For two decades neuroscientists have believed that we are born with all the brain cells we are ever going to possess. As we age, brain power declines because cells are lost, never to be replaced. But new research has turned this idea on its head.

Two US biologists at Princeton, Drs Elizabeth Gould and Charles Gross, found in macaque monkeys new brain cells being produced in their thousands every day. They injected the brains of the monkeys with a chemical that is incorporated into the new DNA formed when cells divide. They found that a stream of new cells were being made and were moving daily into the cortex, the higher level of the brain.

Earlier research had shown that canaries grow new brain cells to learn new songs, and that cells can be generated in the hippocampus, a primitive area of the brain. But the fact that this can also happen in the cortex is striking. It may alter the way scientists think about memory, because it suggests that each day’s supply of new cells are used for storing that day’s experience, and help in understanding degenerative brain diseases.

IT ISN’T often that a new planet swims into our ken. The last was Pluto, discovered in 1930 by the American astronomer Clyde Tombaugh. But if a British astronomer and a team at the University of Louisiana are right, there is a 10th planet out there, so far away and so small that it has never been noticed before.

Dr John Murray of the Open University deduced the existence of the planet from the odd behaviour of comets. He noticed that the orbits of some long-period comets are not arranged at random, but have a pattern consistent with having been disturbed from a “reservoir” of planets called the Oort Cloud that lies in a belt around the edge of the solar system. The pattern could be explained if the disturbance was caused by a planet as large as Jupiter - 90,000 miles in diameter - but 32,000 times further away from the Sun than the Earth is. The Louisiana team drew similar conclusions, but they believe the planet is closer, and about twice the size of Jupiter.

SCIENTISTS are closing in on the first complete sequence of all the human genes. An international team involving scientists from Britain and the US - the Human Genome Project - completed the sequencing of one billion bases (the letters of the genetic alphabet) out of the three billion that human beings are believed to carry.

The British end of the project, based at the Wellcome Trust’s Sanger Centre near Cambridge, also announced the complete sequence of chromosome 22, one of the 23 human chromosomes. The project expects to complete the first draft some time in spring.

Urgency has been added to the task by the challenge offered by a private company, the Celera Corporation, run by Dr Craig Venter. He uses a method relying on massive computer power to assemble short sequences of DNA into the complete genome. It has worked brilliantly on simpler organisms, and he could be set to beat the HGP to the winning post.

Does that matter? The HGP posts its results daily on the Internet. Celera is slower to make data public and has submitted a patent application for 6,000 sequences, a commercially motivated act. The race is thus a test case of two different ways of conducting science: public and private.

Nigel Hawkes is science editor of ‘The Times’