Astronomers said on Thursday they had found six planets orbiting a distant star in the most exciting but also most challenging find since exploration of other solar systems began 15 years ago.

None of the so-called exoplanets, orbiting the star Kepler-11 2,000 light years away, is remotely comparable to the Earth, but the detection of them is a major technical feat, the scientists said.

Five planets are relatively small, ranging in mass from 2.3 to 13.5 times that of Earth, but orbit Kepler-11 at blistering proximity, encircling it at a distance that is even closer than Mercury to the Sun.

Their "year" -- the time it takes to encircle the Sun -- is less than 50 days.

A sixth exoplanet is large and farther out, with an orbital period of 118 days, and is of a yet-to-be determined mass, although it is likely to be a "gas giant" like the outer planets of our own Solar System.

Reporting their find in the British journal Nature, the astronomers said the star system was mind-boggling because the planets were so small, so numerous and yet so densely packed.

"We think this is the biggest thing in exoplanets since the discovery of 51 Pegasi b, the first exoplanet back in 1995," Jack Lissauer of the NASA-Ames Research Center said in a teleconference with the media.

Daniel Fabrycky, a post-doctoral researcher at the University of California at Santa Cruz, described it as "an amazing planetary system."

"Of the six planets, the most massive are potentially like Neptune and Uranus, but the three lowest-mass planets are unlike anything we have in our Solar System," added Jonathan Fortney, the university's assistant professor of astronomy and astrophysics.

The inner planets seem to have an atmosphere of water or hydrogen-helium gas. This is another surprise, given the presumption that a small planet orbiting so close to a star would have a light atmosphere blasted away by solar radiation.

The extra-solar bodies were found by the US orbiting telescope Kepler, which already has a fine haul of these so-called exoplanets to its credit.

Kepler-11's herd of planets orbit in very narrow planes. Sideways on, as viewed from Earth, "the system is flatter than a CD," said Lissauer.