Where should we look? According to Wright, Earth is the obvious, most habitable place in the Solar System, and it’ll be the easiest to search. Humans have dramatically changed the landscape of Earth. Our open pit mines, for example, are a clear indication that an intelligent species dug out a specific mineral from the ground. These might be obvious for millions of years, but over the course of billions of years, plate tectonics will have recycled those regions, absorbing the evidence back into the ground. Radioactive isotopes from ancient nuclear reactors, or fossils of ancient beings will have about the same lifespan. Beyond a few hundred million years, the Earth itself would have completely obscured any evidence of a technological civilization.

Venus is inhospitable today, but it might not have always been the case. Billions of years in the past, when the Sun was cooler, it might have had a thinner atmosphere and milder temperatures. It’s worth searching. That said, it appears that Venus has gone through major geological resurfacing events, where the entire planet’s surface turned inside out. Venus could easily hide its secrets.

Scientists are accumulating more and more evidence that Mars was warmer and wetter in the past, with eras when liquid water could exist on the surface for long periods of time. And unlike Earth and Venus, it doesn’t have active plate tectonics. Landscapes on the surface have remained there for billions of years. Well, okay, they’ve been pounded by meteorites, but they’re still there.