AUSTIN, TEX., JAN. 12 -- It's summertime on Pluto, which, astronomers warn, is no day at the beach.

This revelation is part of an emerging portrait of a planet whose small size, 1,400 miles in diameter, and distance, 2.7 billion miles from Earth, have made it the most difficult to study. Pluto and Neptune are the only planets in the solar system never visited by a space probe.

Now circumstances have enabled astronomers to remove some of its mystery, disclosing a planet covered with bright methane snow where the summer temperatures reach a high of minus 378 degrees Fahrenheit, which is colder than liquid nitrogen.

The findings were presented at the 171st meeting of the American Astronomical Society here today.

Pluto and its moon, Charon, appear to be very different from each other and unusual compared with the rest of the solar system. For example, Pluto is reddish, while Charon is gray and dark and may have water ice, not methane, on its surface.

Pluto is also surprisingly dense, made up of heavy, possibly rocky material, even though bodies in the outer solar system are usually lighter. And Charon is more than half as large as Pluto, leading some astronomers to believe the system is actually a "double planet."

The Pluto Charon system also appears to be the only example in the solar system of a double planet where the atmosphere -- in this case a very thin one, like that of Earth at 60 miles up -- interacts with both bodies, according to Laurence Trafton of the University of Texas. His findings indicate there is a gaseous cloud of methane surrounding the two, he said.

Trafton is part of a team that, as a result of the new information, will search for undetected additional moons of Pluto much further out than Charon.

The astronomers' findings are based on a viewing opportunity, now in progress, that occurs only once every 124 years: this is when Charon passes directly in front of and behind Pluto as viewed from Earth.

Soon after James W. Christy of the U.S. Naval Observatory in Washington discovered Charon in 1978, scientists realized the rare "eclipse" alignment would occur by the mid-1980s.

"Nature was very kind to us in this instance," said Richard P. Binzel, of the Planetary Science Institute in Tucson.

Pluto is now at its closest approach to Earth since 1740.

At such distances, when viewed through ground-based telescopes, Pluto and Charon appear as a single blurry object. Scientists could spot the eclipse only by monitoring the blur diligently through the early 1980s, using a device to measure the total sunlight reflected by the two bodies. When an eclipse finally occurred, in 1985, Charon blocked part of their surfaces and caused a decrease in the reflected light.

When Charon is completely hidden behind the planet, scientists can study the light from Pluto alone, and distinguish the individual properties, such as size and mass, of it and its moon.

Pluto, discovered in 1930, orbits the sun every 248 years. Normally the outermost planet, it is now near its point of closest approach to the sun and inside Neptune's orbit, bringing Pluto a relative heat wave that probably lasts only a few decades of its orbital cycle, the astronomers said.