"We are really in the age of discovery of new worlds." That was Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany, and the Harvard–Smithsonian Center for Astrophysics, during a September 12 press conference in which European researchers announced the discovery of about 50 planets new to science.

There are now 685 exoplanets, or worlds orbiting distant stars, catalogued in the online Extrasolar Planets Encyclopaedia. And the catalogue is growing at an ever-increasing rate. During the planetary conference last week in Wyoming where Kaltenegger and her colleagues rolled out their new findings, the number of known planets jumped by more than 10 percent.

The graph below breaks down the 685 known planets by their year of discovery. The graph is cumulative; it shows the number of known planets up to and including any given year. The slope of the graph shows how quickly the pace of discovery has been ramping up: in 2009, 82 planets were found. Last year the number was 110. And in just the first three quarters of 2011, 163 of the known planets have been discovered.

A trend line computed by Microsoft Excel (red line in lower graph) shows that the growth is not quite exponential, but it's close.

A number of factors can help explain the field's takeoff. One is technological innovation—instruments such as the NASA Kepler spacecraft and the HARPS spectrograph on a European Southern Observatory telescope in Chile have brought dramatic improvements to astronomers' extrasolar vision. Another is manpower—exoplanet research is now a popular field attracting large numbers of young researchers. And a third is a simple snowball effect—early exoplanet discoveries proved that it was possible to detect faraway worlds and prompted other researchers to get in the game.

Some sharp-eyed readers may notice that the graphs stretch back to 1989, whereas the exoplanet community generally refers to 51 Pegasi b, discovered in 1995, as the first exoplanet. The disparity comes from how one chooses to define a planet: Is a planet-size object orbiting a pulsar a planet? Is a giant world roughly a dozen times the mass of Jupiter a planet or a brown dwarf? The catalogue's guidelines are fairly inclusive, as its curator explains here, meaning that some objects in the Extrasolar Planets Encyclopaedia will not meet every researcher's conditions for planethood.