The infernal planet Venus is known for its incredibly dense atmosphere, extremely high air pressure and a surface temperature that is hot enough to melt lead. In other words, here are some of the most hostile surface conditions The solar system.
But adhering to the standard definition of “residential area”, Venus sits inside this region “Goldilocks”. This is because the current definition of a habitable zone explores only the amount of sunlight that reaches the planet. If it is too much or too little, then liquid water cannot exist on the surface, and therefore the planet is not a good candidate for life. According to this simple criterion Venus is fit for habitation; that is, it can potentially maintain liquid water. But this is clearly not the case. So does this make Venus-like planets rare, or should we start questioning our definitions?
A new study using simple models of the evolution of the atmospheres of Venus-like planets has found that these worlds are terribly common.
Astronomers aren’t quite sure what went wrong with Venus long ago. Without detailed surface measurements we have to wonder how this planet turned out to be so ugly.
Because Venus is about the same size as Earth and formed in about the same area, many planetary scientists believe that Venus began very much like our planet, with about the same amount of all the important ingredients: carbon, oxygen, silicon … and water. It is very likely that Venus began with pools – and even oceans – with liquid water on the surface contained in a pleasant atmosphere.
And then something went very, very wrong.
Perhaps the long period of active volcanism has intensified The atmosphere of Venus not repairable. Perhaps it was just normal evolution the sun, its increasing illumination evaporates all the water on the surface. Or maybe it was some process we don’t understand yet.
Regardless of the exact mechanism, Venus survived the escape greenhouse effect. With each increase in atmospheric pressure, the temperature rose, which drove into the air even more gases that feed each other in a catastrophic cycle. Once enough gases were in the atmosphere, Venus could not cool down, trapping almost all the radiation it received from the Sun. As a result, the oceans dried up, sending all the water into the atmosphere, from where it eventually leaked into spaceto never return.
If there was life in these primeval oceans, it is not very good.
The living world
In our search for inhabited worlds, our first step is to identify the planets that lie in the habitable zones of their stars, because that is where the Earth is in its orbit around the Sun, and the Earth is the only place in the universe that is known to contain the life we we know. Yes, there may be other life forms. But life similar to the Earth is something we easily recognize, so it becomes an easy target.
But the planets are complex, making it very difficult to give a simple definition of habitat. There must be water on the surface of Venus; it gets the right amount of sunlight to make it possible. But with the planet’s super-heavy atmosphere, the surface temperature is too scorching to withstand any liquids (except lead, which, while amazing, isn’t very appealing).
Now a group of researchers has tried to find a dividing line between planets similar to Earth and Venus. In an article recently published in the preprint database arXivthey used a relatively simple model of the planetary atmosphere and the kind of radiation that these planets would receive from different species stars.
For each installation, with different types of stars and different orbits around these stars, the researchers started with a mixture of Earth-like atmospheric gases (mostly nitrogen, with a small amount of carbon dioxide) and gradually increased the amount of carbon dioxide to mimic the onset of the greenhouse effect. They then allowed the models to evolve to see what would happen to the composition of the atmosphere over time.
They announced a model of a planet “similar to Venus” when the model exploded and a real escape began. When the planet’s model stabilized and self-regulated, avoiding the escape scenario, they labeled it “Earth-like” and still in a habitable zone.
The banality of evil
Researchers have found that Venus-like worlds are surprisingly common and that large chunks of living space are potentially outside of life.
For example, around a sun-like star, the traditional habitat stretches from 95% of the Earth’s orbital radius to 167%. But these models have found that the outer edge of the “zone of Venus” reaches 135% of Earth’s orbit, which means that our planet may one day be exposed to its own greenhouse effect!
Type F stars, which weigh 1.0-1.4 times more than the sun, performed best, and preserved about 40% of their habitable zones. Small red dwarf stars worst of all, because most of their radiation they emit in infrared ranges that are easily captured by atmospheric gases. For these stars, the Venus zone absorbed all but the extreme edges of the habitable zone.
However, hope is not lost. These models are simple, and the planets are complex. Not every planet that can enter the greenhouse cycle is forced to do so. Interesting atmospheric mixtures, protection from planetary magnetic fields, extra water or plate tectonics can change the trajectories of the planets. Not every planet similar to Venus is doomed to become a hellish world, but we must be careful looking for similar ones to Earth.