In the movie contact the group looking for extraterrestrial signals throught they had something however...?

Question

it turned out to be a known celestial object. What was that object and why were they tricked?

Answer 1

One of the signatures looked for in SETI is a narrow-band radio signal whose doppler signature is inconsistent with it being from the Earth, that is, it has a velocity with respect to the Earth that indicates it is not moving with the Earth. The signal was from a scientific space probe far out among the outer planets of the Solar System. This space probe, being far from Earth and nearly in interstellar space, had a doppler signature that was tagged as "interesting" by the SETI software.

Answer 2

I vaguely remember it being a pulsar.