The number of planets found orbiting other stars has risen steadily since the first two were discovered orbiting pulsars in 1992 and 1993. The number of confirmed extrasolar planets now hovers in the range of 500 – 1100, depending on which astronomers you talk to and which measurement methods one considers verifiable. But the total number of observed planets is set to explode exponentially in number over the next several years, as results pour in from a number of observational surveys, the most ambitious being NASA’s Kepler imaging mission. The data already gathered and made public from this experiment allows us to make some tentative predictions about the total number of planets in the Milky Way galaxy, even if we can’t yet observe them all directly.
The Kepler satellite is currently observing a little over 150,000 stars in our local spiral arm of the Milky Way, in the direction of the Cygnus-Lyra constellation. The system works by watching for temporary dips in the luminosity of those stars, indicating the transit of a planetary body in front of the distant star, something like a stellar eclipse. In order to be observed in this manner, planets have to be orbiting the star in a plane similar to that of our solar system, and that of the entire galaxy. Stars that orbit on oblique planes might not transit their star from our perspective, and thus cannot be measured. Even so, the Kepler data has already returned possible hits on 706 of the total stars under observation. In order to qualify as a hit, the planet must have transited more than once during the observation period, meaning that these are mainly fast moving planets with close orbits. More time will likely increase the total number of positive hits from the Kepler data set, as we capture planets further out with longer duration orbits, like our own Earth.
The most conservative estimate, working from Kepler data released so far is therefore 706/150,000 or 0.47 per cent of observed stars. This would mean that the Milky way galaxy, with its roughly 400 billion stars, contains at least 1.8 billion planets at the low end, and probably many more.
But how many more? Assuming, as astronomers have rightly done, that the actual number of planets far outstrips our ability to observe them, what would be a more reasonable estimate of the total number of planets in our galaxy? We don’t know whether conditions are favorable to planet formation throughout the entire Milky Way. The galactic nucleus might be too energetic to allow planets to form, for example, while the outer fringes of the galactic disk may contain the wrong type of stars for planetary formation. But on the other hand, our observations up until this point have turned up mainly gigantic, massive planets orbiting close to their parent stars, because these are the easiest ones to detect. Astronomers predict that the actual size distribution of extrasolar planets favors smaller, earth-like planets that aren’t as easy to observe with our current techniques. In a recently published paper in Science, astronomers at the University of California, Berkeley estimate that approximately 23% of sun-like stars are host to earth-sized planets, and that multiple planetary systems are quite common. That would put the total number of planets in our galaxy well into the range of 50-100 billion.