2 Planets Found Beyond Solar System

For the first time, astronomers said Thursday, they have whatthey call "irrefutable evidence" that planets exist outside the solarsystem.

The historic discovery of at least two planets, made by a teamled by Alexander Wolszczan of Penn State University, ends a string ofdisappointments in the quest to find the first extrasolar planet and,astronomers say, lends credence to the idea that planets are commonin the cosmos.

The new planets, more than 7,000 trillion miles away and eachabout three times the mass of Earth, are not likely to harbor life,scientists note. The star they orbit is distinctly unsunlike: awhirling dead sphere called a pulsar that, instead of light, emits abarrage of deadly invisible radiation.

But it is the dead star's exotic nature that enabled astronomersto detect its planetary family in the first place - by studyingirregularities in the precise pattern of radio waves the star emits.It is because of that same nature that the prized discovery fell notto optical astronomers, who traditionally have led the century-longsearch for planets around stars other than the sun, but to radioastronomers.

Wolszczan said in an interview his team was able to confirm theexistence of the planets by measurements so fine they detectedchanges in the motions of the pulsar equal to "the crawl of a snail,at 1,200 light-years."

In 1990, Wolszczan first identified the star - in theconstellation Virgo, about 1,200 light-years from Earth in the planeof the Milky Way Galaxy. (One light-year is the distance traveled bylight in a year: 5.9 trillion miles.)

In 1991, aided by Dale Frail of the National Radio AstronomyObservatory in Green Bank, W. Va., he detected the presence of thesuspected planets and, in 1992, announced the finding in the journalNature.

But astronomers had heard similar announcements before. Inevery case, the putative planets had either proved impossible toconfirm, turned out to be something else or been shown to be theproduct of an error.

Since then, the Wolszczan team has bolstered its originalfindings with three more years of even more finely nuanced datadesigned to eliminate any explanation other than planets.

Wolszczan reports on how he confirmed the claim in today's editionof the journal Science. "I'm really glad I don't have to take itback," he said.

The pulsar, known as PSR1257+12, is believed to be the collapsedhusk of a formerly huge star that most likely died in a cataclysmicexplosion called a supernova. Once more massive than the sun, it isnow a dense, lightless object only about 12 miles in diameter.

As it spins on its axis 161 times a second, the pulsar sends outradio waves that sweep Earth in regular pulses, like the beam from alighthouse beacon. This action provides the most precise "clock" inthe cosmos.

But Wolszczan and Frail, in 1991, noticed that the pulses wereslightly irregular, in a regular and predictable way. The teamdetected variations of plus or minus 0.003 second in the arrivaltimes of the radio pulses.

This variation was caused by a wobble, in which the pulsar moved560 miles toward and away from Earth as it spun on its axis.