Ten to 30 Alien Planets for Every Person on Earth?
Jan. 13, 2012
Planets orbiting stars other than the sun may be the rule rather the exception, an international team of scientists has reported. In fact, the Milky Way alone–an average-sized galaxy among the billions of galaxies in the universe–probably contains at least 160 billion planets. And even that, the scientists said, is a conservative estimate.
The scientists reached that number after making a statistical analysis of planets found using gravitational lensing, one of the main methods for hunting for extrasolar planets. They calculated that each star in the Milky Way has an average of 1.6 planets. Then they multiplied this number by the 100 billion stars estimated to belong to the Milky Way. Gravitational lensing, however, is not useful for finding exoplanets less than five times larger than Earth. Nor is it especially useful for finding planets that orbit closer to their star than Venus or farther from their star than Saturn. So the total number of alien planets is certainly much higher.
Three rocky planets smaller than Earth--the first alien planets that small found outside the solar system--have been found orbiting a red dwarf about 120 light-years from Earth. The smallest of the three planets (foreground), which is about the size of Mars, actually orbits the farthest from the star. NASA/JPL-Caltech
Evidence for that conclusion was the discovery of the first exoplanets smaller than Earth outside the solar system. Astronomers studying data imaged by the Kepler space telescope found the three rocky planets about 120 light-years from Earth. One of the planets is about the size of Mars, about half the size of Earth. All three orbit so close to their star, a red dwarf, that their sizzling surface temperatures rule out the existence of life as we know it. The Kepler scientists said the galaxy might have many more small planets than it has large planets. Astronomer John Johnson, one of the authors of the study, said that small exoplanets were “kind of like cockroaches. If you see one, then there are dozens hiding.”
Additional World Book articles:
- Back in Time (Astronomy 1996)
- Back in Time (Astronomy 1999)
- COROT
- In Search of Other Worlds (A Special Report)
- Kepler, Johannes
- Transit
