In my last post, I wrote a bit about Kepler’s search for habitable planets, ones that had the right conditions for human life to exist on them. So what exactly makes a planet the “right” one for us to survive on it? Two conditions, really:
(1) It has to be the “right” size.
(2) It has to be in “right” distance from its sun (star).
What’s the right size for a habitable planet? Anywhere between one-half to twice the size of our Earth is the simple answer. But why that size range and not any bigger or smaller?
We need air to breathe, which means we need the planet to have an atmosphere. If the planet is too small (think Mars), then it does not have enough gravity to hold onto air molecules. So that’s why there’s no atmosphere on Mars — I always wondered about that and I just new that Red Planet wasn’t telling the story right! If the planet is too big (think Neptune), then it will have too much gravity and too much atmosphere for us to survive in. Earth, then, is just the “right” size to hold onto the perfect amount of atmosphere (for us, at least, if not the Klingons and Vulcans).
Now, let’s look at the “right distance” condition, which is more commonly referred to as the habitable zone. Think of this habitable zone in terms of our own solar system orbiting around our own big star (the Sun). Mercury is too close to the Sun, so it’s baked and fried in terms of us trying to live there. Jupiter? Too far away, so too cold. Hard for us to live there without a lot of external support systems in place. Earth? Just the right distance to keep us warm and happy. Look at the Planet Temperature and Size graphic to see some fun facts: lead melts on the planet Mercury. Ouch, that’s hot.
To put this whole concept of “right” conditions in terms of a childhood fairy tale:
Mars? Too small
Neptune? Too big
Earth? Just right
Mercury? Too close
Mars? Too far
Earth? Just right.
So now you know all that, what has Kepler found so far in its almost two years of searching that small patch of sky in the Cygnus starfield? Over 700 planet candidates! Out of these, so far they have 8 planets confirmed. They call them “exoplanets” because they our outside our own solar system. None of these confirmed exoplanets is the right size as you can see. As a matter of fact, they are all rather huge compared to Earth (that little white dot on the right side of the graphic). We’re feeling kind of small right now.
(Both graphics are courtesy of the Kepler Mission web site.)