Monday, December 11

Rogue star not heading for Solar System collision after all – Astronotes

Author Name: Thomas Howarth

What you’ll discover in this blog post:

  • What may happen if a star wanders too close to the Solar System
  • How astronomers look for these incoming stars
  • How astronomers of the Armagh Observatory and Planetarium used the ESO’s VLT to rule out one such close encounter

The white dwarf WD 0810-353 –– the hot and dense corpse of a Sun-like star –– was set for a close encounter with our Solar System in just 29,000 years, the blink of an eye on evolutionary and cosmic time scales. Now, however, using ESO’s Very Large Telescope (VLT), astronomers have found that the dead star isn’t coming our way after all. Breathe a sigh of relief, humanity.

What will life on Earth be like in 29 000 years’ time? It took our species, Homo sapiens, tens of thousands of years to move  from being a part of the natural world as hunter gatherers to begin manipulating it in the form of agriculture. By contrast, it has taken us just 80 years to invent the atomic bomb, walk on the Moon, create the World Wide Web, the iPhone and Facebook, triple the size of the global population and begin seeing the first real approximations to artificial intelligence. The pace of progress is speeding up, not slowing down.

Still, even at the accelerating rate at which our species is advancing, could we do much to stop a rogue star that gets too close to our Solar System? What would happen if one did? This is exactly the situation our descendants might have had to grapple with 29 000 years from now when a white dwarf star known as WD 0810–353 was set to make landfall at the Solar System’s edge. I say “might” because, well, they might not; in fact, they probably won’t. Fresh research using data from ESO’s Very Large Telescope has shown that the calculations were wrong and that WD 0810–353 isn’t heading our way after all.

Close encounters of the stellar kind

“Have you ever wondered if other stars might come so close to our Solar System that they stir up small bodies in the outer Solar System?” says John Landstreet, an astronomer at Armagh Observatory & Planetarium in the UK, and lead author of the paper that was published in The Astrophysical Journal earlier this year. Believe it or not, close encounters with our stellar neighbours are not uncommon for our plucky Solar System, although the result of such a brushing of shoulders
can vary significantly.

By some estimates, upwards of 40 000 stars may have passed through the Oort cloud — a giant shell of icy debris at the distant edge of the Solar System — over the course of the Sun’s lifetime. Our most recent trespasser, known as Scholz’s Star, paid a visit to this region around 70 000 years ago, just as our ancestors were taking their first steps out of Africa.

The Oort cloud inhabits a region between 2 000 and 100 000 AU, with AU being an astronomical unit,
the approximate distance between the Earth and the Sun. Long-period comets — those that take more than 200 years to orbit the Sun — probably emerge from this mysterious region of space, and herein lies the danger of other stars stumbling through it: since objects in the Oort cloud are only loosely bound to the Solar System, it only takes a slight gravitational nudge to alter their orbits. As other stars pass through the Oort cloud, they may potentially send some of the objects there on a collision course with Earth.

Artist’s impression of the Oort Cloud, a vast spherical reservoir of icy bodies in the outer Solar System. Objects in the Oort Cloud can be sometimes nudged towards the inner parts of the Solar System, like the comet marked here with a red line. Credit: ESO/L. Calçada

WD 0810–353, heading our way?

To understand why astronomers thought we might be due a rude intrusion into our peaceful Solar System, you must first meet Gaia, the space telescope launched by the European Space Agency (ESA) back in 2013. Gaia’s mission is to undertake a huge census of more than a thousand million stars throughout the Milky Way and beyond, mapping their motions, luminosity, temperature and composition.

In 2022, astronomers Vadim Bobylev and Anisa Bajkova analysed the vast Gaia dataset looking for stars that might be coming our way. And they stumbled upon the protagonist of our story, the white dwarf WD 0810-353, a very hot and dense  stellar corpse left behind after the death of a Sun-like star.

From the Gaia measurements, WD 0810-353 should come within about 31 000 AU of our Sun approximately 29 000 years from now — dangerously within the Oort cloud. But Gaia had missed a crucial piece of the puzzle: “unusually, this old white dwarf also has a huge magnetic field,” explains Eva Villaver, an astronomer at the Astrobiology Center in Spain and co-author of the study. “In astronomy, magnetic fields are crucial to understand many physical aspects of a star and not considering them can lead to misinterpretations of physical phenomena.” The speed with which WD 0810-353 was moving towards us — its radial velocity — was originally determined using a spectrum of its light from Gaia observations. A spectrum is obtained by splitting light into its component colours. Depending on whether an object is moving towards or away from us, its spectrum will shift to shorter or longer wavelengths, respectively. This process is correspondingly known as a blue- or redshift, since blue light has shorter wavelengths than red.

Schematic representation of blue- and red-shifted spectral lines. Credit: NASA

But the presence of a strong magnetic field can have a profound effect on the spectrum of a star, splitting its spectral lines into several ones and shifting them to other wavelengths. Simply put, if the magnetic field wasn’t taken into account when measuring the white dwarf’s radial velocity, then the question of whether it was coming our way remained an open one.

Crisis averted!

Enter the FOcal Reducer and low dispersion Spectrograph 2 (FORS2), the “swiss army knife” instrument installed on ESO’s VLT at Paranal Observatory in Chile’s Atacama Desert. The team used FORS2 to capture highly accurate spectra of the white dwarf, to see whether its intense magnetic field could be biasing the interpretation of the Gaia data.
Light waves normally oscillate in all directions, but under certain circumstances, like in the presence of a magnetic field, they oscillate along a preferred direction, becoming polarised. The team used the polarised spectrum of WD 0810-353 to model the magnetic field in this white dwarf, and found that the previously reported speed of the star could be explained away by the magnetic field.

The FORS2 instrument (foreground) and its now-retired predecessor FORS1 (background) at ESO’s VLT in Chile. Credit: ESO

“We found that the approach speed measured by the Gaia project is incorrect, and the close encounter predicted between WD0810-353 and the Sun is actually not going to happen,” says Stefano Bagnulo, an astronomer at Armagh and co-author of the study. “In fact, WD0810-353 may not even be moving towards the Sun at all.”

So… crisis averted? Well, in the vastness of space there’s always danger lurking around the corner. It’s almost certain that at some point in our Solar System’s future we’ll get a visit from another star and who knows what havoc that may wreak on our small planet. For now, though, as Bagnulo puts it: “that’s one less cosmic hazard we have to worry about!”

● Research paper
● How polarimetry works

Article originally published by ESO

Author bio:
A chemical engineer by training, Tom received a master’s degree in Advanced Chemical Engineering from the University of Cambridge (UK) where he conducted research into amyloid protein folding using fluorescent lifetime imaging microscopy. It was during the course of his studies that Tom developed a passion for science communication and journalism, which led him to write several articles on a broad range of topics including climate change, emerging technologies and, in particular,
astronomy. Since then, Tom has gone on to work as a contractual research paper editor, freelance journalist and technical PR agent, before arriving at ESO as a science communication intern.