Venus is nearly the same size as Earth and is 25.8 million miles closer to the Sun, so it receives about 40 percent more solar radiation. It's still on the edge of the so-called habitable zone, but the atmosphere is heated in the extreme by greenhouse effects caused by the dense, mostly carbon dioxide atmosphere. The thick clouds also reflect 70 percent of sunlight back into space, making it the brightest object in the sky, other than the moon.

Astronomers figure it wasn't always like that, though. While Venus now has very little water in its atmosphere, it could have had oceans as deep as 525 meters (1,700 feet) billions of years ago. The atmosphere was likely far less thick and toxic, too. The Goddard team plugged all of those factors into climate models used for Earth, and they showed that it had a mean surface air temperature of 11 degrees C (52 degrees F), with max temperatures at 95 degrees F. That means that several billion years ago it could have been just as likely to support life as Earth, if not more so.

The surface of Venus in false color (NASA/JPL)

There are a lot of "ifs," though. The simulation depends on the rotational period and topography of Venus being about the same as they are today. Venus currently takes a ridiculous 243 Earth days to spin once on its own axis (longer than its 225-day year), but planets often spin faster when they're younger. When the simulation was re-run with a much faster rotational period of 16 Earth days, average temperatures were nearly five times hotter -- 133 degrees F, maxing at an inhospitable 183 degrees F. Changing the landscape to be more Earth-like, meanwhile, yielded temperatures warmer by 18 degrees F, which is also borderline for life.

The new models don't prove that Venus was once habitable, but they do show that it could have been. That knowledge may allow astronomers to reconsider exoplanet habitable zones by taking rotation into account. It also provides another theory for how life got on Earth. Scientists already think that meteor impacts spread materials between Mars and our planet, possibly seeding life. If Venus harbored life billions of years ago, it might have done so, too.

The work may also motivate scientists to search for life on Venus as well as Mars. That won't be easy, however, because the Venera probes Russia sent didn't last more than a couple of hours in the extreme temperatures.