Some alien planets may bypass habitability

Earth’s orbit within the sun’s habitable zone means its temperatures are just right for life. But icy worlds located within their star’s habitable zone may abruptly skip from too cold to too hot without going through a habitable stage, a new study finds.

The finding suggests that there may be fewer potentially habitable worlds than scientists previously thought, the researchers said.

Whereas Venus seems too hot for life and Mars seems too cold, Earth lies between Venus and Mars, where temperatures have the potential to be just right for its surface to possess liquid water. This “Goldilocks” zone is also known as the habitable zone, because on Earth, there is life virtually wherever there is liquid water.

Stars like the sun brighten over time. This raises the possibility that a planet or moon that starts out cold and icy around a young dim star, such as early Earth, may eventually warm to a stable habitable state as the star grows more luminous. [How Habitable Zones for Alien Planets and Stars Work (Infographic)]

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However, the new study finds that some planets may go directly from a Mars-like icehouse stage to a Venus-like hothouse phase, bypassing Earth-like habitable conditions.

“Previously, studies suggested that icy planets and moons, such as [Jupiter’s moon] Europa, will become habitable for life after their surface ice or snow melts,” said study lead author Jun Yang, of Peking University in Beijing. “Our work shows that this will not happen.”

The climate of a water-rich planet depends on at least two factors. One is how much ice covers its surface. Ice has a high albedo, meaning it reflects much of the light that falls onto it back to space before the light can warm the planet’s surface. In this way, the ice cools the planet, which leads to the formation of more ice and makes the…

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