Astronomers have identified a quartet of small rocky planets orbiting Barnard’s star - one of our closest stellar neighbors - though they concluded that all of them are too hot to harbor life, much like our solar system’s innermost planet Mercury.

At about 6 light years away, Barnard’s star is the nearest single star - one not orbiting with other stars - to our solar system. Only the three stars in the Alpha Centauri system, about 4 light-years away, are closer. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The researchers used data from the Hawaii-based Gemini Telescope and Chile-based Very Large Telescope to confirm the presence of the four planets around Barnard’s star. A study published last year using data from the Very Large Telescope had indicated the presence of one planet, with hints of three more.

Planets beyond our solar system are called exoplanets. Those orbiting Barnard’s star are among the smallest of the more than 5,800 exoplanets discovered since the 1990s as astronomers refine their ability to pinpoint such little worlds.

Barnard’s star, located in the direction of the constellation Ophiuchus, is a red dwarf, the smallest type of regular star. Its mass is about 16% of the sun’s and it is far less hot. But its four planets are orbiting so close that its heat has created surface temperatures that would seem to preclude life, much like the baked surface of Mercury.

“A key requirement for habitability is the presence of liquid surface water,” said Ritvik Basant, a doctoral student in astronomy at the University of Chicago and lead author of the study published this week in the Astrophysical Journal Letters, opens new tab.

“If a planet orbits too close to its star, any water would evaporate. If it’s too far, it would freeze. It turns out, all four planets orbiting Barnard’s star are too close to their host, making them too hot to sustain liquid water,” Basant said.

This is the only known star with a multi-planet system entirely comprised of planets smaller than Earth. The innermost planet has a mass 26% of Earth, the second has a mass 30% of Earth, the third has a mass 34% of Earth and the outermost of the four has a mass 19% of Earth. Each completes an orbit in just a few days.

To put their mass in perspective, Mars has about 11% the mass of Earth and Mercury has about 6%.

The four planets each travel in nearly perfectly circular orbits around Barnard’s star - all at less than the distance of Mercury’s orbit to the sun.

Astronomers refer to a “habitable zone” that exists around stars at a distance where planetary surface temperatures, like those on Earth, would allow for liquid water. Around Barnard’s star, the researchers ruled out the presence of any Earth-sized planets residing in the habitable zone but have not ruled out the possibility of other small planets in the system.

In the search for life beyond Earth, scientists are looking for potentially habitable planets that are rocky and warm like our own, rather than gas plants. With the new findings, astronomers now know that there are rocky planets orbiting in the Alpha Centauri and Barnard’s star systems, though none in the habitable zone. Two exoplanets have been detected in the Alpha Centauri system, both orbiting the red dwarf Proxima Centauri.

Various methods are used to detect exoplanets. In this study, the researchers used the “wobble” method, formally called “radial velocity.” The presence of a planet gravitationally tugs on its host star, causing the star to wobble ever so slightly. Telescopes can measure this movement, allowing astronomers to infer a planetary presence.

Astronomers have fine-tuned their ability to spot exoplanets through this method, thanks to increasingly sensitive instruments. The outermost of the Barnard’s star exoplanets is the smallest of the approximately 1,100 discovered using this method.

“This study showcases the growing capabilities of next-generation instruments in detecting low-mass planets. The four newly discovered planets orbiting Barnard’s star are all sub-Earth mass planets, a regime that remains largely unexplored beyond the solar system. This marks a significant step forward in the search for Earth-mass planets within the habitable zones of sun-like stars,” Basant said.