Tuesday 24 February 2015

When Schotz’s Star passed through the Solar System’s Oort Cloud.

It is estimated that about 30% of impact craters larger than 10 km on the Earth and the Moon have been caused by long period comets originating in the Oort Cloud (the very outermost part of the Solar System, between about 30 AU and about 120 000 AU from the Sun, which is to say between 30 and 120 000 times as far from the Sun as the Earth. In the 1980s it was theorized that the Sun could have an undetected binary companion orbiting in this part of space, which periodically triggered showers of comets to enter the inner Solar System, triggering mass-extinction events on Earth, though since that time it has become apparent that mass extinction events do not show any such regular pattern and astronomers have failed to find any such companion star.

This still leaves the possibility of close encounters with other stars, with such bodies passing through the Oort Cloud and triggering showers of comets to enter the Inner Solar System. It has been calculated that on average 12 stars pass within one parsec (208 000 AU) of the Sun every million years, one star every 9.2 million years coming within 0.25 parsecs (52 000 AU) of the Sun. Previous studies have found four stars which may in the remote future pass through the Solar System’s Oort Cloud; HIP 85605, which may come within 20 800 AU of the Sun in 332 000 years’ time (though this is highly uncertain as HIP 85605 is a poorly understood binary system whose distance and motion are poorly resolved), HIP 89825 (Gliese 710) which may reach 0.27 parsecs (56 160 AU) from the Sun 1 400 000 years in  the future, HIP 63721 which may also reach 0.27 parsecs (56 160 AU) from the Sun in 146 000 years, and HIP 89825 which may come withion 0.36 parsecs (74 260 AU) from the Sun in 1.5 million years.

In a paper published in the Astrophysical Journal Letters on 10 February 2015 and on the arXiv database at Cornell University Library on 16 February 2015, a team of scientists led by Eric Mamajek of the Department of Physics& Astronomy at the University of Rochester describe the results of a study of a newly discovered body, Scholtz’ Star (WISE J072003.20-084651.2) with the Southern African Large Telescope and Magellan Telescopes which suggest that the body may have passed through the Oort Cloud in the past.

Finder chart of 6 6 arcmin² centred on WISE J072-0846 from SuperCOSMOS Sky Surveys. Scholtz 2013.

Sholtz’s Star currently lies about 7 parsecs (light years) from Earth in the constellation of Monoceras. It is an extremely dim Red Dwarf star lying close to the Galactic Plane, and consequently was not discovered until 2013. It has a low tangential velocity (i.e. it appears not to move very much viewed from Earth), which is unusual in so close a star; all stars are constantly in motion, so if a nearby star appears stationary it is probably moving straight towards or straight away from us. A previous study has suggested that this body is in fact a binary system.

Mamajek et al. conclude that Sholtz’s Star is in fact a binary system, comprising two bodies separated by a distance of about 0.8 AU (80% of the distance between the Earth and the Sun). These bodies have masses of 86 and 65 times that of Jupiter respectively, leading Mamajek et al. to conclude that the larger body, WISE J072003.20-084651.2A (when naming bodies in other star systems stars are given upper case letters and planets lower case letters) is an extremely small Red Dwarf star, while the second body, WISE J072003.20-084651.2B, is a Brown Dwarf, a body too small to fuse ordinary hydrogen in its core, but large enough to fuse the heavy hydrogen isotope deuterium. These bodies are estimated to be about 3-10 billion years old, to have originated within the galactic thin disk (expand) and to form part of the Hercules Stream (expand).

Calculations of the motion of Scholtz’s Star suggest that the body reached a closest distance from the Sun of 0.25 parsecs, or 52 000 AU, from the Sun approximately 700 000 years ago. For comparison the current closest known star, Proxima Centuri, is 268 300 AU from the Sun, while the most distant man-made object, Voyager 1, which has been travelling outwards since 1977, is currently 130.6 AU from the Sun. Despite this close proximity Sholtz’s Star would still have been to dim to be seen by the naked eye, though it would have been brighter than ProximaCenturi. However, unlike ProximaCenturi, Sholtz’s Star is highly active, occasionally producing very bright flares, which may have resulted in it becoming dimly naked eye visible for periods of minutes or even hours.

Finder chart of 6 6 arcmin² centred on WISE J072-0846 from WISE w2-band observation.

An approach at 52 000 AU places Sholtz’s Star within the outer part of the Oort Cloud, where it may potentially have encountered and perturbed the orbits of comets orbiting our Sun, though outside the denser Inner Oort Cloud, which extends to 20 000 AU from the Sun and where the majority of such comets are found.Comets orbiting the Sun at a distance of 52 000 would have an orbital period of about 4.2 million years, thus any such bodies perturbed from their orbit’s by the approach of Sholtz’s Star would take about 2.1 million years to reach the Inner Solar System, arriving about 1 400 000 years in the future. It is estimated that in order to cause a major comet flux (in which the number of long period comets reaching the Inner Solar System increases by a factor of 10 or more), then another star would have to come within 10 000 AU of the Sun, so any flux created by the passage of Sholtz’s Star should be quite small compared to the usual rate of cometry bombardment, generated by galactic tidal effects, presenting only a very minor threat to the Earth.

Mamajek et al. also re-examined the poorly known HIP 85605, concluding that it is both brighter and further away than previously estimate. Mamajek et al. conclude that HIP 85605 is currently 60 parsecs (light years) from the Sun, and that its closest approach to our system will come in 2.8 million years’ time, when it will reach a distance of 10 parsecs (light years). This indicates that the flyby by Sholtz’s Star is the closest known encounter with the Solar System by another star at any point in the calculable past or foreseeable future.

See also…

http://sciencythoughts.blogspot.co.uk/2014/10/the-discovery-of-brown-dwarf-companion.htmlThe discovery of a Brown Dwarf companion to the star ζ Delphini.                                       Brown Dwarfs are objects intermediate to stars and planets in size; they are not large enough to fuse ordinary hydrogen in their cores, but are large enough to fuse the heavier isotope deuterium. These objects are thought to...
http://sciencythoughts.blogspot.co.uk/2014/08/emissions-from-comet-c2002-vq94-linear.htmlEmissions from Comet C/2002 VQ94 (LINEAR).                                                C/2002 VQ94 (LINEAR) was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) team at the Massachusetts Institute of...
http://sciencythoughts.blogspot.co.uk/2014/05/a-small-cold-brown-dwarf-7175-light.htmlA small cold Brown Dwarf, 7.175 light years from Earth.                                                           Brown Dwarfs are curious objects, intermediate between stars and planets. They lack the mass to fuse hydrogen in their cores like true stars, but are massive enough to fuse deuterium (a heavy isotope hydrogen, containing one proton and one neutron in its atomic... 
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