Astronomers are closely studying the closest stars, searching for exoplanets that may be orbiting these stars. Now, a team using the European Southern Observatory’s Very Large Telescope (VLT) has confirmed the presence of such a world orbiting Barnard’s star, the closest single star to our Sun. The planet (called Barnard b) is just over half the mass of Venus and orbits its star once every 3.15 Earth days.

Located a mere six light-years away, Barnard’s star is a red dwarf and the second-closest stellar system to Earth, behind the Alpha Centauri group. The latter is a triple-star system comprised of a binary star system (Alpha Centauri A and B) and a third star (Proxima Centauri) gravitationally bound to the Alpha Centauri system. While Proxima Centaury is the closest known star to the Earth, at a distance of about 4.24 light-years, Bernard’s star is the closest single star to us.

For years, astronomers suspected a planet orbited this star, but no detection had been confirmed—until now.

“We have been always focussing on trying to find Earth-like planets in the habitable zone of nearby stars. We initiated this study five years ago, with the goal of exploring the habitable zone of Barnard’s star and we have discovered a planet with a mass of about half the mass of Venus (or about three times the mass of Mars). This is a tiny planet that we call a sub-Earth,” Jonay González Hernández, a researcher at the Instituto de Astrofísica de Canarias in Spain and lead author of the new study, told ZME Science in an email.

An Elusive Nearby World

Though this planet may be intriguing, it’s far from being Earth-like. Barnard b, as the exoplanet has been christened, sits much closer to its star than Mercury does to the Sun—about twenty times closer. Even though Barnard’s star is cooler than our Sun, the planet’s surface reaches a scorching 125°C (257°F). At this temperature, liquid water, a key ingredient for life as we know it, cannot exist.

However, the possibility of additional planets remains. The study hints at the potential presence of three additional exoplanet candidates orbiting the star. These planets could provide further opportunities to explore how planetary systems form around low-mass stars.

Astronomers used a technique known as radial velocity to detect Barnard b. The team relied on ESPRESSO, an instrument that can detect the slight wobble of a star caused by the gravitational tug of an orbiting planet. Data from other instruments, including HARPS and CARMENES, supported the findings. Previously, there were earlier reports of an exoplanet around Barnard’s star, made in 2018, but could not be confirmed by the new data.

Radial velocity is especially effective for detecting small planets like the one found around Barnard’s star. The precision of the ESPRESSO instrument—capable of detecting velocity shifts as small as 10 cm/s—has allowed astronomers to see through the noise created by stellar activity, which often mimics planetary signals. By employing Gaussian process modeling to account for the star’s magnetic activity, the researchers could isolate the planet’s signal.

“Note that this precision is very similar to the gravitational effect of Earth over the Sun of about 9 cm/s. ESPRESSO instrument is almost one order of magnitude better than previous instruments,” said Hernández, adding that “Barnard’s star is a magnetically active M dwarf but compared to similar stars it is indeed somehow very quiet. However, the challenge is that the stellar effects (that we called stellar activity) on the star’s velocity are about 10 times larger than the effect of the sub-Earth-mass planet (Barnard b) that we have found.”

This discovery adds to a growing list of planets found in our stellar neighborhood, including Proxima b and d, which orbit Proxima Centauri. Each discovery brings scientists one step closer to understanding the distribution of small, rocky planets in our galaxy.

“We know already that most stars have at least one planet. The fact that we are now with ESPRESSO exploring a new domain, with planets in the sub-Earth regime, means that most nearby stars could host planets and planetary systems that were not seen before because we did not have the capability to discover them,” said Hernández.

This discovery marks another milestone in exoplanet research, showcasing the increasing capability of ground-based telescopes in detecting small, rocky worlds outside our solar system. While space telescopes like Kepler and TESS have been instrumental in finding exoplanets, the ESPRESSO spectrograph brings an unmatched level of detail, which could be crucial for future discoveries, including the long-sought Earth twin.

Even more ambitious explorations are expected. The European Southern Observatory’s Extremely Large Telescope (ELT), set to be completed within the next decade, will be equipped with a state-of-the-art instrument known as ANDES. This tool will enable astronomers to study the atmospheres of exoplanets, particularly those in the so-called “temperate zone,” where conditions might allow for liquid water.

While Barnard b may not host life as we know it, these findings remind us that our cosmic backyard is far from empty. As technology advances, we may be on the verge of uncovering even more small, rocky worlds around nearby stars.

“We will continue exploring the habitable zone of Barnard’s star and other nearby stars such as Proxima Centauri, to search for even smaller planets out there,” said Hernández.