This makes it the second closest known exoplanet to us. Previously, an exoplanet was found orbiting in the three-star Proxima Centauri system.
The exoplanet was found after stitching together 20 years of data, including 771 individual measurements, from seven instruments. The analysis that led to the discovery is detailed in a study published Wednesday in the journal Nature.
For years, astronomers thought they would find a planet around the nearby star, but it eluded them.
"The biggest 'kick' about this discovery is the host star," Paul Butler, study co-author and astronomer at the Carnegie Institution for Science, wrote in an email. "Barnard's star is the 'great white whale' of planet hunting."
The planet, known as Barnard's star b, is probably dimly lit by its star and slightly colder than Saturn. The researchers believe that it is an icy desert with no liquid water, a hostile environment where the average surface temperature is around minus-274 degrees Fahrenheit.
The red dwarf star itself emits only about 0.4% of our sun's radiance, so the planet receives about 2% of the intensity that Earth receives from its sun. This is because Barnard's star is in the class of M dwarf stars, cooler and less massive than our sun. It's also an old star that predates our own solar system.
And to look at it through a telescope, the star appears to be moving the fastest among the other stars in the night sky. This is because it's moving quickly in relation to the sun, and it's the nearest single star in the sky to us, Butler said.
"The star is named in honor of the great American astronomer Edward Emerson Barnard, who was a pioneer of stellar photography and astrometry," Butler said. "He recognized that this star had the largest known proper motion a century ago."
The planet is about the same orbital distance from its star as Mercury is from our sun, making a full pass around the star every 233 days. This places it in the "snow line" of the star, where it's cold enough for water to freeze into solid ice. This region in a planetary system is where the building blocks of planets are thought to form, collecting material to become cores. As they migrate closer to their host stars, gathering more material, they become planets.
It's the first time a planet this small and distant from its star has been detected using the radial velocity technique, which Butler helped pioneer. This method is sensitive to the mass of the exoplanet and measures changes in the host star's velocity. Instruments can be used to detect tiny wobbles in the star's orbit that are caused by the planet's gravity.
"I think this discovery shows the power of the [radial velocity] technique for detecting longer period, small planets that are much harder or not possible to detect with missions like Kepler and TESS, which focus on finding transiting exoplanets in shorter orbital periods," Johanna Teske, study co-author and Hubble Fellow at the Carnegie Institution for Science, wrote in an email. "This study sets a wonderful example of collaboration and coordination across multiple teams and multiple data sets, something that doesn't always happen successfully in exoplanet research. It is only by combining data and working collaboratively that this very challenging detection was possible."
These methods haven't always been available to astronomers searching for exoplanets. For most of the past hundred years, the only way was the astrometric technique, in which astronomers look for the host star to wobble relative to background stars, Butler said. It worked only for the nearest stars and was achieved by taking photographs of the star and measuring its positions in relation to one another.
"This made Barnard's star the most important star in the sky because it is the nearest single star in the sky," Butler said.
In the 1930s, Dutch-American astronomer Peter van de Kamp began a quest to study Barnard's star that lasted for most of his 93 years. His claims of how planets could fit in orbit around the star were refuted, and he died five months before the first verifiable discovery of an exoplanet was made in May 1995, Butler said.
"He worked hard at improving the only technique at that time that had a prayer of finding planets, and spent decades collecting the data," Butler said. "Van de Kamp is a true pioneer in extrasolar planets."
Given its proximity to our solar system and its long orbit, future missions and telescopes will be able to provide new insights about Barnard's star b.
"Future space-based telescopes like WFIRST might be able to observe reflected light from Barnard's star off of the planet, and thus tell us something about the composition of the surface and/or atmosphere of the planet," Teske said.