VENUS

Although it's similar in structure and size to Earth, Venus has a thick atmosphere that traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system.It's a cloud-swaddled planet named for a love goddess, and often called Earth’s twin. But pull up a bit closer, and Venus turns hellish. Our nearest planetary neighbor, the second planet from the Sun, has a surface hot enough to melt lead. The atmosphere is so thick that, from the surface, the Sun is just a smear of light. In some ways it is more an opposite of Earth than a twin: Venus spins backward, has a day longer than its year, and lacks any semblance of seasons.

It might once have been a habitable ocean world, like Earth, but that was at least a billion years ago. A runaway greenhouse effect turned all surface water into vapor, which then leaked slowly into space. The present-day surface of volcanic rock is blasted by high temperatures and pressures. Asked if the surface of Venus is likely to be life-bearing today, we can give a quick answer: a hard “no.” Further, Venus may hold lessons about what it takes for life to get its start ­– on Earth, in our solar system, or across the galaxy.

The ingredients are all there, or at least, they used to be. By studying why our neighbor world went in such a different direction with regard to habitability, we could find out what could make other worlds right. And while it might sound absurd, we can’t rule out life on Venus entirely. Temperature, air pressure, and chemistry are much more congenial up high, in those thick, yellow clouds.