Artist's concept of an Earthlike planet (Courtesy of NASA)
Could there be another planet like Earth? Searches targeting thousands of stars nearest our Sun may reveal evidence of planets that could sustain life. Some of these planets might be very much like Earth. "Earth-type" planets must be solid bodies (unlike the gas giants Jupiter, Saturn, Uranus, and Neptune in our outer Solar System) with masses roughly between 0.1 - 10 Earth masses. For life as we know it to exist on the surface of a planet without needing special technology to survive, that planet's temperature and atmospheric pressure must support the existence of liquid water and provide energy for complex life-forming chemical reactions. The planet can't be too close or too far from its star, or the temperature and atmospheric pressure won't be just right. That "just right" distance is known as the habitable zone.
With the help of high-powered telescopes, scientists are hoping to find a planet outside of our Solar System where life could exist. In 2008, using data from the Hubble Space Telescope, scientists observed carbon dioxide on a distant extrasolar planet called HD 189733b, which is orbiting its own star. This discovery is exciting because it shows that we can probe the ingredients of an extrasolar planet's atmosphere. While HD 189733b is about the size of Jupiter and is too hot for life to exist, scientists continue to explore. Using the Hubble Space Telescope and the Spitzer Space Telescope as well as the James Webb Telescope after it is launched in 2013, they continue to search for combinations of gases such as ozone, methane, water vapor, and carbon dioxide that, when found together, would be good biosignatures (signs of life) on far away planets.
Discovering New Worlds
The discovery of new worlds is a great example of how tools, observations, and theories help us explore the Universe. Mercury, Venus, Mars, Jupiter, and Saturn have been observed since the beginning of civilization and can be seen with the unaided eye. The word planet comes from the Greek word for wanderer, which is very fitting since the movements of planets in the sky are different from the stars.
Galileo's observations with telescopes, including his discovery of Jupiter's moons, craters on our Moon, and sunspots laid a foundation for our modern understanding of these "heavenly bodies" as other worlds. His observations were critical to the acceptance of the idea that the Sun rather than the Earth is at the center of our Solar System.
The planet Uranus was the first planet discovered through telescopic observations by William Herschel and his sister, Caroline, in 1781. John Couch Adams and Urbain Le Verrier predicted the existence of Neptune before it was actually observed. They used Isaac Newton's studies of gravity to predict where Neptune should be based on irregularities in the orbit of Uranus. Sure enough, Johann Gottfried Galle and Heinrich d'Arrest discovered Neptune at its predicted position in 1846. Over the past two centuries, numerous small, icy or rocky worlds have been discovered orbiting the Sun, including Pluto, which was discovered by Clyde Tombaugh in 1930 and is called a dwarf planet.
Our Sun is one of several hundred billion stars in the Milky Way Galaxy. The most distant objects that orbit our Sun are billions of miles away, but other stars in our Galaxy are trillions, even quadrillions of miles away! Do these stars have planets, too? In 1995, Didier Queloz and Michel Mayor discovered the first planet
orbiting another star similar to our Sun. Since then, over 400 planets have been discovered orbiting other stars, and this number often increases on the scale of weeks or even days. These planets are known as exoplanets or extrasolar planets, and amateur astronomers can detect a number of them. The Extrasolar Planets Encyclopedia is a terrific place to learn about new discoveries.
Discover more about Earth and extrasolar planets from NASA's Planet Quest.
IYA HOT TOPIC: Explore more Telescopes and Space Probes discoveries on NASA's International Year of Astronomy website.
