Pluto, long regarded as a second-class planet because of its small size, odd orbit and other features more reminiscent of an asteroid than a planet, may be due for a promotion. New evidence shows it has, like all good planets, a significant atmosphere.

The findings, announced last week by a team of astronomers at the National Aeronautics and Space Administration's Jet Propulsion Laboratory, emerged from data collected the last three years by the Infrared Astronomical Satellite in Earth orbit and by ground-based telescopes.

Earlier studies had suggested that Pluto, which is smaller than Earth's airless moon, had a thin atmosphere of methane gas. The new studies show the methane to be far thicker than previously estimated.

The evidence is Pluto's temperature. New measurements show it to be too warm for a body insulated with as little methane as the earlier reports suggested.

Pluto had been considered unplanetlike and more probably an errant asteroid because its orbit is highly elliptical and sharply tilted from the plane of all the other planets. Although it is usually the farthest planet from the sun, its elliptical orbit brought it closer in than Neptune in 1980. Pluto will not regain its ninth-planet position until 1999.

Pluto, at a diameter of 1,370 miles, is the smallest planet and the only one in the outer solar system with a solid surface. Jupiter, Saturn, Uranus and Neptune are all-gas planets. Pluto's composition is believed to be mostly ice and rock.