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Happy Earth Day!

Happy Earth Day! Don't we live on a beautiful planet?

[Earth: The Water Planet. Photo credit: NASA]

Of all the (eight, sadly) planets in our solar system, ours is the only one parked at just the right distance from the sun to support life. This distance is called the Goldilock's Zone - where it's not too hot (close to the sun) and it's not too cold (far away from the sun) but instead (you guessed it) just right for supporting life.

The Goldilock's Zone, also known as the less exciting "habitable zone," is the distance from a star where a planet like Earth can keep liquid water on its surface without it freezing or boiling away, enabling it to support carbon-based life like that on Earth. This zone exists both within a planetary system, like our solar system, and also within a galaxy.

The Zone is not the same for every planetary system. A star that shines 25 percent as brightly as the sun will have a habitable zone that's twice as close to the star as we are to the sun. Planets in that smaller-star system have to be closer to the star to receive enough warmth to keep water in a liquid state. The opposite is true for stars larger than the sun, where planets would need to be further away to prevent water from boiling and evaporating away.

An example of a planet thought to be in a Goldilock's Zone is Gliese 581 g. It orbits the star Gliese 581, part of the constellation Libra, 20.5 light-years away. In 2008, a group from the social networking site Bebo sent a "Message from Earth" in the form of a digital time capsule to Gliese 581 g. The messages were converted into binary code - a series of zeros and ones - and were sent towards the planet by a radio telescope in the Ukraine. The messages zipped by the moon after just 1.7 seconds and Mars after four minutes. It will pass by Gliese 581 g in 2029. Another message, "Hello from Earth" was sent in 2009. Other messages attempting to alert other potential life forms to our presence have been sent out as well.

Both planets and moons are considered when it comes to heavenly bodies that can support life within a Goldilock's Zone. How many planets or moons that could support life have been found thus far? In February, it was announced that NASA's Kepler satellite, which hunts for planets, had found over 1,200 planets, 54 of which could be in the Goldilock's Zone. As exciting as this is, the scientists say it will take years to confirm whether or not these 54 candidates are actually planets. We'll have to wait and see.

Oh, and those messages sent to Gliese 581 g? Don't hold your breath waiting for a reply. If the messages are received and intelligent life does respond, we won't hear back for another 40 years - the time it takes for our message to be received (20 years) and the time it would take for a return message to arrive (another 20 years). In the meantime, you might want to thank your lucky stars that you're on the one known and proven planet that supports life as we know it - Earth.

[The infamous original portrait of Earth, as seen by Apollo 17. Photo credit: NASA]


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