Astronomers using the European Space Agency's XMM-Newton space observatory and the LOFAR telescope have definitively spotted an explosive burst of material thrown out into space by another star – a burst powerful enough to strip away the atmosphere of any unlucky planet in its path.
The burst was a coronal mass ejection (CME), eruptions we often see coming from the Sun . During a CME, massive amounts of material are flung out from our star, flooding the surrounding space. These dramatic expulsions shape and drive space weather , such as the dazzling auroras we see on Earth, and can chip away at the atmospheres of any nearby planets.
But while CMEs are commonplace at the Sun, we hadn't convincingly spotted one on another star – until now.
"Astronomers have wanted to spot a CME on another star for decades," says Joe Callingham of the Netherlands Institute for Radio Astronomy (ASTRON), author of the new research published in Nature. "Previous findings have inferred that they exist, or hinted at their presence, but haven't actually confirmed that material has definitively escaped out into space. We've now managed to do this for the first time."
As a CME travels through the layers of a star out into interplanetary space, it produces a shock wave and associated burst of radio waves ( a type of light ). This short, intense radio signal was picked up by Joe and colleagues and found to come from a star lying around 40 light-years away (just under 15 times the diameter of the Solar System, close by cosmic standards).
"This kind of radio signal just wouldn't exist unless material had completely left the star's bubble of powerful magnetism," adds Joe. "In other words: it's caused by a CME."
A danger to any planets
The matter-flinging star is a red dwarf – a type of star far fainter, cooler, and smaller than the Sun. It is nothing like our own star: it has roughly half the mass, it rotates 20 times faster, and has a magnetic field 300 times more powerful. Most of the planets known to exist in the Milky Way orbit this kind of star.
The radio signal was spotted using the Low Frequency Array (LOFAR) radio telescope thanks to new data processing methods developed by co-authors Cyril Tasse and Philippe Zarka at the Observatoire de Paris-PSL. The team then used ESA's XMM-Newton to determine the star's temperature, rotation, and brightness in X-ray light. This was essential to interpret the radio signal and figure out what was actually going on.
"We needed the sensitivity and frequency of LOFAR to detect the radio waves," says co-author David Konijn, a PhD student working with Joe at ASTRON. "And without XMM-Newton, we wouldn't have been able to determine the CME's motion or put it in a solar context, both crucial for proving what we'd found. Neither telescope alone would have been enough – we needed both."
The researchers determined the CME to be moving at a super-fast 2400 km per second, a speed only seen in 1 of every 20 CMEs taking place on the Sun. The ejection was both fast and dense enough to completely strip away the atmospheres of any planets closely orbiting the star.
In search of life
The atmosphere-stripping ability of the CME is an exciting discovery for our hunt for life around other stars . A planet's habitability for life as we know it is defined by its distance from its parent star – whether or not it sits within the star's 'habitable zone', a region where liquid water can exist on the surface of planets with suitable atmospheres. This is a Goldilocks scenario: too close to the star is too hot, too far is too cold, and in between is just right.
But what if that star is especially active, regularly throwing out dangerous eruptions of material and triggering violent storms? A planet regularly bombarded by powerful coronal mass ejections may lose its atmosphere entirely, leaving a barren rock behind – an uninhabitable world, despite its orbit being 'just right'.
"This work opens up a new observational frontier for studying and understanding eruptions and space weather around other stars," adds Henrik Eklund, an ESA research fellow based at the European Space Research and Technology Centre (ESTEC) in Noordwijk, The Netherlands.
"We're no longer limited to extrapolating our understanding of the Sun's CMEs to other stars. It seems that intense space weather may be even more extreme around smaller stars – the primary hosts of potentially habitable exoplanets. This has important implications for how these planets keep hold of their atmospheres and possibly remain habitable over time."
The finding also informs our understanding of space weather, something that's long been a focus for ESA missions and is currently being explored by SOHO , the Proba missions , Swarm , and Solar Orbiter .
XMM-Newton , meanwhile, is a leading explorer of the hot and extreme Universe. Launched in 1999, the space telescope has gazed into the cores of galaxies, studied stars to understand how they evolve, investigated the environs of black holes, and spotted intense bursts of energetic radiation from distant stars and galaxies.
"XMM-Newton is now helping us discover how CMEs vary by star, something that's not only interesting in our study of stars and our Sun, but also our hunt for habitable worlds around other stars," says ESA XMM-Newton Project Scientist Erik Kuulkers. "It also demonstrates the immense power of collaboration, which underpins all successful science. The discovery was a true team effort, and resolves the decades-long search for CMEs beyond the Sun."
