Sir: What a pity Tom Wilkie ended his otherwise excellent article on spacecraft that have gone to Jupiter and beyond (8 December) with a paragraph about a "tenth planet beyond the orbit of Pluto" that might have been written years ago.
Few planetary scientists now believe that "Planet X" can exist. The strongest evidence against it comes from observations in recent years of dozens of small icy objects, 200km and smaller in size, that are scattered throughout the region of Pluto's orbit and beyond. This is none other than the Kuiper belt, predicted by the Dutch-American astronomer Gerard Kuiper in 1951 as representing debris from the birth of the solar system, too widely dispersed to have aggregated into planet-size objects.
Extrapolating from the very small areas of sky so far surveyed in detail, it has been suggested that there are probably about 100 million of these more than 10km across. Whatever the total, the very presence of the Kuiper belt is a strong indication that there is no large planet out there.
D. A Rothery
Department of Earth Sciences
The Open University
8 DecemberReuse content