The astronomy world was set aflame after news broke in 2016 that an Earth-like planet was orbiting in the habitable zone around Proxima Centauri, our nearest star neighbor at just over four light-years away. Since then, more details have emerged that paint a clearer picture of what the planet, now called Proxima b, might be like.
One 2016 study, conducted by a team of astronomers and astrophysicists from France’s National Center for Scientific Research (CNRS), suggested Proxima b might be an ocean planet reminiscent of the 1995 Kevin Costner film, "Waterworld," covered entirely or almost entirely by a liquid ocean.
“The planet may very well host liquid water on its surface, and therefore also some forms of life,” the CNRS team wrote in a statement. “The planet could be an 'ocean planet,' with an ocean covering its entire surface, and similar water to some icy moons around Jupiter or Saturn.”
The waterworld scenario was just one possible conclusion revealed by the analysis, but it's an exciting possibility to imagine. If true, any creatures that have evolved on Proxima b might have body forms streamlined for movement through water, such as what we see in fish and cetaceans. Or perhaps it's an ocean world bobbing with gelatinous, jellyfish-like aliens.
To reach their conclusions, the team used a compilation of the latest data, best-guess estimates and computer simulations to determine the probable mass distribution of the planet. They calculated Proxima b's radius is likely between 0.94 and 1.4 times that of Earth’s. If it turns out to be at the higher-radius estimates in that range, that's where the ocean world scenario comes in. The planet would be covered by a global sea around 124 miles (200 kilometers) deep.
If Proxima b's radius falls in the lower range, that's exciting, too. It would mean the planet is likely surrounded by a rocky mantle, like Earth. Surface water would probably make up around 0.05 percent of its mass, which is similar to our blue world.
Of course, the planet could also be barren and lifeless. Another study, published in February 2018, provides some cause for caution in setting expectations for the nearest known exoplanet. The study's authors detected a massive stellar flare from Proxima Centauri, and this energetic blast of radiation reached 10 times brighter than our sun's largest flares when observed at similar wavelengths.
The flare increased Proxima Centauri's brightness by 1,000 times over 10 seconds. And according to study co-author Meredith MacGregor, an astronomer at the Carnegie Institution for Science, it raised doubts about Proxima b's habitability.
"It's likely that Proxima b was blasted by high-energy radiation during this flare," MacGregor says in a statement, noting it was already known that Proxima Centauri experienced regular, albeit smaller, X-ray flares. "Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water."
Life finds a way
That still may not rule out life on Proxima b, however. In April 2019, researchers from Cornell University published a paper noting that all life on Earth today evolved from creatures that survived even more UV radiation than Proxima-b and other nearby exoplanets currently experience. The Earth of 4 billion years ago was "a chaotic, irradiated, hot mess," according to a news release from Cornell, yet life nonetheless managed to persist and eventually proliferate.
"Given that the early Earth was inhabited," the researchers write, "we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars. Our closest neighboring worlds remain intriguing targets for the search for life beyond our solar system."
It's impossible to know for sure with current data, but it's still fascinating to imagine a potentially Earth-like world so close to home. And while Proxima b may now seem less promising than originally thought, it's still an encouraging hint of all the diverse exoplanets we're only beginning to discover and understand.
Editor's note: This article was originally published in October 2016, and has been updated with new information.