9781422283714

Mini-planets In the Solar System, the four inner planets—from Mercury to Mars—are quite close together. Then there seems to be a huge gap before we reach Jupiter. We might expect to find another large planet in this gap, but there is not. Instead, there is a swarm of miniature planets, called the asteroids . Even the largest one is much smaller than Earth's Moon. It is incredible that we can spot Saturn with the naked eye at such a distance. But it is the most distant planet that we can see with just our eyes. We need a telescope to see the two planets further out—Uranus and Neptune. How the giants formed The Sun and all the planets formed about 4.6 billion years ago. The planets were born out of a disc of gas, dust, and chunks of rock. They grew in size as the bits of

∆ Neptune and its large moon Triton can only be seen through telescopes.

matter kept hitting one another and sticking together. The inner part of the disc was hot. There, four planets—Mercury, Venus, Earth, and Mars—formed out of rocky lumps. But the gas in this region, mainly hydrogen , did not stay there. The heat and other rays from the Sun forced the gas into the outer part of the disc. There it was much colder. The gas began gathering around the small rocky lumps that had formed there. There were enormous amounts of gas, and the small lumps soon grew into huge bodies of gas. These became Jupiter and Saturn, the giant planets we know today.

∆ Jupiter and Saturn lie far from the Sun.

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