Barnard's star tricked scientists before: why this planet is real.

Scientists say they've finally found a world orbiting Barnard's star, the closest single star to the sun.

This rocky planet, discovered with the European Southern Observatory's Very Large Telescope in Chile, is smaller than Earth, and flies around its tiny cool star every three days. Barnard's star is six light-years away from us, in the constellation Ophiuchus. The only closer stars are the trio that make up the Alpha Centauri system. 

Anyone have déjà vu? Yes, exoplanet hunters have thought before they'd found worlds around this particular star — a popular sci-fi setting — and their discoverers seemed pretty darn sure of their data, too. 

But this one is the real deal, a new research team says. Honest. 

"The discovery of this planet, along with other previous discoveries such as Proxima b and d, shows that our cosmic backyard is full of low-mass planets," said Alejandro Suárez Mascareño, a coauthor of the paper published in the journal Astronomy & Astrophysics today. 

Despite its close proximity to our solar system, Barnard's star is too faint to see with the naked eye. A simple eight-inch amateur telescope should be able to spot it in deep space, though. 

This special star, a target of exoplanet searches for over a century, has a long history of false positive planet detections. Six years ago, an international team of researchers thought they had found a planet three times larger than Earth, just outside the so-called habitable zone, in the Barnard's star system. The authors of that paper, published in the journal Nature, characterized the world as frozen and circling its red dwarf host star every 233 days.

That potential discovery, using the radial velocity method, relied on 771 observations over two decades. Though at the time it was considered an unconfirmed candidate, the lead scientist had said in a statement then that he was "99% confident that the planet is there," though the team would continue to observe the star to rule out other "possible, but improbable" explanations. 

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The radial velocity technique looks for subtle changes in the frequency of starlight as seen from Earth. The gravitational tug of an orbiting planet is thought to cause little wobbles in the light measurements. But sometimes astronomers can be fooled: What might appear to be a planetary signal could in fact be little spots on the surface of the star, causing a false detection. 

Since the 2018 super-Earth candidate was proposed, other astronomers' follow-up studies have formally refuted its existence, including this most recent one. In 2021, a team led by astronomer Jack Rubin conducted nearly 120 new observations with an instrument on the Hobby-Eberly Telescope in Texas and found the mistaken signal was associated with the star's 145-day rotation period. 

The clue that there was a misinterpretation came from the fact that the strongest signal coincided with a period when stellar activity was also strong. In 2022, another team found that the planetary signal was unlikely. 

But that wasn't the first time the star had tricked astronomers into thinking it had planets. Astronomer Peter van de Kamp thought he had discovered two gas giant planets around Barnard’s star, the first as long ago as the 1960s. Some astronomers say he may have died believing he was the first to ever locate exoplanets. His findings were invalidated in later years.

Unlike the previous bloopers, the newly discovered planetary signal — suggesting a world half the mass of Venus — is the first exoplanet to be confirmed in the Barnard's star system with at least two other telescopes. Other verifying observations came from the Roque de los Muchachos and Calar Alto observatories in Spain and the La Silla Observatory in Chile. Astronomers estimate the planet to be about 20 times closer to its star than Mercury is to the sun, with a surface temperature of about 250 degrees Fahrenheit. 

The bad news: Even if the star were about 2,500 degrees cooler than the sun, the planet would still be too hot for liquid water to exist on its surface, and not likely to be habitable, said lead author Jonay González Hernández. But the good news is that the team has found three other planet candidates near the star — and perhaps one of those, if confirmed, could be Earth-like. 

The previously suspected exoplanets were dubbed "Barnard b" until they were ruled out through the complicated and nuanced process of discovering new worlds trillions of miles away.

So, in the famous words of the To Tell the Truth game show, will the real Barnard b please stand up?