Could aliens already know Earth exists?

Yes — it’s plausible that an alien civilization could already know Earth exists, and even that life exists here. Whether they know about humans specifically would depend on how far away they are and what they can detect.

A few ways this could happen:

• Biosignatures: If they had advanced telescopes, they might detect signs of life in Earth’s atmosphere, such as oxygen and methane existing together in a way that suggests biology.
• Transits: From certain locations, an observer could see Earth pass in front of the Sun. Astronomers call this the Earth Transit Zone; star systems in that zone could, in principle, notice Earth this way.
• Technosignatures: A nearby civilization might detect our radio leakage, radar, or other signs of technology, though how far those signals are detectable depends on their equipment and what they are looking for.

The key limitation is distance and light travel time: the farther away they are, the older Earth they see. So they could know Earth exists even if they do not know what Earth is like today.

There’s no confirmed evidence that any extraterrestrial civilization has detected us, but from a physics standpoint, it’s definitely possible.