Astronomers announced today the discovery of three previously unseen moons orbiting Neptune, bringing the total of satellites around that planet to 11.

The moons are the first found at Neptune since the Voyager II mission in 1989 and the first detected from the ground since 1949. Each is roughly 18-24 miles (30-40 kilometers) in diameter. The research was led by Matthew Holman of the Harvard-Smithsonian Center for Astrophysics and JJ Kavelaars of the National Research Council of Canada.

Sunlight reflecting from the newfound Neptunian moons is so minimal that they are about 100 million times fainter than what can be seen with the naked eye under dark skies from Earth.

The discoveries were made by adding together multiple images taken from the 4.0-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, and the 3.6-meter Canada-France-Hawaii Telescope in Hawaii.

All three newly spotted satellites are irregular, meaning they either orbit in the opposite direction of the planet's rotation or carve a path that is highly inclined to the planet's plane of rotation.

Neptune has two other irregular satellites and six regular moons. Triton, the largest moon of Neptune, orbits in the opposite direction and is thought to be a captured object.

In fact, all of the irregular satellites have somewhat traceable histories. The smaller ones are most likely the result of a collision long ago between an existing moon and a passing comet or asteroid, astronomers say.

"These collisional encounters result in the ejection of parts of the original parent moon and the production of families of satellites. Those families are exactly what were finding," said Kavelaars.

The discoveries open a window through which astronomers observe the conditions in the solar system at the time the planets were forming, Holman said.

Other members of the team: graduate student Tommy Grav, University of Oslo & Harvard-Smithsonian Center for Astrophysics; undergraduate students Wesley Fraser and Dan Milisavljevic, McMaster University, Hamilton, Ontario, Canada.

Satellite discoveries around the outer planets are coming at the rate of several per year, thanks to improved telescopes and optics and new computer techniques. Many require additional observations to be confirmed.

In a separate announcement last week, another research team said it had found evidence for the first so-called Trojan satellite of Neptune. Not a true moon, this is an asteroid trapped in an odd gravitational dance with both Neptune and the Sun.

Jupiter plays host to several Trojan asteroids.

"Neptunian Trojans were long suspected to exist and it is gratifying to finally know that they do," said Eugene Chiang of the University of California at Berkeley.