Earth Like Planets
Artist's impression of exoplanets around other stars. Credits: ESA/AOES Medialab
We know that earth is a pretty rare planet in our universe. Only a relative handful of planets are rocky and even fewer are in the habitable zones of their stars. At the same time, there is still a lot that we don't know about our universe outside the solar system.
The challenge in finding earth-like planets is the sheer distances involved. It involves light years even between our sun and the nearest star. We can currently observe stars and other galaxies, but it is even more difficult to find a planet. The first challenge is that planets are often much smaller than the stars they orbit. The earth is one millionth the size of the sun. The second challenge is that planets don't emit or reflect much light. With these two conditions using ordinary astronomy tools to find planets is impossible.
Knowing this how have scientists been able to detect the over 200 extrasolar planets found so far? Scientists have done so indirectly by observing the frequency of light from individual stars. Stars don't only twinkle or dim because of dust in our atmosphere. They also do so when one of their planets pass between them and our point of observation on Earth. One way to detect this is with the Doppler method. It there is a shift detected in the frequency of light from star in a regular pattern then scientist can safely guess that it has a planet orbiting. They can also with further analysis determine the mass of the orbiting planet as well.
Earth like planets have to fit a more stringent set of guidelines than just orbiting a star. First they have to be rocky planets. There are already telescopes that can detect the wave lengths of dust in other star systems. A star that shows evidence of this is likely to have some rocky planets since scientist hypothesize that the inner solar system was formed by collsions between asteroids. This produced dust as a by product.
The next criterion is that the star have a suitable mass. A star that is too massive is likely to be a gas giant. Also there is the fact that it is highly likely that only a planet with gravity close to ours can sustain life. The last criterion is that the planet be in the habitable zone of its star. This is the area close enough to not be frozen but far enough away that it can produce liquid water and have a livable climate.
New Telescopes such as Keppler are helping scientist to learn more about the planets of others stars. If we can find planets like our own we can speculate that we may not be alone in the universe.
If you enjoyed this article there are several others on Universe Today that you will enjoy. There is a great article the first direct spectrum of an exoplanet. There is also an interesting piece about planet formation around massive stars.
There are also some great resources if you want to learn more about earth-like planets and planet detection. You can check out the Goddard Spaceflight Center exoplanet web page. You can also check out the exoplanet web page for the NASA Jet Propulsion Laboratory.
You can also check out Astronomy Cast. Episode 34 talks about the possibility of another Earth.
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