Is there another Earth out there?

Scientists have discovered a planet nearly the size of Earth and within the “habitable zone” surrounding its star — meaning that it would receive enough light and heat to allow for liquid water.  It’s approximately 500 light-years away from us, so we won’t be colonizing it anytime soon, but it is an interesting scientific finding for a number of reasons.

The big question we’d love to answer — eventually — is “is there life elsewhere in the universe?”  “Life” is a big, vague term that could mean whole bunch of things; it could take a form very different than how it has risen on Earth. And it’s hard to look for something that you can’t define.  So scientists have started with a more straightforward question: is there life like us elsewhere in the universe? But that’s still a tall order. So let’s narrow it more, and ask whether there is life like us living in conditions like ours.

There is a lot we know about our planet that makes it particularly conducive to life. It has the right mix of chemicals. It is large enough to have enough gravity to sustain an atmosphere, but not so large that it has massive, crushing gravity (and thereby massive heat). And very, very importantly, it’s just the right distance from the sun, which allows just enough heat to liquefy water without boiling it all off.  We know that liquid water is critical to formation of our kind of life.

So then if we look upward, we can ask “how many planets like Earth are out there?” Thirty years ago, our technology for detecting the existence of planets orbiting other stars was practically nonexistent. Since then, it’s moved radically forward, especially with the assistance of orbital telescopes such as Kepler. We used to think that planets were rare; we have since learned that they are remarkably common and we are constantly finding more. One of the key techniques for doing so is to look for “orbital transit” — stars that get dimmer for short periods of time as a planet passes between us and the star that it orbits.  By analyzing the amount and the duration of the dimming, we can even identify a planet’s size and the distance from its star. (actually, it helps to have more than one observation as well, since that tells us how long it takes the planet to fully orbit its star) Very cool stuff.

So if we have the size of a planet, how far it is away from its star, and the characteristics of the star itself, we can tell how Earth-like it is and whether it’s within the “habitable zone.”

Today’s find is news, because while we’ve found hundreds of planets orbiting other stars, this is the first one that is close to Earth’s size and in the habitable zone. Why do we care? Because at the end of the day, there’s going to be a statistical calculation that gives us an answer to “how many planets like Earth are out there?” Let’s say we did a thorough survey of 1% of our galaxy and in that sample we found 200 Earth-like planets. If we believed the sample was representative of  the other 99% of our galaxy, we’d then estimate that there were 20,000 planets like ours in the Milky Way. Now there are 100-400 billion stars in our galaxy, so it would take a ridiculously long time to survey 1% — that’s 1 to 4 billion stars. But you could easily argue that a much smaller sample, say 1 million and perhaps far less than that, would be just as statistically representative. We’re still in the very early days of being able to survey stars quickly and accurately enough to build a thorough survey, but the fact that we’ve already found one this early in the search is a positive sign that we’ll find many more. If they were extremely rare, then the odds that we’d find one so early in our search would be extremely low — like finding the needle in a haystack within the first handful we grab for.

Perhaps some of the planets will be close to us, in astronomical terms. If you could travel at the speed of light (which we can’t) it would still take 500 years to get to the one planet we’ve found — and the relativistic effects of doing that would be very screwy. Basically, it ain’t gonna happen. Even if we were to send a message there at the speed of light (which we can — radio or light based),  and there were intelligent life on the other side to receive it and respond, it would take a thousand years for one round trip message. Not something that could possibly happen in our lifetime. The first real breakthrough will be if we find one within 50 light-years — then you could imagine a real exchange within someone’s lifetime, or certainly across two generations.

By the way, David Brin wrote an excellent book of fiction playing out the scenario of how life on a set of far-flung planets might eventually interact. I thoroughly enjoyed it.

These are exciting times. We are learning so much, so quickly, about the universe and our place within it.




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