Stars and Planets: February

FOR THE last 20 years, we have encouraged our readers to pose their friends an astronomical trick question: "What's the outermost planet of the Solar System?" The answer has been Neptune. But don't try that one any more. From next Thursday (11 February), the answer will in fact be the obvious one - Pluto.

Pluto takes 248 years to go once around the Sun, and for 228 years of that time it certainly lies farther away than Neptune. But Pluto's oval orbit brings it closer to the Sun than Neptune for 20 years in every revolution - and that's been the case since 1979.

This month, Pluto moves back beyond Neptune, to regain its title as most distant planet. There's no danger of a collision, though, because Neptune lies in a different part of its orbit. Indeed, because Neptune goes around its orbit exactly three times for every two orbits of Pluto, the two planets are a safe distance apart every time that Pluto crosses Neptune's path. That's just as well, because Pluto is so small that even a near miss would be disastrous, with giant Neptune's gravity flinging Pluto out of the Solar System altogether.

Pluto is, so far, the only planet that hasn't been scrutinised by a space probe. Nasa has plans for a Pluto Express mission, designed to study Pluto and its big moon Charon at close quarters. It's a bit of a race against time, though. As Pluto moves away from the Sun, it is cooling down. Its thin atmosphere is starting to freeze on to its frosty surface. And if Pluto has "ice volcanoes" - similar to those that the Voyager 2 spacecraft discovered on Neptune's big moon Triton - these will turn off as the Sun's heat diminishes.

According to Alan Stern, of the Southwestern Research Institute in Boulder, Colorado, the Pluto Express mission will carry on to explore some of the dozens of smaller "ice dwarf" worlds that astronomers have recently discovered beyond Pluto.

The discovery has opened a debate on whether Pluto is really a "planet" at all. It is far smaller than any other planet, and some astronomers say it should be demoted to being merely the largest of the ice dwarfs.

But Stern turns this argument on its head. He believes that the ice dwarfs are only the smaller denizens of a region that must contain much larger worlds - more planets of our Solar System, perhaps even the same size as the Earth. If and when these frozen earths are discovered, Pluto will permanently lose its title as most distant planet.

WHAT'S UP: Venus is becoming more and more prominent in the west after sunset, as a brilliant "Evening Star", though it doesn't appear on the chart because it sets before 22.00. To its upper left is the second-brightest planet, Jupiter, which is now slipping down into the sunset glow.

There will be a spectacular sight on the evening of 23 February, as these two planets graze past one another. They will be little more than a one- tenth of a degree apart - about a quarter of the Moon's width. With binoculars or a small telescope, you'll see both planets in the same field of view, Jupiter with its family of four prominent moons.

At the end of the month, Jupiter lies near Mercury, to the lower right of Venus in the evening twilight glow. Throughout February, Saturn lies to the left of Jupiter, with Mars rising in the east around midnight.

The lack of a full moon on the sky chart is no mistake. January had two full moons, the second of which was on 31 January, and the next is due on 2 March.

New moon falls on 16 February: as seen from a narrow band across Australia, it appears right in front of the Sun. The resulting eclipse, however, is not total. Because we are currently near the Earth's closest point to the Sun, the Sun appears larger than the Moon, and the ring of the sun's surface will be visible all around the Moon's silhouette. This is an annular eclipse, named after the Latin "annulus" for "ring".

A partial eclipse will be visible from South Africa, the Indian Ocean and the whole of Australasia.