Also see Mark Anderson's explanation of why the transit is a critical time for learning to study the atmospheres of other planets.

On the afternoon of June 5 in the United States, a small black dot will crawl across the blazing face of the sun in one of the rarest celestial events: a transit of Venus, when our sister planet passes between us and our star. It won’t happen again for 105 years.

That dot is our closest planetary neighbor and, in many ways, Earth’s near-twin. Venus is 7,521 miles in diameter, just 5 percent smaller. It has an iron core and a thick, heat-trapping atmosphere. It orbits at about three-quarters the distance between Earth and the sun. If astronomers spotted such a planet orbiting another star, they would conclude it was an ideal place to search for life.

But in other ways, Venus is as enigmatic as that black dot against the sun. It is perpetually shrouded in dense clouds of sulfuric acid. The spacecraft that have explored it found no oceans, no protective magnetic field, and no active geology. The handful of old Soviet probes that descended to the planet’s surface perished in an hour or two, destroyed by the 900-degree Fahrenheit temperature (hot enough to melt lead) and atmospheric pressure 90 times that on Earth.

Venus’s contradictions lend it an unmistakable allure: How did a world so similar to our own turn out so different? The answers could tell a lot about the history of the solar system, about where to look for life in other planetary systems, and even about the future of Earth.

Astronomers believe that Earth and Venus were much more like twins when they formed 4.5 billion years ago. Back then the sun was dimmer, and Venus apparently was cooler. Studies of hydrogen molecules in the Venusian atmosphere by NASA’s Pioneer-Venus probe indicate that the planet once had liquid water on its surface, perhaps even expansive oceans.

Life could have started and thrived under those conditions, says planetary scientist David Grinspoon of the Denver Museum of Nature & Science. But at some unknown point in its history, Venus reached a tipping point. The planet overheated and water on the surface rapidly evaporated, filling the atmosphere with water vapor; the vapor trapped more heat, which caused more evaporation, and so on. “It’s a positive feedback loop,” Grinspoon says. “The oceans basically boil.”

From Identical to Evil

Planetary scientists still have to sort out exactly how Venus reached the point of no return. Grinspoon points to one important factor: its distance from the sun. Being 28 percent closer than Earth means that Venus receives about twice as much solar energy. Astronomers think that as the sun gradually grew hotter, it pushed Venus to a threshold temperature that set the runaway greenhouse effect in motion.

To confirm that hypothesis, scientists need to know how Earth-like Venus started out, and then how it transformed from temperate to unbearable. They want to understand how an atmosphere once rich in oxygen evolved into a layer of smothering gases, mostly carbon dioxide.

Determining exactly how Venus went over the edge would clarify whether Venus is the oddity—or Earth is. Last December scientists working with the Kepler space telescope discovered the mission’s first planet in the “habitable zone” around another star, the location where temperatures could be mild enough to allow liquid water on a planet’s surface. Kepler’s goal is to determine the frequency of Earth-size planets in the habitable zones of stars; the news stirred hope that habitable planets are common throughout the galaxy.

But Venus too is an Earth-size planet tucked just within the habitable zone. If not for its thick atmosphere, Venus’s surface would be cool enough to support liquid water. To seek out signs of life on other worlds, astronomers need to learn how to distinguish between planets with Earth-like climates and those that are more like Venus. “That’s why we’re interested in Venus,” says Larry Esposito, a planetary scientist at the University of Colorado at Boulder. “We want to know: Throughout the universe, how many planets are good like us, or 
how many are evil like Venus?”