Wild space weather may mean most of the exoplanets in the Milky Way are uninhabitable, according to new research.
Over the past decade, astronomers have discovered more than 4300 planets orbiting distant stars and the list is growing every week. But this study, published in The Astrophysical Journal, suggests that planets orbiting M-type red dwarf stars will have to contend with violent weather conditions.
The Australian and US team used optical and radio telescopes to observe our closest star – Proxima Centauri, just 4.2 light-years away – as it simultaneously emitted powerful stellar flares and radio bursts, providing the first definitive link between these two events in a star other than our Sun.
But here in our Solar System, these events are linked to coronal mass ejections – massively energetic blasts of ionised plasma and radiation. Lead researcher Andrew Zic, from the University of Sydney, says it’s highly likely similar intense events occur on M-type red dwarfs.
“This is probably bad news on the space weather front,” says Zic. “It seems likely that the galaxy’s most common stars – red dwarfs – won’t be great places to find life as we know it.”
Astronomers previously knew that Proxima Centauri is home to two rocky planets. One is particularly exciting because it exists in the habitable zone around the star, where it receives enough warmth for liquid water to hypothetically exist. However, the habitable zones of small, cool red dwarfs are tucked in close to the star – much closer than Mercury is to our Sun – which could put planets in the path of the star’s ferocious weather.
“What our research shows is that this makes the planets very vulnerable to dangerous ionising radiation that could effectively sterilise the planets,” Zic explains.
This radiation would severely erode a planet’s atmosphere, leaving it exposed to further dangers such as intense X-rays or UV light.
Stars like our Sun also emit such radiation, but since the Sun is much hotter, Earth sits in the habitable zone much further away.
“Further, the Earth has a very powerful planetary magnetic field that shields us from these intense blasts of solar plasma,” says Zic.
Whether exoplanets have magnetic fields or not is still an open question for astronomers.
“But even if there were magnetic fields, given the stellar proximity of habitable zone planets around M-dwarf stars, this might not be enough to protect them.”
Since M-type red dwarfs like Proxima Centauri comprise 70% of the stars in the Milky Way, the finding may narrow down our search for habitable planets and extraterrestrial life.
The observations were made using telescopes across the world, including the Australian Square Kilometre Array Pathfinder (ASKAP) in Western Australia, the Zadko Telescope at the University of Western Australia, and NASA’s planet-hunting Transiting Exoplanet Survey Satellite (TESS). Observing with multiple telescopes over several wavelengths allowed the team to make the crucial link between the optical and radio flares.
“The probability that the observed solar flare and received radio signal from our neighbour were not connected is much less than one chance in 128,000,” Zic says.
Co-author Bruce Gendre from the University of Western Australia says that research is a significant step towards using the radio signals from nearby stars to produce space weather reports beyond our own Solar System.
“Understanding space weather is critical for understanding how our own planet biosphere evolved – but also for what the future is,” he concludes.
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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