The reductionist approach to explaining biological phenomena has displayed its power through the spectacular triumphs of molecular biology. But the approach has its limitations, as discussed at a meeting last month. It is not likely to be useful, or practicable, to explain many biological processes in terms of particle physics. Moreover, exploration of other levels, such as molecules, genes, cells, organisms and populations, may well be more appropriate for an adequate explanation — begging the question, of course, of what constitutes an ‘adequate’ explanation. Paul Nurse The ends of understanding Nature 387, 657 (1997) THE SOLAR SYSTEM An icy object, about 500 km across, has been discovered in a huge, elongated orbit beyond Neptune. It may be a member of a new part of the Solar System, a so-called ‘scattered disk’ of comets produced by the gravitational effects of the young Uranus and Neptune, whose existence has only just now been predicted by theory.