It’s long been said that comets from the Oort Cloud are sometimes gravitationally nudged by passing clouds, and that this stellar trigger causes their plunge toward our part of the solar system. But how often does this happen? On August 31, 2017, the Max Planck Institute for Astronomy announced a new study using data from the ESA satellite Gaia, which has resulted in the first systematic estimate of the rate of these close encounters. Lead author Coryn Bailer-Jones and team have learned that – about every million years – up to two dozen stars pass within a few light-years of the sun, making, these scientists say, for “a near-constant state of perturbation in the Oort Cloud” of distant comets. The results have been published in the peer-reviewed journal Astronomy & Astrophysics (read it on Arxiv.org). These scientists’ statement said:
Comets colliding with Earth are among the more violent and extensive cosmic catastrophes that can befall our home planet. The best known such impact is the one which, 66 million years ago, caused or at least hastened the demise of the dinosaurs (although it is not known whether the blame in this case falls on a comet or an asteroid).
It must be said that, to the best of current knowledge, impacts with regional or even global consequences are exceedingly rare, and occur at a rate of no more than one per million years. Also, monitoring systems give us a fairly complete inventory of larger asteroids and comets, none of which is currently on a collision course with Earth.
Still, the consequences are serious enough that studies of the causes of comet impacts are not purely academic. The prime culprits are stellar encounters: stars passing through our sun’s cosmic neighborhood. The outskirts of our solar system are believed to host a reservoir of cold and icy objects – potential comets – known as the Oort cloud. The gravitational influence of passing stars can nudge these comets inwards, and some will begin a journey all the way to the inner solar system, possibly on a collision course with Earth. That is why knowledge of these stellar encounters and their properties has a direct impact on risk assessment for comet impacts.
Now, Bailer-Jones has published the first systematic estimate of the rate of such stellar encounters. The new result uses data from the first data release (DR1) of the Gaia mission that combines new Gaia measurements with older measurements by ESA’s Hipparcos satellite. Crucially, Bailer-Jones modeled each candidate for a close encounter as a swarm of virtual stars, showing how uncertainties in the orbital data will influence the derived rate of encounters.
Bailer-Jones found that within a typical million years, between 490 and 600 stars will pass the sun within a distance of 16.3 light-years (5 parsecs), or less. Between 19 and 24 stars will pass at 3.26 light-years (1 parsec) or less. All these hundreds of stars would be sufficiently close to nudge comets from the Oort cloud into the solar system. The new results are in the same ballpark as earlier, less systematic estimates that show that when it comes to stellar encounters, traffic in our cosmic neighborhood is rather heavy.
The scientists said their results are valid for a period of time that reaches about 5 million years into the past and into the future. They said that – with Gaia’s next data release, DR2, slated for April 2018 – this could be extended to 25 million years each way. They concluded that astronomers who intend to go further:
… and search for the stars that might be responsible for hurling a comet towards the dinosaurs will need to know our home galaxy and its mass distribution in much more detail than we currently do. That’s a long-term goal of the researchers involved in Gaia and related projects.
Bottom line: A new study from the Max Planck Institute for Astronomy indicates that – about every million years – up to two dozen stars pass within a few light-years of our sun, making, these scientists say, for “a near-constant state of perturbation in the Oort Cloud.”
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.