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
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.
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.
Lineweaver and Daniel Grether