In September 1859, the most powerful explosion in recorded history burst from the surface of the sun, releasing more energy than a billion of the largest nuclear bombs that have ever been in the U.S. arsenal. When the magnetic eruption reached the Earth, it induced currents in telegraph wires across the globe, shooting sparks out of pylons and delivering an electrical shock to some operators. Last month, researchers at the University of Colorado announced unprecedented observations of an even more powerful event that came from the nearest star to the solar system, Proxima Centauri.
Sun-like stars are born with strong magnetic fields, which gradually decay throughout their lifetimes. In their youth, turbulent motions near the surface of a star frequently twist up their magnetic fields and produce violent eruptions that send harmful radiation and charged particles bursting into space. By the time stars reach middle-age like our sun, these stellar flares become much less frequent and typically release less energy. The most powerful explosions on the sun occur about once in a century, while more feeble events are happening all the time.
Stars are formed in a variety of sizes, and the smallest of them age more slowly than the largest. The lifetime of a star is determined by the amount of fuel around the nuclear-fusion-powered core, and by how quickly they burn through that fuel. Like automobiles, tiny Prius-like stars are more efficient than their bloated SUV-like cousins. The sun is about halfway through its 10-billion-year lifetime, while smaller red dwarf stars like Proxima Centauri can live a thousand times longer. Since the universe has only existed for less than 14 billion years, even the oldest red dwarf stars are still in their infancy and display the magnetic exuberance that is typical of youth.
Proxima is the nearest component of a triple star system about four light years (25 trillion miles) away in the direction of the constellation Centaurus. The other two stars, known as alpha Centauri A and B, orbit each other every 80 years while distant Proxima swings around the pair every half-million years. Two planets have been discovered around Proxima, one of which has a size similar to the Earth and orbits in the so-called habitable zone where liquid water could theoretically exist. However, this zone is snuggled quite close to faintly shining Proxima compared to our solar system, making the frequent and energetic flares from the young red dwarf even more problematic.
“It is hard to imagine that lifeforms like we have on Earth would be able to survive,” says Meredith MacGregor, an assistant professor in the Department of Astrophysical and Planetary Sciences at CU Boulder. “Any life would have had to evolve to be able to handle continual large doses of ultraviolet radiation.”
MacGregor is a Boulder native, and she was an eager participant in science fair competitions starting in elementary school. As a high school junior, she won a top award at Intel’s International Science and Engineering Fair for a novel physics experiment she conducted in the basement of her family home. She earned her undergraduate and doctoral degrees from Harvard, and was a postdoctoral fellow at the Carnegie Institution for Science before joining the faculty at CU a few months ahead of the pandemic.
Two years ago, MacGregor coordinated an intense campaign to monitor the brightness of Proxima over several months using nine different telescopes, measuring light from the ultraviolet to high-energy radio waves. The idea was to capture the wide spectrum of radiation produced by the frequent stellar flares on Proxima to better understand the physics that drives these energetic events. During the campaign, Proxima released a flare that was roughly 100 times more powerful than the 1859 event on the sun.
“The star went from normal to 14,000 times brighter when seen in the ultraviolet over the span of a few seconds,” explains MacGregor. “Proxima Centauri’s planets are getting hit by something like this not just once in a century, but at least once a day if not several times a day.”
Although the observations are a bad sign for the prospects of life on Proxima’s planets, humanity may get a chance to see for themselves later this century. In 2016, venture capitalist Yuri Milner announced an initiative to send a fleet of microchip-sized space probes to fly past Proxima Centauri. Each “starchip” would weigh about as much as a penny, attached to a reflective “light sail” measuring 15 feet across. Once in orbit with their light sails unfurled, high-powered lasers on Earth would accelerate the tiny spacecraft to 15-20% of the speed of light, allowing them to make the journey to the nearest star in 20-30 years.
The project is estimated to cost up to $10 billion, with the first launch anticipated in 2036. Considering the duration of the mission, and the additional four years to return data from that distance, future astronomers could release the first images of the crispy planet next-door sometime in the 2060s. Yes friends, there’s a lot of space between the stars.
Travis Metcalfe, Ph.D., is a researcher and science communicator based in Boulder. The Lab Notes series is made possible in part by a research grant from the National Science Foundation.