Life has altered our planet to such an extent that its effects are visible from space. And not just from low Earth orbit. The signature of life is visible light years from here, thousands of billions of kilometers away. If intelligent civilizations have emerged on other stars, the signs of life on Earth should have been all too clear.
These signs are called biosignatures. Earth’s atmosphere contains high levels of oxygen and ozone, highly reactive molecules that should have long ago reacted with other compounds and disappeared if they were leftovers from the formation of the Solar System. Instead, their presence suggests that something on Earth’s surface is producing them in prodigious quantities. The same is true of methane, which breaks down easily in sunlight.
Another is phosphine, a toxic, flammable gas with a characteristic smell of garlic or rotting fish. Phosphine is highly reactive and so survives for only ...
