If there is, we do not know of one yet. But we
must emphasize the YET. The technology to
determine the composition of the atmosphere of an
exoplanet is just now becoming sufficiently good
enough to detect free oxygen, diatomic oxygen
In our solar system NO OTHER planet has free
diatomic oxygen in its atmosphere. We believe
that free oxygen originates exclusively by
photosynthesis. Therefore if we did detect
free oxygen in the atmosphere of another planet,
we would have strong reasons to surmise, but not
prove, that life existed there.
In science, we do not have all the answers.
But the FUN part is that we can ask questions and
then use our powers of reasoning and the
experimental method to address very difficult
questions. In many cases, if we study long and
hard enough we can answer these questions.