Other Worlds Around Other Stars
The detection and study of exoplanets—planets orbiting other stars—is expanding into the realm of Earth-like planets, less than 15 years after the discovery of the very first planet orbiting a star like the Sun. More than 400 planets are known, most discovered by the ground-based Doppler spectroscopic technique, in which telescopes look for a slight variation in radial velocity in stars like the Sun, and in smaller stars. An operating “transit” telescope in space today is capable of detecting planets the size of our own and smaller (Figure 2.1.1). NASA’s Kepler mission, launched March 6, 2009, observes more than 100,000 stars in the “Orion arm” of our Milky Way galaxy for a telltale dip in their light output which, if regular and repeatable, represents the passage or transit of a planet in front of the star. A French and European Space Agency precursor to Kepler, called COROT, has during its 2½ years of observations already detected planets as small as about 1.7 times the diameter of Earth. With these missions in operation, we will know in the next 5 years just how common Earth-size planets located on short orbits close to their stars might be in our galactic neighborhood.
FIGURE 2.1.1 Kepler measurements of the light from HAT-P-7. The larger dip is that due to a planet about 1.4 times the radius of Jupiter transiting in front of the star, reducing the light of the star by about 0.7 percent. Such a drop has been observed from ground-based telescopes. However, the smaller drop, about 0.013 percent of the light of the star, is seen by Kepler as the planet itself passes behind the star—hence Kepler is directly detecting the light of the planet itself. Such accuracy and precision are beyond ground-based telescopes and are sufficient to detect an Earth-size planet in transit across Sun-like stars. SOURCE: NASA press release and W.J. Borucki, D. Koch, J. Jenkins, D. Sasselov, R. Gilliland, N. Batalha, D.W. Latham, D. Caldwell, G. Basri, T. Brown, J. Christensen-Dalsgaard, et al., Kepler’s optical phase curve of the Exoplanet HAT-P-7b, Science 325:709, 2009.