- Astronomers
have identified the closest known flyby of a star to our solar system: A
dim star that passed through the Oort Cloud 70,000 years ago. A group
of astronomers from the US, Europe, Chile and South Africa have
determined that 70,000 years ago a recently discovered dim star is
likely to have passed through the solar system's distant cloud of
comets, the Oort Cloud (image above). No other star is known to have
ever approached our solar system this close - five times closer than the
current closest star,
.
In
a paper published in Astrophysical Journal Letters, lead author Eric
Mamajek from the University of Rochester and his collaborators analyzed
the velocity and trajectory of a low-mass star system nicknamed
"Scholz's star."
The star's trajectory suggests that 70,000 years ago
it passed roughly 52,000 astronomical units away (or about 0.8 light
years, which equals 8 trillion kilometers, or 5 trillion miles). This is
astronomically close; our closest neighbor star Proxima Centauri is 4.2
light years distant. In fact, the astronomers explain in the paper that
they are 98% certain that it went through what is known as the "outer
Oort Cloud" - a region at the edge of the solar system filled with
trillions of comets a mile or more across that are thought to give rise
to long-term comets orbiting the Sun after their orbits are perturbed.
The star originally caught Mamajek's attention during a discussion with co-author
Valentin D. Ivanov, from the
European Southern Observatory.
Scholz's star had an unusual mix of characteristics: despite being
fairly close ("only" 20 light years away), it showed very slow
tangential motion, that is, motion across the sky. The radial velocity
measurements taken by Ivanov and collaborators, however, showed the star
moving almost directly away from the solar system at considerable
speed.
"Most stars this nearby show much larger tangential
motion," says Mamajek, associate professor of physics and astronomy at
the University of Rochester. "The small tangential motion and proximity
initially indicated that the star was most likely either moving towards a
future close encounter with the solar system, or it had 'recently' come
close to the solar system and was moving away. Sure enough, the radial
velocity measurements were consistent with it running away from the
Sun's vicinity - and we realized it must have had a close flyby in the
past."
To work out its trajectory the astronomers needed both
pieces of data, the tangential velocity and the radial velocity. Ivanov
and collaborators had characterized the recently discovered star through
measuring its spectrum and radial velocity via Doppler shift. These
measurements were carried out using spectrographs on large telescopes in
both South Africa and Chile: the Southern African Large Telescope
(SALT) and the Magellan telescope at
Las Campanas Observatory, respectively.
Once
the researchers pieced together all the information they figured out
that Scholz's star was moving away from our solar system and traced it
back in time to its position 70,000 years ago, when their models
indicated it came closest to our Sun.
Until now, the top candidate
for the closest known flyby of a star to the solar system was the
so-called "rogue star" HIP 85605, which was predicted to come close to
our solar system in 240,000 to 470,000 years from now. However, Mamajek
and his collaborators have also demonstrated that the original distance
to HIP 85605 was likely underestimated by a factor of ten. At its more
likely distance - about 200 light years - HIP 85605's newly calculated
trajectory would not bring it within the Oort Cloud.
Mamajek
worked with former University of Rochester undergraduate Scott Barenfeld
(now a graduate student at Caltech) to simulate 10,000 orbits for the
star, taking into account the star's position, distance, and velocity,
the Milky Way galaxy's gravitational field, and the statistical
uncertainties in all of these measurements. Of those 10,000 simulations,
98% of the simulations showed the star passing through the outer Oort
cloud, but fortunately only one of the simulations brought the star
within the
inner Oort cloud, which could trigger so-called "comet showers."
While
the close flyby of Scholz's star likely had little impact on the Oort
Cloud, Mamajek points out that "other dynamically important Oort Cloud
perturbers may be lurking among nearby stars." The recently launched
European Space Agency
Gaia satellite
is expected to map out the distances and measure the velocities of a
billion stars. With the Gaia data, astronomers will be able to tell
which other stars may have had a close encounter with us in the past or
will in the distant future.
Currently, Scholz's star is a small,
dim red dwarf in the constellation of Monoceros, about 20 light years
away. However, at the closest point in its flyby of the solar system,
Scholz's star would have been a 10th magnitude star - about 50 times
fainter than can normally be seen with the naked eye at night. It is
magnetically active, however, which can cause stars to "flare" and
briefly become thousands of times brighter. So it is possible that
Scholz's star may have been visible to the naked eye by our ancestors
70,000 years ago for minutes or hours at a time during rare flaring
events. The star is part of a binary star system: a low-mass red dwarf
star (with mass about 8% that of the Sun) and a "brown dwarf" companion
(with mass about 6% that of the Sun). Brown dwarfs are considered
"failed stars;" their masses are too low to fuse hydrogen in their cores
like a "star," but they are still much more massive than gas giant
planets like Jupiter.
The formal designation of the star is "WISE
J072003.20-084651.2," however it has been nicknamed "Scholz's star" to
honor its discoverer - astronomer Ralf-Dieter Scholz of the
Leibniz-Institut für Astrophysik Potsdam (AIP) in Germany - who first
reported the discovery of the dim nearby star in late 2013. The "WISE"
part of the designation refers to NASA's
Wide-field Infrared Survey Explorer (WISE)
mission, which mapped the entire sky in infrared light in 2010 and
2011, and the "J-number" part of the designation refers to the star's
celestial coordinates. -
Daily Galaxy.