Exoplanets: How Many Exoplanets have been found? What is the Kepler Mission? Is there Life Outside the Solar System?
An exoplanet, or extra-solar planet, is a planet outside our solar system. Astronomers are interested in exoplanets because of the possibility that such planets could support extra-terrestrial life. It is also interesting to compare other solar systems to our own to see just how unique or commonplace our solar system is.
At the time of writing, 562 exoplanets have been discovered.
How Are Exoplanets Discovered?
The first exoplanets were identified by noticing anomalies in the orbits of binary stars. The gravitational pull exerted by an exoplanet causes the orbit of a binary star to be disrupted. Astronomers can make detailed measurements of the star's radial velocity (its velocity towards or away from Earth) by looking for a Doppler shift in the spectrum of the light radiated by the star - if the spectrum is blue-shifted then the star is moving towards us, and it is red-shifted if the star is moving away. By looking for tiny anomalies in these radial velocity measurements, astronomers can deduce the presence of an exoplanet.
It is also possible to "see" a exoplanet as it passes in front of the star, by measuring a very slight decrease in how bright the star appears.
The Kepler telescope was launched into orbit around the Earth in 2009. Its purpose is to continuously monitor over 100,000 stars to look for variations in their brightness that could indicate an exoplanet passing in front of the star. It has already detected signs of over 1200 exoplanets, and that is by watching only a tiny fraction of the sky. Although some of these results may be false positives, it appears that there is a huge number of exoplanets out there.
Extra-Terrestial Life on Exoplanets
The discovery of so many exoplanets is exciting because of the possibility that some of them could support extra-terrestrial life.
Most of the early exoplanet discoveries turned out to be gas giant planets, similar to Jupiter and Saturn in our solar system. Our solar system appeared to be unusual in that the closest planets to our Sun are small, rocky planets rather than gas giants. However, there may be a selection bias at work: larger planets are easier to detect, so it makes sense that we would see more of these compared to Earth-like rocky planets.
Sixty-eight of the exoplanets detected by Kepler are Earth-sized. Could one of these have the correct atmosphere and planetary conditions to support life?
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