WASHINGTON - An international team of researchers have announced the discovery of 104 new planets outside our solar system, including four that could have Earth-like, rocky surfaces.

Scientists on Monday discovered the exoplanets using the Kepler space telescope as well as ground observations by Earth-based telescopes, including four on the summit of Mauna Kea in Hawaii.

The US$600 million Kepler mission has allowed scientists to discover more than 4,600 planets -- 2,326 of them confirmed -- since it launched in 2009.

"The diversity of planets is astounding," said Evan Sinukoff, an astronomer at the University of Hawaii who contributed to the research.

"We discovered many planets about twice the size of the Earth orbiting so close to their host stars that they are hotter than 1000 degrees."

The latest trove includes 21 situated within their sun’s habitable zone -- the distance from a star that could permit liquid water to exist and support life.

The four potentially rocky planets -- ranging from 20 to 50 per cent larger than Earth -- orbit tightly around the same star in a planetary system about 400 light-years from Earth.

Though the planets rotate around their star even closer than Mercury orbits the sun, two of the planets may have surface temperatures similar to Earth’s, as their star is cooler than our sun, astronomers said.

The unmanned Kepler mission has been scanning 150,000 stars in the Cygnus constellation for signs of orbiting bodies, particularly those that might be able to support life.

It works by observing a dimming in the light of a star, known as a transit, each time an orbiting planet passes in front of it.

In 2013, the space telescope suffered a problem with the reaction wheels that typically keep the spacecraft steady.

NASA subsequently set the spacecraft on a new mission called K2, to study supernovas, star clusters and far-off galaxies.

Scientists verified the latest exoplanets as part of the K2 mission. – AFP