We've observed an alien world orbiting Barnard's Star, a tiny red dwarf that's only six light-years away, making it the second-closest known exoplanet beyond our solar system. Known as a "super-Earth," the planet (designated Barnard's Star b, or GJ 699 b) is thought to be at least 3.3 times the mass of Earth and orbits its star once every 233 days.
Barnard's Star is the fourth-closest star to our sun. Alpha Centauri's triple-star system (including Alpha Centauri A and B, plus Proxima Centauri) are the only stars nearer. Proxima Centauri is the closest star to have a known exoplanet in orbit, Proxima Centauri b. That world is slightly more massive than Earth, is located only 4.2 light-years away and has been pelted by solar flares, dashing hopes of it hosting life.
While an exciting (and historic) find, you can forget about Barnard's Star b bearing any resemblance to our planet. Barnard's Star is a very low mass and dim red dwarf that produces only 0.4 percent of the radiant power our sun generates. That means its "habitable zone" is extremely compact, and the exoplanet orbits beyond the star's "snow line." The snow line around any star is the distance beyond which water cannot exist in a liquid state on a planet's surface. The exoplanet has a predicted surface temperature of -170 degrees Celsius (-274 F), making it wholly incompatible for life (as we know it, anyway).
Still, this new exoplanetary discovery is exciting. Super-Earths are like nothing we have in our solar system and have only been discovered orbiting other stars more distant than Barnard's Star. These alien worlds occupy the mass range between the small rocky planets (like Earth, Mars and Venus) and the larger gaseous planets (like Neptune). Knowing that we have one of these strange exoplanets so nearby could allow us to get to know this planetary species a little better.
Although it's on our interstellar doorstep, discovering the Barnard's Star super-Earth took an international team of astronomers using decades of spectroscopic data of the star to find it.
"For the analysis we used observations from seven different instruments, spanning 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies," said Ignasi Ribas, of the Institut de Ciènces de l'Espai (ICE, CSIC), Spain, in a statement. Ribas is the first author of the study published in the journal Nature.
The radial velocity method used in exoplanet hunting requires precise observations of a star's spectrum. When starlight is received by telescopes, its spectrum can be split into its component wavelengths — such as infrared, visible and ultraviolet. However, if astronomers record observations of this starlight over many years, they may notice slight periodic frequency shifts. This is how we find exoplanets actually: As they orbit their host stars, their gravity causes their stars to wobble, pulling them toward and away from the telescope on Earth, creating a frequency shift that corresponds to the exoplanet's mass orbital period. Unlike NASA's Kepler and new Transiting Exoplanet Survey Satellite (TESS) – which both detect the slight dimming of starlight as an exoplanet orbits in front of its host star (known as a "transit") – the radial velocity method doesn't depend on detecting this dip in light to realize the presence of exoplanets around stars.
"This technique has been used to find hundreds of planets," said collaborator Paul Butler, of Carnegie Institution for Science and one of the pioneers of the radial velocity method, in a statement, "We now have decades of archival data at our disposal. The precision of new measurements continues to improve, opening the doors to new parameters of space, such as super-Earth planets in cool orbits like Barnard's Star b."
Since this exoplanet is so close, astronomers hope that they will be able to use it as a target for the next generation of space telescopes, such as the planned NASA Wide Field Infrared Survey Telescope (WFIRST). This makes Barnard's Star b a prime candidate for us to use powerful spectroscopic techniques to, one day, peer into its atmosphere (if it has one) and understand what it's really made of.