institute of Astronomy, Bailer-Jones analysed 50,000 stars from the Hipparcos and Tycho-2 catalogues, both formed with data from the European Space Agency’s Hipparcos satellite, which tracked 2.5 million stars.
The study included new radial velocity data for each star, and computed star velocities with respect to our Sun. Rather than make easy assumptions (such as capping maximum radial velocities), or choosing arbitrary distances, the study worked with the uncertainties inherent in the data to produce a range of perihelion distances and timescales. Velocities were also calculated using probabilistic techniques to further improve accuracy.
An artist's depiction of the Oort Cloud, indicating its size compared to the inner Solar System, including the Kuiper Belt of icy objects. Image credit: NASA
But the caveats continue. The paper is open about the fact that 50,000 stars is a small sample size and that Hipparcos is prone to a selection bias of the brightest stars only. Many of the close-encounter candidates are also dwarf stars smaller than our Sun, meaning that their influence on the Oort cloud will be less than for a larger star.
There is also the question of the “Galactic Tide”, which is a difficult variable to factor. The gravitational effect of the Milky Way Galaxy itself can tidally disrupt the Oort Cloud, further complicating the influence exerted by close-passing stars.
The paper, published in the journal Astronomy & Astrophysics, mentions that the European Space Agency’s Gaia spacecraft, whose mission is to catalogue over a billion objects, will produce much higher-quality data when the archives are released in 2016.
Gaia promises better accuracy for parallax, proper motion and radial velocity data down to much lower magnitudes. This would undoubtedly improve forecasts for future close-encounters. Whether the Gaia archives can link past encounters with cometary impacts on Earth remains to be seen.
Intriguingly, the paper’s largest perturbation find (the star Hip103738), which occurred 3,850,000 years ago, correlates well with two impact craters from that period, listed in the Earth Impact Database.