The first Earth-sized planets discovered were orbiting a pulsar (PSR B1257+12), which had gone supernova a long time ago. These are bombarded with lethal radiation from the star. Needless to say, they are dead worlds, highly unlikely to support life.
The discovery was made more than a decade ago, prior to the discovery of the first planet orbiting a sunlike star: the planet (51 Pegasi b, Bellerophon).
Other than these planets, astronomers have found mostly massive planets in Jupiter's class (Jupiter's mass: 313 x Earth's mass). But using the radial velocity method, astronomers are today able to detect Neptune-sized planets (called Super Earths) with a mass of about 15 times more than the Earth.
However, even smaller planets are possible to be found using the photometric transit method in which a planet crosses in front of the disc of a star. This method is used by the successful Kepler mission. Statistics say that planets the size of Earth should be more common than large Jupiter class planets, but finding them proves challenging. Though, astronomers are discovering smaller and smaller worlds and the goal is to find an earthlike planet in the
habitable zone around a sunlike star. They also hope that the planet is in an earth-like stable environment with plenty of water.
The most Earth-like extrasolar planet found today orbits the star GJ 876, which is a red M-class dwarf (M 3.5 V to be more accurate, and with a mass 1/3rd of the sun), the most common type of stars in the galaxy GJ 876 is located in the constellation of Aquarius at a distance of 15.2 light years. Not only does the system host two Jupiter like planets (which seems to be very unusual around red dwarfs), but they also host a terrestrial planet the size of the Earth (announced on June 13, 2005), which orbits the parent star from a distance of about 3.2 million kilometres ( 0.021 AU), which allows it to complete an orbit in just two days.
The temperature at this distance would be from 200 to 400 degrees Celsius. The mass of the planet is thought to be 5.9 Earth masses, and it was detected using the radial velocity method.
Previous: Gas Giants.
1 Jupiter Mass = 1.9 x 10^27 kg,
1 Earth Mass = 5.94 x 10^24 kg,
1 AU = 150 million km.
Above: Recently astronomers discovered the largest extrasolar planetary system with six confirmed planets. It's called Kepler 11b as it was discovered by the space based observatory Kepler. The system is very compact (all discovered planets orbit correspond to the orbit of Venus, or tighter). The planets are Neptune-sized, or smaller.
This illustration is available upon request, as a Print (6000x3750 pixels, 300 dpi).
Space art 1: An extrasolar terrestrial planet as seen from directly above the north pole. A moon is visible at bottom/right.
Space art 2: An extrasolar terrestrial planet, which orbits at the outer edge of the parent star's habitable zone, where water would be close to freezing point. Furthermore, this planet has entered an ice age, making it almost completely covered with ice.
Space art 3: The terrestrial planet orbiting the star GJ 876. It is thought to be between 200 and 400 degrees Celsius hot.
Space art 4: This depiction of an extrasolar planet has a more artistic nature. The title of the image is "Cold of Space", and is also available for wallpaper.
Space art 5: The image above illustrates a planet about to get absorbed by the sun that it orbited around, but through gravitational interactions with other objects in the system moved towards the star. Soon it will be no longer, but the lithium it contains will shine in the star's spectrum, witnessing about this terrible incident. Lithium is a substance that does not naturally linger in stars. The title of the image is "Ash to Ash, Dust to Dust", and is also available for wallpaper.