A scientist called Professor Carl Murray was the first to see the new object and said: "It was an incredible privilege to be the first person to spot it."
Although scientists aren't completely sure that it is a moon, as it could just be a "clump" of space material.
There could be a second object too, as when scientists studied more detailed photos of the first object they saw it in a place they didn't think it should have been.
That means either the object has a very unusual orbit or there is a second object as well.
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I watched Rosetta launch into space
