There'll be fireworks when the ice dwarf cometh

A frozen world at the edge of the solar system may turn into a comet to dazzle our descendants. Heather Couper and Nigel Henbest report
After the ninth planet, Pluto, was discovered in 1930, most astronomers thought there must be at least one more planet farther out. Now, it seems, there is no 10th planet. Instead, the outer edges of the solar system are populated with thousands of icy lumps.

In 1977, at the Palomar Observatory in California,Charles Kowal was hunting for the distant "Planet 10" when he came across a small world much closer in. It followed an oval orbit between the paths of Saturn and Uranus, and was 200km across - too tiny to be classed as a planet. Kowal named it Chiron, after the mythological centaur.

Fifteen years later, researchers using a powerful telescope in Hawaii found a new world beyond Pluto. But it was similar in size to Chiron, and too small to be classed as Planet 10.

To date, astronomers have found 19 small worlds in the outermost reaches of the solar system. They range in size from 100km to 300km across. Lying so far from the Sun, they must consist largely of solid ice. The numbers found to date suggest that the total population of these "ice dwarfs" must be more than 10,000.

The ice dwarfs are chunks left over from the formation of the planets: closer to the Sun, billions of ice dwarfs came together in the remote past to form the giant worlds of Jupiter, Saturn, Uranus and Neptune. Many astronomers now arguethat Pluto, the runt of the solar system at just 2,300km across, is not really a planet but the largest of the surviving ice dwarfs.

The discovery of the ice dwarfs has helped to solve another of astronomy's long unanswered questions: where do comets come from?

When the space probe Giotto sped through Halley's Comet in 1986, it proved that the show put on by a comet - its great, gassy head and long tail - consists of gas, steam and dust erupted from an icy nucleus just a few kilometres across. The reservoir of small icy bodies beyond Pluto is the origin of many comets - those that orbit the Sun more or less in the same plane as the planets' orbits. (Comets with more tipped-up orbits come from a huge spherical cloud much farther out.) Every so often, Neptune's gravity stirs up some of the smallest ice dwarfs and sends them on orbits towards the Sun. As they approach the Sun's heat, the ice and other frozen gases begin to boil away to create the comet's glorious show.

Now we are able to put Chiron in context. Recent calculations show that its orbit is unstable: it cannot have been following the same path since the birth of the solar system.Chiron was once an ice dwarf much farther from the Sun. Neptune's gravity pulled it inwards, then Uranus dragged it closer still to the Sun.

Next year, Chiron reaches the closest point to the Sun in its 50-year orbit - just inside the orbit of Saturn. Even this far out, the Sun is boiling away its frozen ices to create a glowing "atmosphere": Chiron is showing incipient signs of cometary activity. It is actually a giant comet nucleus.

Several thousand years from now, Saturn's gravity may tip Chiron into the inner part of the solar system. Then the fireworks will really commence. Chiron is 25 times wider than the nucleus of Halley's Comet and contains thousands of times more ice. Comet Chiron will blaze in our skies for our descendants a thousand times brighter than any comet in recorded history.

The planets

After passing behind the Sun on 14 April, Mercury makes a rare appearance in the evening sky. By the end of the month, you will find it setting in the north-west some two hours after the Sun. It will reach a brilliancy of magnitude of -1.5, brighter than Sirius, the brightest star in the sky. But at a low altitude, Mercury will not look so spectacular.

The other inner planet, Venus, is a dawn object. Look for it rising about an hour before sunrise early in the month. Saturn, difficult to spot, is close to Venus on 13 April.

Mars, so prominent earlier in the year, is now fading as the Earth draws away from it. But it is still a very obvious red object in evening skies, moving from Cancer to Leo this month. On 10 April, the Moon will be just to the south of the planet.

Jupiter rises in the early hours of the morning and starts off close to the red star Antares in Scorpius (literally "rival of Mars"). The giant planet moves "backwards" against the stars (to the west) as Earth, in its faster orbit, overtakes it.

Eclipses

There are two eclipses in April, neither visible from Britain. During a partial eclipse of the Moon on 15 April, only 12 per cent of the Moon will be covered by the Earth's shadow. This eclipse is visible from the western parts of North and Central America, the Pacific Ocean, Australasia, Antarctica and Eastern and Central Asia. Two weeks later, on 29 April, there will be an annular eclipse of the Sun. These take place when the Moon is at the far point of its orbit and looks slightly smaller than the Sun in the sky. As a result, the Moon cannot completely overlap the Sun during an eclipse, and the edge of the Sun shines around the Moon as a bright ring or "annulus". It can be seen from South America, on a line crossing from the Peru-Ecuador border through northern Brazil. Elsewhere in the southern Pacific and in Florida, the West Indies, the Atlantic and North Africa (extreme west only), the eclipse is partial.

Shooting stars

The Lyrids - meteors radiating from the constellation of Lyra - peak on 22 April. Although the shower normally yields only 10 meteors an hour, there have been more prolific displays in the past, the last being in 1982.

The stars

The familiar shape of Leo is currently distorted by a red interloper - the planet Mars. But there is no mistaking the leonine shape riding high in the south this month, with the "sickle" making up his head.

Like Lyra, Leo is a radiant for shooting stars - the annual shower peaks on 18 November. The meteors, of course, do not actually emanate from the constellation, but appear to radiate from it because of the effects of perspective. The Leonids, like the Lyrids, usually yield only 10 meteors an hour. But there have been sensational meteor "storms" in the past, when up to 100,000 shooting stars have fallen to earth in the course of an hour. The last was visible in the United States in 1966.

Diary (all times BST)

8 April 6.35am Moon at first quarter

14 April Mercury at superior conjunction

15 April 1.08pm Full Moon; partial lunar eclipse

22 April 4.18am Moon at last quarter; maximum of Lyrids meteor shower

29 April 6.36pm New Moon; annular eclipse of the sun

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