Star paid our sun a visit

Astronomers from the South African Astronomical Observatory (SAAO) and Southern African Large Telescope (SALT) together with collaborators in the U.S. and Europe have recently discovered a dim star which they calculate passed through the edge of our solar system 70,000 years ago.

A close pass by astronomical standards, the star passed Earth at a distance of around 8 trillion kilometres (52,000 times the distance between the Earth and the Sun, or 0.8 light years). At that distance astronomers think that the star passed through the “outer Oort Cloud” system – a region at the edge of the solar system filled with trillions of frozen comets larger than 1 km in diameter in size. At that time, the star referred to as “Scholz’s star” was the closest known star to our solar system (other than our Sun) with potential to disturb the Oort Cloud. No other star is known to have ever approached as close to our planetary system, and the currently nearest star, Proxima Centauri, is over five times as distant at 4.2 light years away.

“We were alerted by European colleagues to a new discovery of a nearby star,” says SAAO and SALT astronomer Dr. Petri Vaisanen. Using the SALT telescope in Sutherland the team immediately measured the star’s speed relative to Earth. In technical terms, it turned out its “radial velocity” was very high, while the “tangential velocity” was small. Essentially, it is moving very rapidly away from us, while its “sideways” or “tangential” motion across the sky is slow.

“After these measurements the realisation of what might have happened was intriguing,” continues Vaisanen. “We were dealing with stuff from science fiction movies here. The star must have zoomed past us relatively recently. How close? Did it disturb any comets’ orbits which could then “rain down” into the inner solar system posing serious threat to life on Earth?

Led by Eric Mamajek from the U.S., the astronomers proceeded to calculate the most likely path of the star through space over time. They wanted to estimate the distance between the Scholz’s star and the Sun, at the moment of closest approach to see what effects the star may have had on our solar system. A massive slow moving object entering the inner solar system could trigger so-called “comet showers” – periods with an enhanced number of comets whose orbits are perturbed by the passing object bringing them into the inner solar system. These events are extremely rare, but they could potentially lead to extinction events on Earth due to comet impacts, such as those that are thought to have wiped out the dinosaurs. Artist’s concept showing the scale of our solar system. The scale bar is in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance (logarithmic scaling). One AU is the distance from the Sun to the Earth, which is about 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU. The solar system is considered to reach as far as the Oort Cloud, the source of long period comets. Beyond the outer edge of the Oort Cloud, the gravity of other stars begins to dominate that of the Sun. Note the position of Voyager 1, the most distant man made spacecraft in space. (Credit: NASA, creative commons licence).

The results, to be published in Astrophysical Journal Letters, show that luckily for our planet, Scholz’s star is a relatively low mass object that passed at high speed through the outer part of the Oort comet cloud at the edge of the solar system. The astronomers report that they carried out 10,000 simulations of the star’s possible path through space and none of them yielded an encounter that should have triggered a significant comet shower.

“We are happy that there are no apocalypse dates to report,” says Vaisanen, “but it is very interesting that after hundreds of years of astronomy with telescopes we can still uncover surprising stellar size objects right in our neighbourhood.

Scholz’s star close pass of only 0.8 light years distant, puts it as the closest known stellar flyby to the solar system. While the close flyby of Scholz’s star likely had little impact on the Oort Cloud and the rest of the solar system, there may be many other stars that may have had a significant effect, or may do so in the future. The recently launched European Space Agency (ESA) Gaia satellite will allow astronomers to measure the distances and the velocities of billions of stars, to find out if they may have had a close encounter with us in the past or will do so in the future.

Artist’s impression of the Scholz’s binary star, a small red star with a brown dwarf companion (at right), as it visited the outskirts of the solar system just 70 000 years ago. The bright lone star to the left is the Sun 0.8 light years away at closest pass. (Credit: Michael Osadciw, University of Rochester).

Background Information

Scholz’s star is a small, dim red dwarf type star in the constellation of Monoceros, about 20 light years away. Its formal designation is “WISE J072003.20-084651.2”, however it has been nicknamed “Scholz’s star” to honour astronomer Ralf-Dieter Scholz who discovered the star in late 2013. Further observations by Valentin Ivanov, Petri Vaisanen and Alexei Kniazev using SALT, and by Adam Burgasser using the Keck telescope, demonstrated that Scholz’s star is actually a binary star system consisting of a low-mass star (about 8.2% the mass of the Sun, or 86 times the mass of Jupiter) and a “brown dwarf” companion with mass about 65 times the mass of Jupiter (6.2% the mass of the Sun). Brown dwarfs are considered “failed stars” – with masses too low to burn hydrogen deep in their cores to shine like a “star”, but much more massive than gas giant planets like Jupiter.

This image shows the movement of Scholtz’s star (centre) against the backdrop of more distant stars. This image was made by co-adding three different images of the same star-field taken many years apart. The red point in the middle shows the position of Scholtz’s star sixty years ago. The blue point shows the star’s present position. The white arrow shows how the star will move across the sky over the next 200 years. (Credit: V. D. Ivanov, DSS, and 2MASS).

The Oort Cloud is a vast distant spherical shell surrounding the entire solar system filled with trillions of long period comets. Though not directly observed, its existence is required to explain the orbital characteristics and numbers of comets with orbital periods around the Sun of many thousands to millions of years. Computer simulations of the Oort cloud show that these comets usually remain in the Oort Cloud unless they are gravitationally perturbed by encounters with nearby stars. When stars pass through the Oort Cloud they change the orbits of some of the comets, causing them to enter the inner solar system and we observe these as long period comets.

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