"More than 100 planets have been detected around other stars in the last 10 years.... None of these are thought to support life because they are [gas giants].... Earth-sized planets have not been discovered [yet because] it is harder to spot smaller planets--much harder."
THE SEARCH FOR LIFE outside the Earth is more active than ever as telescopes probe for intelligent life on other planets while robots scour the surface of Mars. There even are desktop computers that predict which stars might be orbited by Earth-like planets. Moreover, planets similar to Jupiter are being discovered around other stars on a monthly basis.
Imagine a planet orbiting a faraway star. It is a bit larger than Earth and is completely covered by a miles-deep ocean. We think that such "water world" planets exiSt around some stars. We also surmise that there are others with less water, maybe even much drier than Earth. Can they harbor life? To answer this, we need an understanding of life here on our own planet. How and when did life originate on Earth? Where did Earth come from? Are there "Earths" orbiting other stars in our galaxy?
Earth formed and resides in the "habitable zone." the distance from a star at which liquid water may exist on the surface of a planet. All life on our globe requires some interaction with water. If a planet is too close to its parent star, like Venus, for example, water will evaporate from its surface; if it is too far, like Mars, then water only can exist as ice. Like Goldilocks' third bowl of porridge, Earth lies where the temperature is just right. So, the quest for life on planets around other stars begins as a search in the habitable zone.
Astronomy has been an active science for thousands of years, and high-powered telescopes have existed for almost a century. However, planets around other stars only have been detected in the last decade. Why can't we just use our big, fancy telescopes to take pictures of other planets? Because a planet orbiting another star appears about 1,000,000,000 times fainter than the star! It is washed out completely by the light of the star and impossible to see. It is like trying to hear a whisper from across the stadium during the Super Bowl.
More than 100 planets have been detected around other stars in the last 10 years. However, none of these are thought to support life because they are massive balls of gas hundreds of times as large as Earth with no surface to stand on! The reason that Earth-sized planets have not been discovered is that it is harder to spot smaller planets--much harder. The method used relies on the wobble of a star as a planet goes around it.
For example, picture a seesaw with a football player on one end and a kitten on the other: to balance the seesaw, the fulcrum needs to be placed very close to the football player. The kitten travels way up and down, but the muscular athlete moves very little. The idea is the same for a planet orbiting a star. Like the kitten, the planet is much less massive than the star, and it moves very far compared with the star. Yet, like the football player, the star does move, albeit in a much smaller orbit. The star's wobbling can be detected with extremely sensitive instruments, and the presence of the planet can be deduced--remember, we merely can see the light of the star, not the planet. It is easier to locate more massive planets since the star's wobble is greater, just like the football player's motion is greater if he is balanced by a 10-year-old boy instead of a kitten. The wobble of a star due to the orbit of a terrestrial planet is so small that, employing this method, finding such a planet is nigh impossible.
New techniques, however, always are on the horizon, and a pair of upcoming space missions hope to detect a terrestrial planet around another star: the Europeans are launching Corot in 2006 and NASA is unleashing Kepler in 2007. The hope is to uncover …