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Earth is outside habitable zone?

Kepler has been looking for rocky worlds that could possibly retain liquid water on their surface
Kepler has been looking for rocky worlds that could possibly retain liquid water on their surface

Well, first, the BBC asks: What makes a planet habitable?


Water in liquid form is thought to be a necessity for life on Earth.

Based on this, let’s look at the classical definition for the habitable zone as the region around a star, such as our own Sun, where the temperature of any orbiting planet permits water in liquid form.

But, as it happens, there are difficulties.

What if the planet sports a blanket of white clouds? Clouds are reflective and therefore will cool the planet, acting to push the habitable zone closer to the star.

Amusingly, if we calculate this “equilibrium temperature” for the Earth, taking into account its beautifully reflective clouds, then it turns out that we live outside the classical habitable zone!

The same calculation for Venus gives an expected equilibrium temperature of about -10̊C, but in reality it is more like 450̊C.What happened?

Good thing to find out. Further complexities:

Just to complicate matters, the habitable zone also depends on the type of star the planet orbits. The more massive and hotter the star, the further out the habitable zone will lie.

Conversely, small cool stars will have a habitable zone that is much closer in.

I haven’t even begun discussing some of the “rare-Earth” arguments that point out a range of factors that affect the Earth that may be necessary for life, but that may be rare for other planets. More.

This piece, by Dr Christopher Watson is senior lecturer in extrasolar planets and low-mass stars at the Astrophysics Research Centre, Queen’s University, Belfast, is a useful corrective to the usual hype and hooplah every time a planet is discovered with some feature or other vaguely reminiscent of Earth.

The hype often sounds like someone spotting a broken skateboard in the driveway, and describing it as a four-wheel drive vehicle.

See also: Don’t let Mars fool you. Those exoplanets teem with life!

How do we grapple with the idea that ET might not be out there?


Copernicus, you are not going to believe who is using your name. Or how.

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