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…
The 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...
Emissions 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...
A 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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