Our nearest neighboring star, Proxima Centauri, may be harboring an extensive solar system, as scientists believe they’ve found a third planet orbiting it. It’s a find that re-emphasizes just how commonplace planets outside our Solar System may be — and it provides us with a third possible world nearby to study and potentially explore.
Located a little more than 4 light-years from Earth, Proxima Centauri has long captured the imagination of scientists and sci-fi enthusiasts as a prime place to visit if we ever venture far outside our Solar System. The celestial object became even more intriguing in 2016 when astronomers found a planet orbiting around it. Called Proxima b, the planet is located in the star’s habitable zone, where temperatures may be just right for water to pool on the surface. Just a few years later, a second planet, called Proxima c, was discovered around the star, too.
With the discovery of this third likely exoplanet, called Proxima d, Proxima Centauri is possibly home to a wide array of worlds. While we do not have the means to travel to Proxima Centauri yet in any kind of reasonable time frame, these planets might be the first places we’d visit if we ever do develop such a capability. For now, their close proximity to Earth makes them prime candidates for follow-up study and observations. Astronomers say they could help us in our ever-evolving quest to understand how planets form around distant stars.
“I think it helps us indeed understand planet systems as a whole or more,” João Faria, lead author of a study in Astronomy & Astrophysics describing the discovery and a researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal, tells The Verge. “The more we find, the more we can understand them.”
Faria and his team stumbled on evidence for Proxima d inadvertently. The discovery came to light as they were trying to confirm that the first exoplanet, Proxima b, actually existed. In 2019, while working with the European Southern Observatory’s Very Large Telescope, or VLT, in Chile, they observed Proxima Centauri to home in on Proxima b. But in their data, they noticed the hint of another exoplanet in the system.
“It came as a surprise initially,” says Faria. The team kept spying on the star with the VLT from 2019 to 2021 to figure out more about this mysterious third planet.
Finding exoplanets is an incredibly tough business since these distant objects are often completely drowned out by the light from the stars they orbit. Typically, astronomers must infer that an exoplanet is present. One way to do that is by watching how a star’s brightness changes over time. If a planet passes in front of a distant star relative to Earth, the world momentarily blocks some of the star’s light, causing the star to dim ever so slightly. It’s a phenomenon we can pick up and use to learn more about the exoplanet.
Astronomers aren’t sure yet if Proxima d passes in front of Proxima Centauri from our vantage point on Earth. Instead, the team found evidence for the world by watching Proxima Centauri’s ever-so-slight wobbles, the same way Proxima b was found. Even small exoplanets have a gravitational effect on their stars, albeit a small one. Their presence can cause the parent star to teeter slightly, typically in a pattern we can view from Earth. This technique is how Faria and his colleagues were able to determine that Proxima d likely exists.
For now, Proxima d is just a candidate, so its existence isn’t confirmed yet. But based on those wobbles, we know a few enticing things about Proxima d if it’s real. It’s thought to be about a quarter of the mass of Earth, making it the lightest planet in the Proxima Centauri system and among the lightest exoplanets ever found. “If confirmed this discovery would enable astronomers to study an exoplanet with a mass smaller than that of the Earth, right next door in our neighboring planetary system,” Diana Dragomir, an assistant professor at the University of New Mexico specializing in exoplanets, who was not involved in the study, writes in an email to The Verge.
On top of that, the planet is practically hugging its parent star. Located at less than a tenth the distance from Mercury to the Sun, the exoplanet takes just five days to orbit around Proxima Centauri. That may seem fast, but Proxima b only takes 11 days on its orbit. Because Proxima d is so close to Proxima Centauri, the discovery team believes that the planet’s surface is probably far too hot for water to remain as a liquid — placing it outside the habitable zone.
Even if Proxima d was in the habitable zone, it’s very unclear if life could survive on the surface. Proxima Centauri is a red dwarf, much smaller and dimmer than our Sun, meaning the type of light these planets receive is very different than the kind we get on Earth. Plus, red dwarf stars are known to flare much more frequently and more powerfully than stars like our Sun, meaning Proxima Centauri is constantly raining down radiated particles onto its nearby planets. Even without all those flares, these planets are bombarded with significantly more radiation than we receive here on Earth because of their close orbits.
But even if life as we know it may find it hard to survive there, scientists are eager to learn more about planets around red dwarfs like Proxima Centauri. And a close solar system like this one is a great place to study first. “We still have questions about what kind of planets form around these stars,” Faria says, “if they’re the same as the planets in the Solar System or if they’re different.”