Is Earth really in the habitable zone, and can an exoplanet in the habitable zone be not habitable?

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Everyone knows that stuff like Co2 warms the atmosphere (shocking I know). But I recently heard that if there was none of the natural Co2 we already have, the average temperatures on earth would be around -15 degrees Celsius (could be wrong about that number). I would assume that this is not ideal for life, and if this is the case, is Earth really in the habitable zone, or is it just habitable because of the greenhouse gases in the atmosphere that make it warmer? Is the variablity of the amount of greenhouse gases accounted for when calculating how big the band of the zone of habitability?

Edit: Got my answer, thanks everybody!

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“The habitable zone” is just a broad description for the rough distance from a star that planet should be to *have a chance* of being habitable. It can still be a completely hellish place quite easily. Mars isn’t habitable, but it is in the habitable zone, for example. And as you say, if Earth’s atmosphere were different it also wouldn’t be habitable, even though it is in what we call the habitable zone. It’s really just that within this zone *it is possible* for a planet to be habitable.

The habitable zone is merely a possibility that a planet in the area could have liquid water, which is the critical component to life as we know it. This is determined by the star’s luminosity (brightness) and the planet’s orbital radius. It is not an actual guarantee that the planet is habitable. For instance, Mars is technically in the habitable zone, yet it’s clearly not capable of supporting life as we know it. Likewise Venus skirts the edge of the habitable zone, but its atmospheric pressures, atmospheric composition, and temperature is incredibly destructive.

That zone of habitability (sometimes called the “Goldilocks Zone”) is actually tied to whether there can be liquid water on the surface of a planet that has enough atmospheric pressure.

That condition is tied to the distance from the sun and how much energy it emits, and only accounts for the possibility that if there’s enough of an atmosphere, there would be the possibility of liquid water on the surface.

As to the temperature ranges that support life, plenty of life exists at temperatures of -15 degrees. The question of whether life as we understand it could evolve in such conditions is different, but many of the oldest forms of life are “extremeophiles”, existing in conditions we would normally not associate with being able to support life.

And lastly, carbon is extremely common- it would be unlikely to find any planet with an atmosphere that doesn’t have at least some CO2, or systems that emit CO2, like volcanoes.

>Is the variablity of the amount of greenhouse gases accounted for when calculating how big the band of the zone of habitability?

Yes. It’s the range in which it’s considered *possible* with our current knowledge of different climates.

To answer the somewhat underlying second question about greenhouse gasses, it’s important to remember that greenhouse gas emissions are not entirely man made. The atmosphere containing these greenhouse gasses is definitely a part of how our planet is survivable and there are natural ways these gasses are created.

Of course that doesn’t excuse the fact that we humans can over produce them, but the presence of CO2 isn’t itself a bad thing and yes that contributes to why we can survive on earth.