From Max Planck Institute for Astronomy: “Heavy stellar traffic, deflected comets, and a closer look at the triggers of cosmic disaster”

Max Planck Institute for Astronomy

Max Planck Institute for Astronomy

August 31, 2017

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Bailer-Jones, Coryn
Coryn Bailer-Jones
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Markus Pössel
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Image of the Week
Close stellar encounters from the first Gaia data release
Figure 1: The open circles show the time (horizontal axis) and distance (vertical axis) of the closest approach of stars to the Sun. Negative times indicate times in the past from today. Each point has been calculated as the median of the distribution of a swarm of surrogate particles which have been integrated through a Galactic potential. The “error” bars show the limits of the 5% and 95% percentiles of these distributions (which together form an asymmetric 90% confidence interval). That is, the swarm is used to propagate the uncertainties in the TGAS measurements to uncertainties in the perihelion parameters. The background colour (scale on the right) indicates the estimated completeness of the TGAS survey. That is, if all TGAS stars had radial velocities (and most do not), this gives the probability that a star with any particular perihelion parameters would be present in TGAS. Image credit: Coryn Bailer-Jones

Image of the Comet C/2012 S1 (ISON), taken with the TRAPPIST–South national telescope at ESO’s La Silla Observatory on the morning of Friday 15 November 2013, whose likely origin is the Oort cloud. This comet is definitely not colliding with Earth, but it shows the typical appearance of comets entering the inner solar system, including the typical tail made of gas and dust. Image: TRAPPIST/E. Jehin/ESO

ESO Belgian robotic Trappist-South National Telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

As stars pass close by our solar system, they can nudge comets from the distant Oort cloud into the inner regions around the Sun. Thus, stellar encounters are an important factor in determining the risk of large cosmic impacts on Earth. Now, Coryn Bailer-Jones from the Max Planck Institute for Astronomy has used data from the ESA satellite Gaia to give the first systematic estimate of the rate of such close stellar encounters. Every million years, up to two dozen stars pass within a few light-years of the Sun, making for a near-constant state of perturbation. The results have been published in the journal Astronomy & Astrophysics.

Oort Cloud NASA

ESA/GAIA satellite

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, to use a unit more common in professional astronomy) 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 current results are valid for a period of time that reaches about 5 million years into the past and into the future. With Gaia’s next data release, DR2 slated for April 2018, this could be extended to 25 million years each way. However, astronomers intending to go even 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 – a long-term goal of the researchers involved in Gaia and related projects.
Background information

The research described here is published as C. A. L. Bailer Jones, The completeness-corrected rate of stellar encounters with the Sun from the first Gaia data release in the journal Astronomy & Astrophysics.

E-print on arXiv
ESA picture of the week
ESA press release

See the full article here .

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