As noted previously for the star Kepler 37, stars with several planets orbiting them have been observed. Direct images of multiple planets orbiting a star have been taken. However, most are detected using the transit method. To do this, astronomers watch the largest planet transit a star several consecutive times. The interval between transits is the length of the year on that planet. When there are other planets in a star system, including ones that don’t transit their star as seen from Earth, their gravitational tugs on the observed planet change the length of time it takes to orbit the star. So, if the length of a year for a transiting planet changes, then astronomers can deduce that there must be other planets in that system. In 2010, star Kepler 9 was observed to have two transiting planets and at least one nontransiting planet.

Three planets, Gliese 581e, c, and b, with masses of 1.7, 5.5, and 15.8 Earth masses, respectively, have been discovered orbiting another Gliese catalog star, Gliese 581. All of these planets orbit their stars closer than Mercury orbits the Sun, and, hence, they are likely to be in synchronous orbits. This means that they rotate at the same rate that they orbit their star; the same side of each planet continually faces the star just as the same side of our Moon continually faces Earth. Most, if not all, of these planets are believed to be composed primarily of rock and metal with water on or near their surfaces. The stars they orbit are much cooler than our Sun.

As of 2014, 460 stars have been observed with two or more planets that orbit them; several are shown in Figure 5-8.