science 2008-2009: 5: another green world (and another and another)
By Jon Agar, on 26 September 2009
There are five classical planets. The eighteenth, nineteenth and twentieth centuries add one each. The detection of hordes of fainter planets in the 1990s and 2000s has had some dramatic consequences.
First, the discovery, many by Caltech astronomer Michael Brown, of a string of “trans-Neptunian objects” has forced scientists to rethink what a “planet” is. The International Astronomical Union created in 2006 a new category of “dwarf planet”, placing Pluto and Ceres with some of these new objects, now given names: Eris, Sedna (possibly), Haumea and Makemake. The demotion of Pluto is a reminder of the revisable character of even the most apparently basic or venerable of scientific categories.
Second, beginning in 1995, planets around other ordinary stars have been detected. These “exoplanets” have typically revealed themselves through the tiny wobbles, detectable with fine spectroscopic measurement, induced in the movement of their home stars. By 2008, more than 300 exoplanets had been identified, including one, HD 189733b, with a trace of water vapour in its spectrum. This discovery prompted serious speculation about “biomarkers” to look for in exoplanets.