An artist’s impression shows the newly found half-Earth-mass exoplanet Barnard b, with its red dwarf sun in the background. Credit: ESO/M. Kornmesser
As lone stars go, there’s nothing quite so distinctive as Barnard’s Star. After the Alpha Centauri system, it is our closest neighbor, a red dwarf one-fifth the size of the Sun and 4,500 degrees Fahrenheit (2,500 degrees Celsius) cooler. Just 6 light-years away, it outpaces all other stars by shifting across the night sky by an apparent distance wider than half a Moon per century, a phenomenon known as “proper motion.”
A paper published this week in Astronomy and Astrophysics has given Barnard’s Star even more shine with the announcement of a long-sought-after exoplanet that scientists are calling Barnard b. Less than half the mass of Earth, it circles its host star every three days on an orbit 20 times closer in than Mercury’s is to the Sun. The discovery spotlights both the rapidly improving science of exoplanet detection and the potential for finding life in our own backyard — especially around the Milky Way’s most abundant stars, red dwarfs.
Back and forth
The hunt for worlds surrounding Barnard’s Star dates to the 1960s. A string of apparent discoveries and breathless announcements were all dashed within years due to errors in instrumentation and computational methods. As recently as 2018, a supposed exoplanet thought to orbit the star every 233 days was soon found to be a phantom, an artifact of insufficient precision and noise from factors like starspots (the extrasolar equivalents of sunspots).
Related: The complicated history of planets around Barnard’s star
Precision was key to this new study’s success, and its discovery of Barnard b is testament to how far exoplanet science has advanced in a few short years.
“Our goal was to design an instrument able to see the signal of an Earth,” says Jonay González Hernández, the study’s lead author, who was also involved in the 2018 paper.
That signal, known as radial velocity, is the wobble in its host star that a planet’s gravity exerts as it orbits. For Earth, that tug on the Sun moves our star back and forth at roughly 3.5 inches (9 centimeters) per second. Until 2018, instruments could not distinguish a wobble this minuscule from factors such as starspots — the dark spots can effectively mimic a star’s wobble as they rotate in and out of view.
To measure such a small aberration, González Hernández needed an instrument 10 times more sensitive than anything then available. He and his colleagues joined with teams in Switzerland, Italy, and Portugal to build an instrument called ESPRESSO — the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations — under the aegis of the European Southern Observatory (ESO).
“ESPRESSO is very precise, and this has made all the difference,” González Hernández says. Fitted on one of the four 8.2-meter telescopes of ESO’s Very Large Telescope in Chile, ESPRESSO features optical fibers connected to a core instrument housed in a sealed vacuum chamber under meticulously controlled environmental conditions. This minimizes errant noise from fluctuations in temperature and atmospheric pressure that can throw off measurements.
Radial velocity spectrographs work by capturing the spectral fingerprints of starlight produced by a combination of chemical elements. Different types of stars have characteristic spectra generated by the abundance of various elements in their atmospheres. By measuring the changes of thousands of spectral lines, scientists are able to calculate radial velocity based on the shifting of the spectrum toward redder or bluer wavelengths.
Using a sophisticated technique that compiles and isolates the predictable effects of planetary motion from the more unpredictable dynamics of stellar motion, the team managed to crack the 80-year-old case of Barnard’s Star.
“We finally started to see something when we reached 100 measurements,” says González Hernández, “and then we were sure that the planet was there.”
Scientists discovered that Barnard’s Star wobbles a full 20 inches (50 cm) per second — five times Earth’s effect on the Sun — placing Barnard b well within the detection range of ESPRESSO. The planet’s short period also allowed the team to capture hundreds of orbits during the four-year study, resulting in an extremely robust dataset.
An alien world
With a surface temperature estimated at 260 F (126 C), Barnard b is an unlikely candidate to host life. But the authors are already searching for Earth-sized companions that may be waiting in the wings. These may even lie in Barnard’s Star’s habitable zone, the Goldilocks-like region where watery planets with temperate climates might thrive.
“In a star as cold as Barnard,” says co-author Alejandro Suárez Mascareño, “the habitable zone corresponds to orbital periods between 10 and 40 days.”
After spending years tracking down Barnard b, González Hernández and Suárez Mascareño also wonder what the view from the surface of the exoplanet might be like. By their calculations, Barnard’s Star would appear eight times larger than the Sun from Earth and feature huge starspots visible when the star rises and sets.
“You would see the surface of the star changing over days or months,” says Suárez Mascareño, “and that means that irradiation on the planet could change with stellar rotation and even create seasons.”
Caltech astrophysicist Jessie Christiansen, who is not an author on the paper, is encouraged by the discovery of Barnard b.
“It is very exciting that the closest star systems have rocky planets in them, and even more exciting that it seems rocky planets are very common in our galaxy,” says Christiansen, who is the project scientist of the NASA Exoplanet Archive where more than 5,700 exoplanets have already been cataloged. “Every new discovery seems to give us the same answer, that everywhere we look with enough precision, we are finding rocky planets. This leads to the next huge question. How many of them are habitable? And from there, how many have life?”
