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Astronomers have discovered not one, but four tiny planets orbiting our cosmic neighbor, Barnard's Star. The planets, each about a fifth the mass of Earth, orbit their host star in just a few days — probably too hot for life, but an important milestone in the discovery of smaller planets.
New planets around Barnard's Star
Astronomers have found compelling new evidence that Barnard's Star, the second-closest star system to Earth, contains not one but four small planets. Each of these planets is only about 20-30% the mass of Earth and orbits very close to its star, completing a full orbit in just a few days. Because of their proximity, they are likely too hot to support life. However, their discovery marks an important milestone in the detection of smaller planets around nearby stars.
"This is a truly exciting discovery—Barnard's Star is our cosmic neighbor, yet we know so little about it," said Ritwik Basant, a graduate student at the University of Chicago and first author of the study. "It signals a breakthrough in the accuracy of these new instruments over previous generations."
This new discovery builds on a previous study that used a different telescope and found strong evidence for at least one planet around Barnard's Star, with possible signs of others. The latest findings, published March 11 in The Astrophysical Journal Letters, researchers from the Gemini Observatory, the National Science Foundation's NOIRLab, Heidelberg University, and the University of Amsterdam participated.
The long search for planets
For a century, astronomers have studied Barnard's Star in the hope of finding planets around it. First discovered by E. E. Barnard at Yerkes Observatory in 1916, it is the closest system to us that has the same configuration as ours, with only one star. (The closest star system to us, Proxima Centauri, has three stars orbiting each other, which changes the dynamics of planet formation and orbits.)
Barnard's Star is an M dwarf star, which we now know is extremely large in the universe. So scientists would like to learn more about what kinds of planets it contains. The problem is that these distant planets are too tiny to be seen next to the brightness of their stars, even with our most powerful telescopes. That means scientists have had to come up with a way to find them.
Hunting for planets with star oscillations
One such effort was led by Professor Jacob Bean of the University of Chicago, whose team built and installed an instrument called MAROON-X, which is attached to the Gemini telescope atop a Hawaiian mountain and designed specifically to search for distant planets.
Because stars are much brighter than their planets, it’s easier to look for planetary influences on their stars, like following the wind by watching a flag move. MAROON-X is looking for one such effect; the gravity of each planet slightly shifts the star’s position, making it appear to wobble back and forth. MAROON-X measures the color of the light so precisely that it can pick up on these subtle shifts and even determine the number and masses of planets that must be orbiting the star to have this effect. Basant, Bean and the team carefully calibrated and analyzed data taken on 112 different nights over three years. They found strong evidence for three planets around Barnard’s Star. When the team combined their findings with data from another group’s November experiment, obtained by an instrument called ESPRESSO on the Very Large Telescope in Chile, they saw strong evidence for a fourth planet.
More than just gas giants
The scientists say these planets are likely rocky, not gaseous, like Jupiter. That will be difficult to determine with certainty; the angle at which we see them from Earth means we can't watch them transit in front of their star, which is the usual way to tell if a planet is rocky. But by gathering information about similar planets around other stars, we can make better guesses about their composition. However, the team was able to rule out other planets in the habitable zone around Barnard's Star with a fair amount of confidence.
"Really fascinating"
Barnard's Star is called the "great white whale" of planet hunters; several times over the past century, groups have announced evidence suggesting planets around Barnard's Star, only to be disproven. But these latest findings, independently confirmed in two separate studies by the different instruments ESPRESSO and MAROON-X, represent a much higher degree of certainty than any previous results.
"We were observing at different times of the night on different days. They're in Chile; we're in Hawaii. Our teams weren't coordinating with each other at all," Basant said. "That gives us a lot of confidence that these aren't phantoms in the data."
An exciting future for Discovery
These are some of the smallest planets found using this observational technique. Scientists hope this will mark a new era in the search for new and exciting planets in the universe. Most of the rocky planets we've found so far are much larger than Earth, and they seem to be fairly similar across the galaxy. But there's reason to believe that smaller planets will have a more diverse composition. As we find more of them, we can start to learn more about how these planets form and what makes planets habitable.
The thrill of the unknown
The find itself was also fascinating, scientists say.
"We worked very intensively on this data in late December, and I thought about it the whole time," Bean said. "It was like we suddenly knew something that no one else knew about the universe. We just couldn't wait to unlock this secret.
"A lot of what we do can be incremental, and sometimes it's hard to see the bigger picture," he said. "But we've found something that hopefully humanity will know about forever. That sense of discovery is incredible."
