Astronomers have discovered two small moons orbiting Uranus, bringing to 17 the number of moons circling the seventh planet.

The larger moon measures only 100 miles across, the smaller only 50 miles. Unlike the planet's other moons, they orbit Uranus at a crazy angle, alternately rising high above and then plunging far below the plane occupied by most solar system objects.

Planetary satellites like the two new ones, known as irregular moons, have already been discovered orbiting Jupiter, Saturn and Neptune.

``I think it's great,'' said Jonathan Lunine, an astronomer at the University of Arizona. ``Finding where irregulars are tells you about the formation of the solar system.''

The larger moon appears red in photographs, suggesting to astronomers that it may be covered with hydrocarbons produced by the cosmic ray bombardment of methane ice on the object.

Astronomers discovered the moons using the Hale telescope at California's Palomar Observatory. The moons were first spotted on the nights of Sept. 6 and 7, and their existence was confirmed with observations made last weekend. The International Astronomical Union announced the discovery Friday.

The moons are in oblong orbits that take them millions of miles from Uranus. They were probably captured by the giant planet early in the history of the solar system, said Philip Nicholson, an astronomer at Cornell University and one of the moons' discoverers.

``It's going to be difficult to learn more because the objects are so faint,'' Nicholson said.

Nicholson discovered the moons with Cornell astronomer Joseph Burns, Brett Gladman of the University of Toronto and J.J. Kavelaars of McMaster University in Canada.

The moons are yet to be named. Tradition dictates that, like Uranus' other 15 moons, they receive names from Shakespeare or Alexander Pope.