Astronomers at the German Aerospace Center's Institute for Planetary Research have discovered GJ 367b, a rocky exoplanet orbiting its star so closely that it completes a full revolution in just 7.7 hours. Ultrashort-period planets like this remain poorly understood, making this TESS detection a significant breakthrough.
Ultrashort-period exoplanets orbit their stars in less than one Earth day. Precise measurements of their masses and radii offer key insights into formation and evolution processes. More examples like this one help refine our models. In a new study published in Science, researchers detail the detection of GJ 367b.
GJ 367b circles a red dwarf star, about half the Sun's size, located just 31 light-years away. This rocky world is roughly 70% Earth's size and finishes one orbit in just 7.7 hours.
Led by astronomer Kristine W.F. Lam, the team identified GJ 367b using data from NASA's TESS satellite, which detects planets via the transit method—spotting brief dips in a star's brightness as a planet passes in front.
Follow-up observations with the HARPS instrument at the European Southern Observatory in Chile measured the planet's mass at about half Earth's. Combined with its size, this yields a density higher than Earth's.
"Such density indicates the planet is dominated by an iron core," the authors note. "These traits mirror Mercury's oversized iron-nickel core, unique among Solar System terrestrials."
GJ 367b is no candidate for life as we know it. Its proximity to the star means it receives 500 times more radiation than Earth, with no atmosphere to protect it.
Likely tidally locked, it shows just one face to its star, akin to the Moon-Earth system. Dayside temperatures may soar to 1,500 degrees Celsius.
How GJ 367b formed remains unclear. It may have assembled from iron-rich materials or be the stripped core of a once-larger world eroded by stellar radiation or impacts. "If a remnant of a gas giant core, the original planet was no bigger than Neptune," the team explains.
At just 31 light-years away, GJ 367b is prime for study with upcoming telescopes like the James Webb Space Telescope. Observations may reveal other worlds in the system's habitable zone.
