This artist’s concept pictures the planets orbiting
Barnard’s Star, as seen from close to the surface of one of them.
Image credit: International Gemini
Observatory/NOIRLab/NSF/AURA/P. Marenfeld
The Discovery
Four rocky planets much smaller
than Earth orbit Barnard’s Star, the next closest to ours after the three-star
Alpha Centauri system. Barnard’s is the nearest single star.
Key Facts
Barnard’s Star, six light-years
away, is notorious among astronomers for a history of false planet detections.
But with the help of high-precision technology, the latest discovery — a family
of four — appears to be solidly confirmed. The tiny size of the planets is
also remarkable: Capturing evidence of small worlds at great distance is a tall
order, even using state-of-the-art instruments and observational techniques.
Details
Watching for wobbles in the light
from a star is one of the leading methods for detecting exoplanets — planets
orbiting other stars. This “radial velocity” technique tracks subtle shifts in
the spectrum of starlight caused by the gravity of a planet pulling its star
back and forth as the planet orbits. But tiny planets pose a major challenge:
the smaller the planet, the smaller the pull. These four are each between about
a fifth and a third as massive as Earth. Stars also are known to jitter and
quake, creating background “noise” that potentially could swamp the
comparatively quiet signals from smaller, orbiting worlds.
Astronomers measure the
back-and-forth shifting of starlight in meters per second; in this case the
radial velocity signals from all four planets amount to faint whispers — from
0.2 to 0.5 meters per second (a person walks at about 1 meter per second). But
the noise from stellar activity is nearly 10 times larger at roughly 2 meters
per second.
How to separate planet signals from
stellar noise? The astronomers made detailed mathematical models of Barnard’s
Star’s quakes and jitters, allowing them to recognize and remove those signals
from the data collected from the star.
The new paper confirming the four
tiny worlds — labeled b, c, d, and e — relies on data from MAROON-X, an
“extreme precision” radial velocity instrument attached to the Gemini Telescope
on the Maunakea mountaintop in Hawaii. It confirms the detection of the “b”
planet, made with previous data from ESPRESSO, a radial velocity instrument
attached to the Very Large Telescope in Chile. And the new work reveals three
new sibling planets in the same system.
Fun Facts
These planets orbit their red-dwarf
star much too closely to be habitable. The closest planet’s “year” lasts a
little more than two days; for the farthest planet, it’s is just shy of seven
days. That likely makes them too hot to support life. Yet their detection bodes
well in the search for life beyond Earth. Scientists say small, rocky planets
like ours are probably the best places to look for evidence of life as we know
it. But so far they’ve been the most difficult to detect and characterize.
High-precision radial velocity measurements, combined with more sharply focused
techniques for extracting data, could open new windows into habitable,
potentially life-bearing worlds.
Barnard’s star was discovered in
1916 by Edward Emerson Barnard, a pioneering astrophotographer.
The Discoverers
An international team of scientists led by Ritvik Basant of the University of Chicago published their paper on the discovery, “Four Sub-Earth Planets Orbiting Barnard’s Star from MAROON-X and ESPRESSO,” in the science journal, “The Astrophysical Journal Letters,” in March 2025. The planets were entered into the NASA Exoplanet Archive on March 13, 2025.
Source: Discovery Alert: Four Little Planets, One Big Step - NASA Science
