If there are aliens in our galaxy, they might already know about us.

In a new study, two astronomers have identified 1,004 sun-like stars that could all have habitable Earth-like planets in their orbits. Any intelligent aliens on those exoplanets should be able to see Earth and spot the chemical signs of life here.

That’s because, from those planets’ perspectives, Earth passes in front of the sun each time it orbits — a tiny, dark spot in front of the blazing star.

This appearance is called a transit; astronomers on Earth use these tiny drops in the brightness of other stars to identify planets passing in front of them. In just 11 years, scientists have discovered more than 3,000 planets using this method.

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By measuring how the light from a star changes when a transiting planet passes in front of it, scientists can determine the exoplanet‘s size and, sometimes, the composition of its atmosphere. So it follows that intelligent aliens could do the same about Earth. If they did, they would spot signs of life: Plants on our planet fill the atmosphere with oxygen, and bacteria produce nitrous oxide, a gas which is unlikely to appear without biological processes.

Read more: Business Insider