By cosmic standards, Proxima Centauri is right next door. At 4.2 light-years away, it is the closest star to our sun, which makes the planets around Proxima the closest planets to us of any in the universe.
Right now, one of the best ways to get information about our neighboring planets is from a very sophisticated telescope instrument in Chile that astronomers use to peer beyond Earth’s atmosphere. Exoplanets are everywhere, and researchers have detected several thousand of them around the Milky Way, but there’s something special about finding an exoplanet close to home. And that instrument—the Echelle Spectrograph for Rocky Exoplanet and Stable Spectroscopic Observations, or ESPRESSO—recently gave astronomers that extra kick, evidence of a new potential planet.
Even this close, we can’t swing by for a visit, of course, not with current technology. But if human beings ever embark on an interstellar journey, Proxima Centauri is the place they’ll go. Researchers, no doubt fueled by actual cups of espresso, are already thinking deeply about what it would take to reach the cosmic neighborhood where the star resides, starting with tiny robots that could fly faster than traditional spacecraft and shorten the journey from thousands of years to mere decades. And, as with any long, expensive, and high-stakes journey, it's nice to do a bit of research ahead of time about what you might encounter at your destination.
The new maybe-planet isn’t a planet until further study, but if the data check out, the discovery would mean that Proxima Centauri has its very own planetary system, with worlds brewed from their star’s leftover gas and dust just as our own planets were. In 2016, astronomers found Proxima b, a planet about the size of Earth, orbiting in the star’s habitable zone, and have since confirmed its existence. They’re still working to confirm Proxima c, the larger planet they detected in 2019. Now they’re tentatively excited about this latest candidate planet, which they’re calling Proxima d, and which looks like an even better prospect than Proxima c.
Astronomers can’t actually see any of the Proxima planets, but they can detect signs of their presence by observing the movements of their star. As the planets go round, their gravity tugs at Proxima Centauri, causing the star to wobble ever so slightly. João Faria, an astrophysicist at the Instituto de Astrofísica e Ciências do Espaço, in Portugal, and his colleagues were studying Proxima Centauri in this way, trying to glimpse Proxima b to help confirm its existence, when they found a surprise wobble that they believe comes from another, smaller planet.
According to the data, Proxima d is about a quarter of Earth’s mass and orbits just outside of Proxima Centauri’s habitable zone, locked into a special configuration around its star. “One side of the planet is always in daytime, and one side is always in nighttime,” Emily Gilbert, a Ph.D. candidate in planetary science at the University of Chicago who studies exoplanets, told me. Proxima d takes just five days to loop around its star, and given the proximity, the planet probably doesn’t have an atmosphere, Gilbert said; it would be boiled away on the day side, and frozen off on the night side.
With news of Proxima d now out in the world, it’s up to other astronomers to analyze the available ESPRESSO data and also make their own observations of the maybe-planet. To learn more about this maybe-planet, particularly its composition, astronomers would need to spot Proxima d crossing in front of its star and examine how much starlight the planet blocks. Perhaps, along the way, they will uncover a new mysterious signal, as Faria and his team did when they observed Proxima b. More worlds could very well be circling Proxima Centauri, escaping detection. “I really wouldn’t want to bet, but I think this star can still be hiding some surprises,” Faria told me.
Proxima is actually part of a triple-star system, known all together as Alpha Centauri. Alpha Centauri A and B orbit close together, while Proxima circles them farther out. Astronomers have attempted to detect planets around stars A and B as well, but the latest detection doesn’t look too promising, and the last one before that was determined to be a spurious signal in the observations. Still, Alpha Centauri A and B are particularly enticing in the search for life beyond Earth because they resemble our own sun. Proxima, on the other hand, is a red-dwarf star—smaller, dimmer, and prone to frequent flares that emit radiation. “Red dwarfs are not very good for habitability,” Eduardo Bendek, a research scientist at NASA’s Jet Propulsion Laboratory, told me. “They sterilize the surface of planets around them.” But Alpha Centauri A and B are too bright for our best exoplanet-detecting instruments, and it’s tricky to determine whether any wobbling in their motions is the result of a lurking planet or just the stars themselves tugging at each other, Bendek said.
Bendek is working on one of several missions percolating in the space community that target our neighboring triple-star system. His project involves building a brand-new telescope specifically designed to observe nearby, bright stars such as Alpha Centauri A and B. Another ambitious effort involves a flyby of tiny, laser-propelled spacecraft that could beam home pictures. For decades, the stars of the Alpha Centauri system, particularly Proxima, have played in science fiction as the destination for humankind’s first interstellar trip. Our first attempt won’t be as grand as what those stories imagined. It will instead resemble our earliest voyages to explore the planets in our own system, when spacecraft flew past without stopping, trying to snap many pictures as they went. “This is basically the only system where we would have even a tiny chance of getting something there in our lifetimes,” Gilbert said.