NASA scientists were conferring today about a nearby planet that is shockingly similar to Earth. It is just 5% smaller in radius and 15% smaller in mass. It is almost the exact same age as our planet, and gets its warmth from an identical star. The only thing that’s a bit off is that it orbits a bit closer to its star than Earth does, so it receives nearly twice as much radiation. On the other, it also reflects away a lot of that radiation. Its theoretical (equilibrium) temperature is just below freezing, so with a little natural greenhouse warming it would be quite an inviting place.
If we found it orbiting another star, this world would surely be hailed as the most Earthlike exoplanet known: the best place yet to search for alien life.
No doubt you sense there is a catch, and indeed there is. The world I’m talking about is Venus. It is not orbiting another star; it is the planet closest to home right here in our own solar system. But I’m not just being coy. Despite its proximity, Venus is a profound enigma. It really should be a hospitable world, but the truth is that it is more like hell on almost-Earth. Understanding why that is–why our planet went right while Venus went terribly wrong–is crucial for finding out whether habitable planets are common or rare throughout the universe.
The ways in which Venus diverged from Earth are as dramatic as they are perplexing. Venus has a crushing atmosphere tinged with sulfuric acid clouds and dominated by carbon dioxide. It has a year-round surface temperature of about 450 degrees Celsius (850 degrees Fahrenheit), far hotter than an oven set to “broil.” It has no appreciable magnetic field to protect it from charged particles that blow out from the sun. It has no plate tectonics to renew its geology. It rotates so slowly that one “day” takes 243 Earth days, and its rotation is backwards compared to that of almost all the other planets. It has no moon.
Some of these traits may be connected. Perhaps Venus’s lack of a moon has something to do with its slow rotation. Its slow rotation, in turn, might be related to its lack of magnetic field…which in turn might be related to its thick, dry atmosphere…which in turn might be related to its lack of dynamic geology. There might be a chain reaction at work.
I’m saying “might” a lot because there is still a tremendous amount that scientists do not know about Venus. The perpetual cloud cover presents a huge challenge in studying its surface features. Even worse, landing a probe on its surface is exceedingly difficult. Atmospheric pressure on the ground is more than 90 times that at sea level on Earth, and then there are those searing temperatures, which quickly fry electronics and melt mechanical components. There is also a sense of gloom hanging over Venus, since the odds of finding anything alive there are just about zero. For these reasons, Venus has attracted a tiny fraction of the attention lavished on Mars.