Unlike most GS-13s, Bob Harrington and Tom Van Flandern don't spend their time worrying about agency funding, regulations or other earthly concerns. The subject they are tackling is whirling in the outer reaches of the solar system, if it exists at all.

Harrington and Van Flandern, astronomers at the U.S. Naval Observatory on Massachusetts Avenue, are searching for a 10th planet, more than three times the size of the Earth but invisible to the naked eye. Its discovery could explain some of the riddles of the solar system.

The two spend most of their time with computers rather than telescopes, trying to calculate the paths of the stars and planets for nautical almanacs and star maps that help navigators.

Because they found they could not predict the orbits of Uranus and Neptune precisely, a few years ago they developed their theory of a 10th planet whose gravitational field skews the orbits of the others.

The two had been working independently at the observatory. Van Flandern was writing a computer model to show what kind of planet could cause the orbits of other planets to fluctuate. Harrington was preparing a computer model to show how a 10th planet might have hurled past Neptune billions of years ago, distorting the orbits of its moons and sending one moon, now the plant Pluto, into a path around the sun.

They found that their calculations of the character and orbit of the 10th planet matched almost exactly, even though they arrived at their figures in different ways.

The orbit of the 10th planet, they calculate, ranges from 4 billion miles from the sun to 10 billion miles away. The Earth, in contrast, is a paltry 93 million miles from the sun.

The two say they believe the temperature of the 10th planet hovers slightly about absolute zero, or almost 460 degrees below zero Fahrenheit.

Harrington calls the planet "IX," using the Roman numeral 9, since, he says, Pluto is really just a moon. Van Flandern, more serious about such things, merely calls it "the 10th planet."

They acknowledge that many scientists believe they are chasing shadows, but they insist that an unknown planet is the best explanation for variations in the orbits of Uranus and Neptune that have puzzled skywatchers for decades.

When Pluto was discovered in 1939, many scientists assumed it was the missing link causing the fluctuations in the orbits of the known planets. However, astronomers later found that Pluto was too small to have such an impact: a thousand times too small, Harrington said.

The men are trying to refine their models further, and hope soon to begin looking for the planet through telescopes, once they have a more precise idea of where to look.

"Its still a puzzle," Harrington said. "We're having a basic re-discussion of everything we know about the solar system."