Its route suggests it entered Earth's orbit three years ago, he said
Earth has acquired a second "mini-moon" about the size of a car, according to astronomers who spotted the object circling our planet.
The mass - roughly 6-11 feet in diameter - was observed by researchers Kacper Wierzchos and Teddy Pruyne at the Nasa-funded Catalina Sky Survey in Arizona on the night of February 15.
"BIG NEWS. Earth has a new temporarily captured object/Possible mini-moon called 2020 CD3," likely to be a C-type asteroid, Wierzchos tweeted on Wednesday.
The astronomer said it was a "big deal" as "this is just the second asteroid known to orbit Earth (after 2006 RH120, which was also discovered by the Catalina Sky Survey)."
Its route suggests it entered Earth's orbit three years ago, he said.
The Smithsonian Astrophysical Observatory's Minor Planet Center, which collects data on minor planets and asteroids, in an announcement said "no link to a known artificial object has been found," implying it was likely an asteroid captured by Earth's gravity.
"Orbit integrations indicate that this object is temporarily bound to the Earth."
Tech entrepreneur Elon Musk said the car-sized object was not the Telsa Roadster he launched into space in 2018, which is now orbiting the Sun.
"It's not mine," he tweeted.
Earth's new neighbour is not in a stable orbit around the planet and is unlikely to be around for very long.
"It is heading away from the Earth-moon system as we speak," Grigori Fedorets, research fellow at Queen's University Belfast in Northern Ireland, told New Scientist magazine, and was likely to escape in April.
The only other asteroid known to orbit Earth, 2006 RH120, rotated the planet from September 2006 to June 2007.