Scientists have confirmed the existence of four small, rocky planets orbiting Barnard’s Star — the second closest star system to Earth — using a specialized instrument on the mighty Gemini North telescope in Hawaii. Just six light-years away from us, all the worlds are too hot to support life as we know it.
This find is particularly exciting, explained Ritvik Basant, who is a Ph.D. student at the University of Chicago and an author on a paper about the new discovery. This is because, he said, Barnard’s Star is essentially our cosmic neighbor, yet we don’t know very much about it.
There have been many claims of exoplanets orbiting Barnard’s Star over the years, dating all the way back to the 1960s. Barnard’s Star is a red dwarf, also known as an M-dwarf, and is noticeable for having the fastest proper motion, in reference to its motion visible in the night sky, of any star so far discovered.
Most recently, in 2024, astronomers using the ESPRESSO spectrograph on the Very Large Telescope in Chile claimed the detection of one planet, and evidence for a further three. Now, a team led by Jacob Bean and Basant at the University of Chicago has confirmed beyond a shadow of a doubt the existence of all four planets.
“Barnard’s Star’s proximity allowed us to observe it even during bad weather nights, as its brightness made it accessible even under suboptimal conditions. This enabled us to collect more data, ultimately leading to the detection of these very low-mass planets,” Basant told Space.com.
A key tool used in the team’s observations was the MAROON-X spectrometer, which is a visiting instrument on Gemini North. MAROON-X measures the “radial velocity” — the slight wobble back and forth of Barnard’s Star as it revolves around the center of mass shared between itself and the four orbiting planets. They’re all much less massive than Earth. In fact, they are some of the least massive exoplanets ever detected.
The innermost planet in the system is planet d (the planets are named in order of discovery, not distance from the star), which has a mass just 26% that of Earth’s and orbits Barnard’s Star every 2.34 days at a distance of 1.7 million miles (2.8 million kilometers/0.0188 astronomical units). Next up is planet b: the planet first identified in the ESPRESSO data in 2024. This planet has a mass 30% that of Earth’s, and orbits its star every 3.15 days at a distance of 2.13 million miles (3.4 million kilometers/0.0229 AU).
Planet c is the heavyweight of the bunch, with a mass 33.5% that of Earth’s. It orbits Barnard’s Star at a distance of 2.55 million miles (4.1 million kilometers/0.0274 AU) and has an orbital period of 4.12 days.
The first three planets were confirmed using just the MAROON-X observations. To confirm the fourth planet, e, the MAROON-X data had to be combined with ESPRESSO’s measurements to reveal a planet with just 19% of Earth’s mass, orbiting Barnard’s Star every 6.74 days at a distance of 3.56 million miles (5.7 million kilometers/0.0381 AU).
These worlds are incredibly compact in terms of distance to one another, with just 372,820 miles (600,000 kilometers) between planets d and b, and 434,960 miles (700,000 kilometers) between b and c. For comparison, the mean distance between Earth and our moon is just 238,600 miles (384,000 kilometers). Imagine having a planet on our doorstep at just twice that distance!
Yet, that is how things are arranged around Barnard’s Star.
For an even starker contrast, NASA’s Parker Solar Probe, which actually dives into the solar corona, gets as close as 3.9 million miles (6.2 million kilometers) to the surface of our sun. The orbits of all four planets around Barnard’s Star could easily fit inside Parker’s Solar Probe’s orbit. And, to further the contrast between our solar system and Barnard’s Star’s planetary system, the closest planet to the sun in our Solar System, Mercury, has a mean distance of 36 million miles (58 million kilometers) between itself and the sun.
The small separations between the planets around Barnard’s Star also bring to mind another system of worlds around a red dwarf, TRAPPIST-1, where seven planets are packed within 5.75 million miles (9.267 million kilometers) of their central star.
A red dwarf like Barnard’s Star is very different to our sun, however. It has just 16% of our sun’s mass, and 19% its diameter. As such, its planetary system is scaled down. Red dwarfs can also be very volatile, spewing clouds of charged particles and flares of radiation more frequently than our sun does, which could strip nearby worlds of their atmospheres. However, red dwarf activity does decrease with age, and the Barnard’s Star system is about 10 billion years old.
That said, none of the planets found so far would be habitable to life as we know it anyway, since they are too close and too hot. Instead, the habitable zone around Barnard’s Star would coincide with worlds farther out, with orbital periods of between 10 and 42 days. So far, no planets have been found that far out from the star.
“With the current dataset, we can confidently rule out any planets more massive than 40 to 60% of Earth’s mass near the inner and outer edges of the habitable zone,” sBasant said.. “Additionally, we can exclude the presence of Earth-mass planets with orbital periods of up to a few years. We are also confident that the system does not host a gas giant within reasonable distances.”
MAROON-X was able to gather 112 radial velocity measurements of Barnard’s Star throughout the period 2021–2023. Meanwhile, ESPRESSO has recorded 149 radial velocity measurements of the fleet-footed but diminutive star. This isn’t enough to completely rule out the possibility of any more small planets that might be lurking in the habitable zone.
“We also have additional data from 2024 that was not used in this discovery,” said Basant. “If I had to choose a number, I would estimate that 50 more data points would be ideal for achieving the best sensitivity possible with current instruments.”
MAROON-X is specifically designed for measuring radial velocities of red dwarf systems. The focus on red dwarfs is two-fold. One reason is that they are the most populous type of star in the galaxy and make up the majority of the closest stars to us. Second, their small masses make it easier to detect wobbles in their movements caused by Earth-size rocky planets. Placed on an eight-meter class telescope such as Gemini North, and able to view into the near-infrared where red dwarfs such as Barnard’s Star are brighter, MAROON-X is perfectly placed to seek these scaled-down planetary systems.
“This discovery was possible due to a combination of factors,” said Basant. “If I had to choose one, it would be the unprecedented precision of next-generation instruments like MAROON-X and ESPRESSO.”
Unfortunately, the four planets of Barnard’s Star do not transit, or pass in front of their star, from our point of view. This means that we cannot observe secondary eclipses (where the planets move behind their star, allowing us to subtract the star’s light from the combined light of the star and planets, to be left with just the light of the planets) or transit spectroscopy (where starlight is filtered through planetary atmospheres, if they have one, revealing molecules that may be present).
However, “hile these planets do not transit, their thermal emission can be studied with [the James Webb Space Telescope], though this remains challenging,” says Basant.
In the meantime, Basant, Bean and their team intend to keep looking for more planets orbiting Barnard’s Star. After all, we’re practically neighbors — and it’s about time that we found and got to know this planetary system next door.
The findings were published on March 11 in The Astrophysical Journal Letters.
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