A red dwarf and its brown dwarf companion buzzed through the outer Oort Cloud some 70,000 years ago, around the time when modern humans began migrating from Africa into Eurasia.
Most of space is empty. So in a galaxy bustling with hundreds of billions of stars, there’s too high a separation between them for any physical run-ins. Even close encounters are few and far between.
But studies of a nearby, low-mass star hiding among the confusion of the galaxy’s disk shows that space might be a little less empty than previously thought.
A year ago, astronomer Eric Mamajek (University of Rochester) heard about a faint star, while chatting with his colleague. This star, nicknamed Scholz’s star, sparked his interest: it was close — only 20 light-years away — yet its proper motion was surprisingly slow, meaning that it inched sluggishly across the sky.
The latter doesn’t mean that the star isn’t moving, but that much of its movement is hidden in its radial velocity, the motion along our line of sight and into the plane of the sky. It became clear that the star had recently passed close to the Solar System and was now moving rapidly away.
Putting the star’s approximate distance and velocity into a “toy code,” Mamajek had a rough answer within 20 minutes: the star had almost certainly sped near the Sun tens of thousands of years ago.
To calculate the star’s trajectory more precisely, and to see just how close it had come, Mamajek needed data on the star’s current position and its motion, both along and into the plane of the sky. A team led by Adam Burgasser (University of California, San Diego) gathered the necessary data.
The star’s proper motion along the plane of the sky can only be measured by waiting long enough for the star to change its position appreciably. Luckily, images as far back as a photographic plate from 1955 had serendipitously captured the star. Between 1955 and 2014, the star had moved roughly 6 arcseconds. (For comparison, your little finger held up to the sky at arm's length covers a full degree, or 3600 arcseconds.)
Burgasser’s team measured the star’s parallax — that tiny back-and-forth motion we see as Earth moves from one end of its orbit to the other — to give the star’s current distance. And spectroscopy showed the slight Doppler shift in the star’s spectral lines as it moves away from us, providing the star’s radial velocity.
Most surprisingly, Burgasser’s team showed that Scholz’s star, a red M-class star, actually has a smaller brown dwarf companion.
With all the pieces of the puzzle, Mamajek and his colleagues were able to trace all the possible paths Scholz’s star may have taken. The team simulated 10,000 orbits for the star to take into account the uncertainties in the star’s position, distance and velocity, as well as the effect of Milky Way’s gravitational field.
Of all those simulations, 98 percent show that the star had passed through the outer Oort Cloud. Its closest approach was probably between 0.6 and 1.2 light-years away, when it scraped the Oort Cloud 70,000 years ago at 83 kilometers per second.
Until now, the top candidate for the closest flyby had been the so-called “rogue star” HIP 85605, discovered by Coryn Bailer-Jones (Max Planck Institute of Astronomy) in a study that analyzed the trajectories of 50,000 nearby stars. That star was predicted to pass 0.13 to 0.65 light-years from our Sun in 240,000 to 470,000 years.
Bailer-Jones, however, noted that the original distance to HIP 85605 was highly uncertain. So Mamajek and his colleagues determined a more likely distance and its newly calculated trajectory doesn't bring it within the Oort Cloud at all.
Although Bailer-Jones agrees with the team’s assessment of the rogue star, he also warns that even though Scholz’s star currently holds the record, it doesn’t hold it by much. A second star, known as Gliese 710, has a more precisely calculated trajectory that shows it flying by almost as close as Scholz’s star. Both close approaches come within each other’s uncertainties.
Nonetheless, the discovery of another close-pass star proves an interesting point. “This is by no means a statistical survey,” says Mamajek. But, he continues, it’s an example of what are likely many more undiscovered nearby stars, whose trajectories might bring them close to the Sun.
The European Space Agency recently launched the Gaia satellite to map out the distances and velocities of billions of stars, bringing low mass stars into focus.
Close encounters could perturb comets in the Oort cloud, shaking them up and sending them our way. “But there is no need to worry,” says coauthor Henri Boffin (European Space Observatory). “Even if the Oort cloud was perturbed, it takes millions of years for a comet in the cloud to reach the Earth.”
Adam Burgasser et al. " WISE J072003.20-084651.2: An Old and Active M9.5 + T5 Spectral Binary 6 pc from the Sun." Astrophysical Journal. February 19, 2015.
Eric Mamajek et al. “The Closest Known Flyby of a Star to the Solar System.” Astrophysical Journal Letters. February 12, 2015.