If you could see an alien planet from Earth, you wouldn't necessarily need to look for cities. You could just check for oxygen in the atmosphere.

What's Oxygen Got To Do With Anything?

Oxygen in the atmosphere? What would that prove? Well, if there are aliens, they're undoubtedly different from what we're used to on Earth. But one thing we know is that oxygen is the same everywhere.

Oxygen gas is highly reactive. That means it combines very rapidly with other elements, such as carbon and hydrogen. Now when the Earth was formed, any free oxygen must have quickly combined with other elements to form less reactive compounds. None would have been left over in the atmosphere.

Then Where Did It Come From?

The oxygen that we find in our atmosphere now has only been around since plants first evolved. Plants give off oxygen as a by-product, just as you and I give off carbon dioxide.

Without life on Earth, there'd be no oxygen in the atmosphere. Aha! So if we find oxygen around some alien planet... That's a good place to look for life.