Mass as a function of period for the 74 exoplanets detected as of Nov. 2001. Regions where planets are “Detected”, “Being Detected” and “Not Detected” by the Doppler surveys are shaded differently and represent the observational selection effects of the surveys. The number in the upper left of each small box gives the number of planets in that box. The increasing numbers from left to right and from top to bottom are easily identified trends that can be extrapolated into the region around Jupiter (right).

Scientists at the Anglo-Australian Telescope and other astronomical observatories around the world have been scanning the sky for tell-tale signs of planets around nearby stars. So far these planet hunters have found 74 extra-solar planets orbiting 66 nearby stars (some stars have two known planets; one has three). The existence of the 74 planets has been inferred from ‘wobbles’ in the positions of the 66 host stars as each star and its planet, or planets, orbit their common centres of gravity. The bigger the planet and the shorter its ‘year’, or orbital period, the easier it is to detect, so only planets much bigger than Jupiter or in closer orbits than Jupiter have been detected so far.

However, new analysis of these planets suggests that Jupiter seems to be a typical planet – much more typical than indicated by previous analyses. We have analysed the latest data on the masses and orbital periods of all the recently detected extra-solar planets and carefully edited the data to correct for the limitations of the detection techniques, which are not yet able to detect Jupiter-sized planets.
Although Jupiter-like planets taking 12 years to orbit their host star have not yet been detected, we were able to make a simple extrapolation of the trends identified in the current data. Correcting for the limitations of the detectors in a simple new way gave the result that Jupiter is not an exceptional planet. Jupiter-like planets are 50 per cent more common than indicated by previous analyses.

If someone like us were doing a similar survey from one of these other planets, using instruments as sensitive as ours, and looked at our Sun, they would not yet have found evidence of any of our planets. Planet hunters should begin to find Jupiter-like extra-solar planets within the next few years. Similar analyses to answer the question ‘How typical is Earth?’ cannot yet be done using this technique, but the larger estimate for the number of Jupiter-like planets suggests a similarly larger estimate for the number of Earth-like planets.

Charley Lineweaver and Daniel Grether