Astronomers spotted a new visitor in Earth's gravity - a mini-moon. Photo / via Twitter

Astronomers have spotted a new temporary visitor in Earth's gravity – a mini-moon.

The tiny cosmic object - called 2020 CD3 and discovered by the Nasa-funded Catalina Sky Survey - has been orbiting our planet for three years.

The asteroid is said to be about the size of a washing machine and has a surface brightness similar to C-type asteroids, which are carbon rich and very common.

BIG NEWS (thread 1/3). Earth has a new temporarily captured object/Possible mini-moon called 2020 CD3. On the night of Feb. 15, my Catalina Sky Survey teammate Teddy Pruyne and I found a 20th magnitude object. Here are the discovery images.

— Kacper Wierzchoś (@WierzchosKacper) February 26, 2020

Although there are more than a million known asteroids in the solar system, this is the second one to orbit Earth, the Daily Mail reported.

The mini-moon, also known as C26FED2, was seen by astronomers Kacper Wierzchos and Teddy Pruyne from the Catalina Sky Survey at the University of Arizona on February 15, 2020.

It was then spotted four times by February 17, which was enough evidence for experts to confirm it is orbiting our planet, with astronomers noting it is "temporarily bound to the Earth".

"BIG NEWS. Earth has a new temporarily captured object/Possible mini-moon called 2020 CD3. On the night of Feb. 15, my Catalina Sky Survey teammate Teddy Pruyne and I found a 20th magnitude object," Wierzchos shared in a tweet on February 25th, after the Minor Planet Center, a branch of the International Astronomical Union classified the asteroid as a temporarily captured object.

The asteroid is expected to continue orbiting Earth for a few more months before breaking free and venturing back into deep space, where it will resume its solo journey around the Sun.

(2/3) The object has just been announced by the MPC and its orbit shows that it entered Earth's orbit some three years ago. Here is a diagram of the orbit created with the orbit simulator written by Tony Dunn:

— Kacper Wierzchoś (@WierzchosKacper) February 26, 2020

(3/3) The object has a diameter between 1.9 - 3.5 m assuming a C-type asteroid albedo. But it's a big deal as out of ~ 1 million known asteroids, this is just the second asteroid known to orbit Earth (after 2006 RH120, which was also discovered by the Catalina Sky Survey).

— Kacper Wierzchoś (@WierzchosKacper) February 26, 2020

The last mini-moon to appear in Earth's gravity was last year and it fell from the sky over Australia.

The fireball was first spotted by Australia's Desert Fireball Network in August 2016 and at the time astronomers thought it was a normal meteor.