Looking beyond the known planets of the Sun, out among an orbiting multitude of small icy bodies, astronomers have seen and measured a miniplanet, more than half the size of Pluto, that is the largest object in the solar system to be detected since the discovery of Pluto in 1930.

A ball of ice and rock about 800 miles in diameter, the object is well over a billion miles farther out than Pluto and some four billion miles from Earth. It is in a region known as the Kuiper Belt, where comets originate and nearly everything is a relic of the solar system's formative epoch.

Some planetary researchers also think this discovery undermines the belief that Pluto is a planet.

Though objects in the region had been detected by telescope, no object that far away in the solar system had ever been seen before, astronomers said in announcing the discovery yesterday. The announcement was at a meeting in Birmingham, Ala., of the Division of Planetary Sciences of the American Astronomical Society.

Two astronomers at the California Institute of Technology, using a telescope at the Palomar Observatory near San Diego, detected the faint light of the object in June and calculated its distance and its unusually circular orbit. The planetary researchers, Dr. Michael E. Brown and Dr. Chadwick Trujillo, have proposed naming the object Quaoar (pronounced KWAH-o-ar), the creator god of an Indian people who originally inhabited the Los Angeles basin.

The discovery was not complete until the Hubble Space Telescope got clearer images. Its camera was able to show the full disc of Quaoar, rather than just a pinpoint of light, enabling astronomers to measure its width.

Although not considered large enough to be a full-fledged planet, planetary scientists said, Quaoar is expected to yield new insights into the mysterious populations at the fringes of the planetary system.

''It's indicative that there are larger and larger objects out there to be found,'' Dr. Brown said in a telephone interview. ''I would be very surprised if there aren't things larger even than Pluto.''

The first object in the Kuiper Belt was observed by Dr. David C. Jewitt of the University of Hawaii and Dr. Jane X. Luu, now at Leiden University in the Netherlands. In 1951, the Dutch-American astronomer Gerard Kuiper hypothesized that other small bodies would be found beyond the orbit of Neptune, the outermost giant planet of the Sun.

Dr. Jewitt praised the new discovery, calling this ''an awesome new object'' but no surprise. The new miniplanet, he and other astronomers noted, is not even the largest object already observed in the Kuiper Belt.

In the opinion of many planetary astronomers, that distinction belongs to Pluto itself. With a diameter of 1,400 miles, Pluto is the smallest of the nine planets and, they feel, should probably be classified as a Kuiper Belt object, not a true planet. Scientists suspect that it originated in that region and was ejected into a Neptune-crossing orbit by some long-ago gravitational disturbance.

''Quaoar definitely hurts the case for Pluto being a planet,'' Dr. Brown said. ''If Pluto were discovered today, no one would even consider calling it a planet because it's clearly a Kuiper Belt object.''

Whether Pluto is a planet or not, Dr. Jewitt said, recent studies have led to predictions of up to 10 objects in the Kuiper region that are as larger or larger than Pluto. They will be more distant than Pluto, thus fainter and harder to detect.

Of the 662 Kuiper Belt objects cataloged in the last decade, the previously largest ones (Pluto excepted) had been about 540 miles in diameter, and many of the visible ones are no more than 100 miles across. Scientists estimate that there could be several billion of these icy objects bigger than one-half mile wide.

When Dr. Brown and Dr. Trujillo first caught sight of Quaoar, they searched the Palomar archives and soon found images of it taken as early as 1982, but not recognized at the time. The images were from a survey by Dr. Charles T. Kowal, then a Caltech astronomer searching in vain for a hypothesized massive but elusive planet known as Planet X.

Dr. Kowal, who is now at the Applied Physics Laboratory of Johns Hopkins University, reacted philosophically to the news of the discovery he had somehow overlooked 20 years ago. ''It's a good argument for keeping old photographic plates,'' he said. ''I'm glad my old work is useful.''

The record of previous sightings allowed the astronomers to establish the distance and plot the object's orbital track. They could estimate that it completes an orbit of the Sun every 288 years.

''It's probably been in this same orbit for four billion years,'' Dr. Brown said.

Not only is the object's orbit unusually circular, compared with Pluto's, its course sticks closer to the relatively flat orbital plane in which all the planets except Pluto are found. Quaoar's orbit is tilted by 7.9 degrees from that plane; Pluto's tilt is about 17 degrees.

Astronomers said the discovery was not so much a triumph of new optics as of a highly focused search strategy and computers that allow speedy analysis of vast quantities of data that previously could only be skimmed. It made Dr. Jewitt wonder, ''Who knows what else is out there?''

Photos: CALL IT QUAOAR -- After astronomers at the Palomar Observatory spotted the faint light of a miniplanet in June, the Hubble Space Telescope sighted the entire disk. The drawing at right indicates its size. (Photograph from NASA; illustration by NASA/Greg Bacon/Modis)(pg. F2)