An Earth-Sized Exoplanet in Our Cosmic Neighborhood

Astronomers have found a temperate world orbiting a star just 11 light-years away.

Discovering an exoplanet isn’t what it used to be. Since the first detection of a planet around another star in 1995, astronomers have found thousands more, thanks in large part to the Kepler Space Telescope, which—truly an overachiever—has discovered nearly 5,000 potential worlds and verified about half of them. Even the discovery of the most exciting kind of exoplanet—an Earth-sized world orbiting inside a star’s habitable zone—has become routine: Kepler has confirmed the existence of more than 30 of them.

But that doesn’t mean the discoveries are boring.

A rocky planet similar in size and temperature to Earth is orbiting a nearby star called Ross 128, astronomers announced Wednesday. The star is a red dwarf, a type of star that is smaller and cooler than stars like our sun, located just 11 light-years from our solar system.

This makes the planet, which has been named Ross 128 b, the second-closest temperate, Earth-sized planet to our little corner of the universe. The closest is Proxima b, which resides four light-years away in the habitable zone of Proxima Centauri, our nearest star, and was discovered last year.

Ross 128 b was found by ESO’s High-Accuracy Radial-Velocity Planet Searcher (HARPS), an instrument located at the La Silla Observatory in Chile, and the discovery is described in a paper in Astronomy and Astrophysics. Astronomers at HARPS spent 10 years gathering and refining data, which included more than 160 measurements of Ross 128, before they found the signal that turned out to be an exoplanet, said Xavier Bonfils, the paper’s lead author, an astronomer at Institut de Planétologie et d’Astrophysique de Grenoble in France. “To detect such a small signal requires a lot of data,” Bonfils said.

Stars like Ross 128 move ever so slightly because of the gravitational tug of nearby objects. These tiny movements can be seen as changes in a star’s light spectrum; when the star moves toward an observer—in this case, the HARPS instrument—its light looks slightly bluer, and when it moves away, it turns redder. If these shifts occur in regular patterns, that means there’s an exoplanet lurking. Astronomers can analyze this information to determine some of the planet’s properties. Nicola Astudillo-Defru, an astronomer at the University of Geneva and Bonfils’s coauthor, said he reacted with “a big wow” when the data showed a rocky and temperate world. “I feel so excited that I mailed Xavier minutes later with a clear URGENT in the subject,” he said in an email.

Astronomers say Ross 128 b completes an orbit around its star every 9.9 days. They estimate the exoplanet has an equilibrium temperature somewhere between -60 degrees and 20 degrees Celsius (-76 degrees to 68 degrees Fahrenheit).

Astronomers don’t know whether Ross 128 b resides in its star system’s habitable zone, that sweet spot where temperatures are just right for liquid water to pool on the surface of a rocky world. But they’re still hopeful that it might be able to support life. Red dwarfs like Ross 128 are dimmer than sun-like stars, emitting infrared instead of visible light. So while Ross 128 orbits 20 times closer to its star than the Earth does to the sun, the exoplanet receives only 1.38 times more stellar radiation than Earth. Bonfils said Ross 128 is “quieter” than other red dwarfs, like Proxima Centauri, which can unleash flares that bathe orbiting planets in ultraviolet and X-ray radiation.

“Some even think that the atmosphere can erode, can evaporate, due to these activities,” Bonfils said. The quiet nature of Ross 128 may have created a comfortable cosmic environment for life on the exoplanet to take hold, despite its planet’s close proximity to the star.

Ross 128 popped into headlines in July of this year, when astronomers at the Arecibo Observatory in Puerto Rico said they had detected a mysterious radio transmission coming from the star for about 10 minutes in mid-May. Stars can emit electromagnetic radiation in the form of radio waves, but this signal was recorded at a frequency scientists hadn’t seen before in red dwarfs. Arecibo and other telescopes quickly trained their eyes on Ross 128 for more observations, but the transmission never appeared again. The astronomers entertained several possible explanations, including—much to the internet’s delight—extraterrestrials. They eventually said the signal probably came from one or more geostationary satellites. But the discovery of an Earth-sized exoplanet around the star could restart some of that conversation.

“We are considering additional follow-up in light of the new discovery at radio and optical wavelengths,” said Andrew Siemion, the director of the Berkeley SETI Research Center who runs the Breakthrough Listen Initiative, aimed at finding evidence of extraterrestrial civilizations. The Breakthrough Listen team helped Arecibo astronomers observe Ross 128 this spring. “Nearby exoplanets are particularly exciting from a SETI perspective as they permit us to search for and potentially detect much weaker signals than from more distant targets.”

While Ross 128 is, in cosmic terms, just down the street from us, it would take a spaceship traveling at the speed of light 11 years to reach it. Humans, at least the kind Earth has today, will never reach their stellar neighbor. But they can get a closer look. In the next decade, powerful telescopes like ESO’s Extremely Large Telescope will be able to sniff out the atmospheres of distant exoplanets like Ross 128 b for hints of biomarkers like oxygen. This technology will usher in a new era in the search for exoplanets, in which astronomers learn not only that more are out there—and certainly, more are—but discover what they’re truly made of. The detection of life-giving molecules in the atmosphere of a distant world will be a momentous occasion. Perhaps thousands of those discoveries, on many worlds, will follow. Perhaps someday, that will start to become routine, too.