This chart was assembled from data on the incomparable Extrasolar Planets Encyclopaedia , maintained and curated by the astronomer Jean Schneider. It depicts the 520 exoplanets detected between 1992 and 2010, divided up by detection technique. The accompanying data sheet includes a few notes and caveats about the assumptions I used to generate the chart. There's also an interactive version available.
Finding 520 planets in less than 20 years is an impressive testament to the skill of modern-day planet-hunters. And the rapid, recent acceleration of detections suggests that the next 20 years will see several thousands of additional planets added to our catalog. But given that there are hundreds of billions of stars in our galaxy alone, shouldn't we actually be finding more planets?
Let's assume that planets are an almost inevitable side effect of a star's formation from a collapsing cloud of gas and dust—a hypothesis that is less controversial by the day.
Now, there are still several ways for a star to be bereft of planets. A star can form simultaneously with one or more companion stars, and the group will be collectively bound together by its own gravity; in this case, the gravitational interactions of the stars may disrupt planet formation. Similarly, the gravitational nudge of a passing star could scatter planets from around their sun, casting them away forever into the inky void. Gravitational nudges could also make planets fall into their host stars, the planets' constituent atoms lending a dirty sheen to the surfaces of their suns, like oil slicks on oceans. These and other planet-stripping scenarios are unavoidable occurrences in the universe, so not all stars have planets, but it's still a good bet that most do.
Standing beneath a pristine night sky in the northern hemisphere, an unaided human can, at best, see perhaps 2,500 stars. Another 2,500 or so can be seen in the skies above the southern hemisphere. These ~5,000 stars are visible by virtue of their being very bright or very close to us, so that they flood our eyes with photons, particles of light.
Photons are like currency for astronomers: In general, the more you have, the more you can do. Consequently, the stars we can see with the naked eye on a dark night are, on balance, the easiest for astronomers to study. So if most of these stars have planets, and the stars are so easy to observe, why are there only 500-odd exoplanets known today, and less than 100 known around naked-eye stars, rather than 5,000 or more?
The answer comes in three parts: First, just as there are more small pebbles than giant boulders in the world, small planets are probably far more common than the large ones that are easier to detect.
Second, we haven't been looking long enough or hard enough to detect all these smaller planets around nearby stars; the first exoplanets were only discovered in 1992, and only in the past couple of years have we gained the capability to reliably detect tinier, more prevalent worlds.
Finally, and most importantly, each detection method we use has its own unique observational biases that can blind it to the presence of exoplanets large and small.
It's been my experience that most misunderstandings of exoplanetary discoveries are caused by a lack of familiarity with the capabilities and limitations of each detection method. If you want to be savvy about the search for life beyond our solar system, if you want to be immune against exoplanetary hype and flim-flam, then you need to know the basics of how astronomers find planets in the first place. Tune in tomorrow for a beginner's guide to planet hunting!