Astronomers have discovered a very rare molecule known as phosphine on Venus, or at least in the clouds of its upper atmosphere. Phosphine is a gas made up of hydrogen and phosphorus that doesn’t appear naturally on Earth and has only been manufactured industrially in oxygen-free environments. Finding phosphine on Venus suggests the possibility of acid-resistant microbes, or living microorganisms, floating in the clouds above the planet’s scorching surface. If the discovery holds up to further investigation, it will mark the first confirmed evidence of extra-terrestrial life in the universe.