Venus has a thick atmosphere, crushingly high air pressure, and hot surface temperatures that can melt lead. It has some of the most inhospitable surface conditions in the solar system.

Venus sits inside the "Goldilocks" region, which is ahabitable zone. The current definition of a habitable zone only looks at the amount of sunlight reaching a planet. The planet is not a good candidate for life if liquid water is too much or too little. According to this criterion, Venus can potentially support liquid water. But it does not. Does this make Venus-like planets rare, or should we start questioning our definitions?

New research using simple models of the atmospheres of Venus-like planets has found that they are frighteningly common.

What makes a planet hospitable? Our assumptions may be wrong.

The runaway greenhouse 

Astronomers don't know what went wrong with Venus. We are left to guess how this planet turned out, without detailed measurements of the surface.

Because Venus is the same size as Earth and formed in the same neighborhood, many planetary scientists believe that Venus has the same amount of carbon, oxygen, and water as our own planet. Venus probably started out with pools and oceans on its surface.

Something went very, very wrong.

A long period of active volcanism may have beefed up Venus. The sun's increasing illumination may have caused it to evaporate all of the water on the surface. Maybe it was a process that we don't understand.

Venus had a runaway greenhouse effect. With every increase in atmospheric pressure, the temperatures rose, which caused even more gases to enter the air, feeding off each other in a disastrous cycle. Venus was unable to cool off because it received too much radiation from the sun. As a result, the oceans dried up, sending all of the water into the atmosphere, where it eventually leaked into space, never to return.

The primordial oceans were not having a good time.

The living world

Earth is the only place in the universe known to host life, so we need to identify planets that are in the right zone of their stars. There may be more than one form of life out there. Earth-like life is the kind we would most easily recognize, so it is an easy target.

It is difficult to provide a simple definition of the habitable zone on planets. Venus should have water on its surface because it gets the right amount of sunlight. With the planet's super thick atmosphere, the temperatures on the surface are too hot to support any liquids.

A group of researchers tried to find a dividing line between Earth and Venus. A paper recently published to the preprint database arXiv used a relatively simple model of planetary atmospheres and the kind of radiation those planets would receive from different kinds of stars.

The researchers started with an Earth-like mix of atmospheric gases and gradually increased the amount of carbon dioxide to mimic the beginnings of a runaway greenhouse effect. They let the model evolve to see what would happen as time went on.

When the models blew up, they declared a model planet like Venus. They designated the model planet as Earth-like if it self-regulated and stayed within the habitable zone.

The banality of evil 

The researchers found that Venus-like worlds are common and that some parts of the zone are off-limits to life.

Around a sun-like star, the traditional habitable zone stretches from 85% of Earth's orbital radius to 161%. The models found that the outer edge of the Venus zone reached 135% of Earth's distance from the sun.

Roughly 40% of F-type stars' habitable zones were able to survive. Small red dwarf stars emit most of their radiation in the infrared bands, which are easily locked away by atmospheric gases. The Venus zone consumed most of the edges of the habitable zone for these stars.

Hope is not lost. The models are simple and the planets are complex. Not every planet can enter a runaway greenhouse cycle. Extra amounts of water or plate tectonics may alter planets trajectory. We have to be careful when searching for Earth-like planets because not every Venus-like planet is doomed to become a horrible world.

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