Brighter than any object in the night sky except the moon, our nearest planetary neighbor is in many ways like Earth. Venus is almost the same size as Earth. And like Earth, it has a rocky surface with a metal core. The orbit of Venus is only about a quarter of the way from Earth to the sun at about 108 million kilometers (67 million miles).

But Venus is truly an evil twin! Clouds of sulphuric acid send corrosive droplets to the rocky ground. An extremely thick atmosphere of carbon dioxide blocks out almost all visible sunlight, but lets plenty of infrared radiation through—the kind of heat you'd expect from being that close to the sun.

To make matters worse, carbon dioxide is a prime greenhouse gas. So most of that heat is trapped. This greenhouse effect makes Venus much hotter than Earth, with a surface temperature of about 470 degree Celsius (880 Fahrenheit). And all that carbon dioxide makes the surface pressure on Venus extreme—equal to the squeeze you'd feel about a kilometer (half a mile) below the surface of one of Earth's oceans.