While Venus Simmered, Fast-Cooling Earth Thrived

For rocky planets like Earth the key to holding on to water, a prime ingredient for life, is: location, location, location.

For rocky planets like Earth the key to holding onto water, a prime ingredient for life, is: location, location, location.

This is the conclusion arrived at by a team of Japanese researchers studying why Earth and its sister world Venus ended up so different.

Scientists believe the second and third planets from the sun started more or less as twins, with similar elements and environments, including magma oceans that likely lasted hundreds of millions of years.

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But Venus, positioned roughly 67 million miles from the sun, took more time to cool off and solidify its core, thanks to a steamy lower atmosphere that ended up costing the planet all its water. Meanwhile, 25 million miles farther away, Earth cooled more quickly, enabling it to retain its water, a new computer model shows.

Lead researcher Keiko Hamano, with the University of Tokyo, said the idea for the study stemmed from the realization that baby planets' atmospheres and interiors would interact with each other.

He integrated previously separate studies on planet evolution and came up with a simple model that explains why terrestrial bodies like Venus and Earth, although both located within the sun's so-called "habitable zone," where liquid water can exist, ended up so different.

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The researchers identified two types of rocky bodies, based solely on distance to a parent star. One type, like Venus, stays molten for a prolonged period of time, which allows its volatiles to escape. The others, like Earth, solidify within several million years, which allows them to keep their water and form oceans.

The study presumes that Earth, located about 93 million miles away from the sun, and Venus, about one-third closer, formed roughly to their present locations.

"There's no reason to think otherwise," planetary scientist Linda Elkins-Tanton, with the Carnegie Institution for Science in Washington D.C., told Discovery News.

The distance of a planet to its host star is just one of many factors likely to impact its ability to host life. In addition to having the right temperature and conditions for surface water, a planet's composition, ability to experience plate tectonics and formation of a protective magnetic field also may be key to habitability, Elkins-Tanton points out.

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"We would like to expand this study to explore how other parameters, such as planetary mass, the type of a host star and other gaseous components affect early evolution of terrestrial planets," Hamano added in an email to Discovery News.

The research is of interest to scientists looking for planets, particularly habitable ones, beyond the solar system.

"The habitability of Earth and the inhospitability of Venus may be the inevitable result of our planetary sibling order next to the sun rather than later evolutionary bifurcations. If so, simi­lar patterns of habitability are likely to be found in exoplanets," Elkins-Tanton writes in a paper published along with Hamano's research in this week's Nature.

Earth and Earth's "twin" Venus -- a planet with an atmosphere that contrasts dramatically with its terrestrial sibling.