LONDON (AP) _ Astronomers who announced last year that they had found the first planet outside the Earth's solar system say they made a mistake - the planet does not exist.

Professor Andrew Lyne and Dr. Matthew Bailes said in a letter published in today's Nature magazine that they had erred in the calculations that brought them worldwide publicity.

The weekly, in an editorial, commented that ''such ups and downs are inseparable from science.''

It congratulated the men, who both work at Manchester University's Nuffield Radio Astronomy Laboratories at Jodrell Bank, saying they ''should take pride in the directness of their acknowledgment of their mistake.''

Jodrell Bank is known for the pioneering work of its giant radio telescope, completed in 1957. Its 250-foot-wide parabolic reflecting bowl has a range 1,000 times greater than most of the best optical telescopes.

Tim Lyster, editor of the journal Astronomy Now, said: ''This mistake shows that astronomers are human like the rest of us.''

Nature published a letter last July from the astronomers and a third Jodrell Bank researcher, S.L. Shemar, announcing their ''discovery.''

The team led by Lyne had been analyzing radio waves emitted from a star named PSR1829-10, which is 30,000 light years away.

A light year is the distance light and radio waves, traveling at 186,287 miles per second, cover in one year.

The astronomers found a discrepancy in the times the signals arrived on Earth and attributed this to disturbances in the star's motion caused by a planet 10 times heavier than Earth.

In the letter published today the astronomers said their computer program did not take fully into account that the Earth's orbit around the sun is an ellipse, not a circle.

''But when our error in our calculation of the Earth's orbit is taken into account, this movement of the star disappears and there is no need to postulate the existence of the planet,'' they said.

''At least we hadn't got around to giving it a name,'' The Times of London quoted Bailes as saying.

The Daily Telegraph quoted him as saying the error does not invalidate subsequent reports by other astronomers of planets around other stars.

These astronomers did not make the error the Britons made, and it is probable the Milky Way teems with such planets, the newspaper quoted Bailes as saying.