Just six light-years from the solar system -- in cosmic terms, our backyard -- exists a world about half the size of Venus that's 125 °C on the surface and where a year lasts three Earth days.
The planet, called Barnard's b, orbits Barnard's star, a point of light in our night sky that all astronomers well know as one of the fastest-moving stars. It appears to move because it's so close to the solar system. Only the three-star Alpha Centauri star system, which is 4.1 light-years distant, is closer. In 2016, astronomers found a planet orbiting the nearest star to Earth in this system, called Proxima Centauri.
Barnard's b is around 20 times closer than Mercury is to the sun. Hinted in 2018 and now confirmed using the European Southern Observatory's Very Large Telescope in Chile, the closest individual star to us has been studied for decades. In a paper published in Astronomy & Astrophysics, Barnard's b has been confirmed, culminating in five years of observations. "Even if it took a long time, we were always confident that we could find something," says Jonay González Hernández, a researcher at the Instituto de Astrofísica de Canarias in Spain, and lead author of the paper.
Barnard's star is a red dwarf star, a low-mass, cool star that comprises about 70% of all the stars in the Milky Way. They're too dim to be visible to the naked eye.
The researchers focused on the so-called "Goldilock's Zone" around Barnard's star, where the temperature allows liquid water to exist on the planet's surface. However, their search led them much closer to the host star. "Barnard b is one of the lowest-mass exoplanets known and one of the few known with a mass less than that of Earth," said González Hernández. An exoplanet is a planet that orbits a star other than our sun. "But the planet is too close to the host star, closer than the habitable zone ... even if the star is about 2,500 degrees cooler than our sun, it is too hot there to maintain liquid water on the surface." Since water is thought to be required for life, so it's lifeless.
Barnard's b was detected using the radial velocity method -- a measurement of the host star's wobble that can only exist if there's a slight gravitational pull from one or more orbiting planets.
In addition to discovering Barnard's b -- a completely different signal from that detected in 2018 -- the researchers also found traces of three more exoplanets in orbit around the same star. "We now need to continue observing this star to confirm the other candidate signals," said Alejandro Suárez Mascareño, a researcher also at the Instituto de Astrofísica de Canarias and co-author of the study. "But the discovery of this planet ... shows that our cosmic backyard is full of low-mass planets."
Barnard's star has long fascinated astronomers because of its apparent motion against the backdrop of other stars. Most stars do not appear to move a discernable amount in a human lifetime. Its velocity with respect to the sun is about 311,000 miles per hour (500,000 km/h), and it travels a distance equivalent to the moon's diameter across the sky every 180 years. That's the fastest apparent motion of any star.