“Goldilocks was hungry. She tasted the porridge from the first bowl.
‘This porridge is too hot!’ she exclaimed.
So, she tasted the porridge from the second bowl.
‘This porridge is too cold,’ she said.
So, she tasted the last bowl of porridge.
‘Ahhh, this porridge is just right,’ she said happily and she ate it all up.” -Goldilocks and the Three Bears
Life as we know it on Earth all makes great use of one particular molecule in one particular phase:
Liquid water! Having a liquid phase to water requires an atmosphere with enough pressure (and something with even 1% of Earth’s atmosphere will do it), but it also requires the right temperature to be a liquid.
And this is important! Water in its solid phase doesn’t really lend itself — in our experience — to the flourishing of successful lifeforms.
And it’s the same deal at the other extreme: water vapor — water in gaseous form — doesn’t have the same properties as liquid water. And we don’t know of any lifeforms that make use of water vapor for vital biological processes the way they do with liquid water.
And that’s what happens if the temperature gets too hot: liquid water boils and goes into a gaseous phase.
(Although it makes life taste delicious, it doesn’t, in our experience, make life from scratch.)
And as far as we know — which isn’t all that far — the Earth is the only planet with liquid water on its surface, and the only planet with life on it.
Earth has the right temperature range for its pressure — between 0 and 100 degrees Celsius at 1 atmosphere — for liquid water to exist in vast quantities. What’s more, is that Earth has had these conditions for the entire 4.5 billion year lifetime of our Solar System!
Sure, there are some other worlds that may have had liquid water once.
Venus, for example, is estimated to have had the right conditions for liquid water for about the first 300 million years of the Solar System. But the atmosphere got too thick, and the once-Earthlike temperatures skyrocketed. Thanks to the extreme greenhouse effect it experiences, the average temperature on Venus is now about 500 degrees Celsius, which is far too hot for liquid water.
And the opposite is true for Mars! Despite being so much farther away from the Sun, Mars is still close enough that, if it had an Earth-like atmosphere, it probably had liquid water on its surface for more than a billion years! (Dried up riverbeds seem to support this.)
But with most of its atmosphere lost due to the solar wind, Mars is now a frozen wasteland, with no liquid water on the surface.
But, as you know, we are not limited to the Solar System, and each shining light in the night sky holds the promise of a possibly habitable world!
Because every star has what we call a habitable zone (or Goldilocks zone) around it, where the temperature is just right for a planet with an Earth-like atmosphere to have liquid water!
Can you guess why we’re excited about this latest, sixth planet orbiting Gliese 581?
Because, not only it is a rocky planet like Earth, but because it’s smack in the middle of its habitable zone!
Now this doesn’t mean that this planet has life on it, or even liquid water, but it does make it the best known candidate outside of our Solar System.
More importantly, it’s the first great candidate, and it’s only 20 light years away! (The closest star outside of our Solar System is already over 4 light years away, so this really isn’t that far!)
We think that probably about 10-20% of all stars have planets in their habitable zones, but this is the first time this scientific idea has been verified outside of our own Solar System! What’s the next step? The next step, of course, is to build a good enough telescope with a spectrometer (or a differential brightness mechanism to look for clouds) to look for signatures of liquid water on this world; but I think this step is totally one worth celebrating! Pretty soon, it’ll be time to call the new alien ambassador…