Why are the outer planets so massive?
If you could have taken a trip on Voyager 1 to look at Jupiter, the Lord of the Heavens, you would have seen something quite different from Earth and our nearer neighbours, Mercury, Venus and Mars.
The inner planets are rocky and tiny when compared with such mammoths as Jupiter, Saturn, Uranus and Neptune, which look today much as they did when they were first formed some 4600 million years ago. These remote planets are sometimes called ‘the gas giants’, because they are composed largely of hydrogen and helium, which make up about 98 per cent of the Universe. Hydrogen and helium are made of very light atoms. The lighter an atom, the more quickly it will move. If it is heated, it moves faster still. Only a planet of vast size and with a strong gravitational force can keep such unruly atoms in check; it holds on to them more easily if it is cold.
Astronomers have for long debated why the gas giants exist at all, and why such a change in character takes place in the distance between Mars, the last of the rocky planets, and Jupiter, the first of the gas giants. Many believe that the solar system formed from a vast cloud of swirling dust and gas. Most of the dust and gas collapsed to the centre of this mass, and formed the Sun. Part of the rest was pulled together by gravitational attraction, and made up the planets.
The Sun contracted, became hotter, and erupted in an atomic fire, blowing away most of the nearby dust and gas. As they warmed up, the planets closer to the Sun lost their hydrogen and helium, leaving only their rocky cores. The outer planets, too far from the Sun’s heat, kept their lighter gases. This explains their immense size. Jupiter, the largest planet in the solar system, has a diameter of about 143 000 km (89 000 miles), eleven times that of the Earth. To fill its great volume would require more than a thousand Earths.
Vast as they are, the outer planets are extremely light in density only 1.4 grams per cu cm, about a quarter the density of typical material from the Earth. Among these remote giants is an oddball that doesn’t quite fit the pattern. Pluto, the Guardian of the Dark, is smaller than our Moon and extremely cold. It is believed to be completely covered by frozen methane, a gas that turns to ice at -182.5°C (-296.5°F). Astronomers puzzle over how this midget got out there among the giants. One theory is that it was once a moon of Neptune, and during an extraordinary close encounter between that planet and Triton (Neptune’s largest moon) escaped to take up its own lonely orbit far out in space.
The inner planets are rocky and tiny when compared with such mammoths as Jupiter, Saturn, Uranus and Neptune, which look today much as they did when they were first formed some 4600 million years ago. These remote planets are sometimes called ‘the gas giants’, because they are composed largely of hydrogen and helium, which make up about 98 per cent of the Universe. Hydrogen and helium are made of very light atoms. The lighter an atom, the more quickly it will move. If it is heated, it moves faster still. Only a planet of vast size and with a strong gravitational force can keep such unruly atoms in check; it holds on to them more easily if it is cold.
Astronomers have for long debated why the gas giants exist at all, and why such a change in character takes place in the distance between Mars, the last of the rocky planets, and Jupiter, the first of the gas giants. Many believe that the solar system formed from a vast cloud of swirling dust and gas. Most of the dust and gas collapsed to the centre of this mass, and formed the Sun. Part of the rest was pulled together by gravitational attraction, and made up the planets.
The Sun contracted, became hotter, and erupted in an atomic fire, blowing away most of the nearby dust and gas. As they warmed up, the planets closer to the Sun lost their hydrogen and helium, leaving only their rocky cores. The outer planets, too far from the Sun’s heat, kept their lighter gases. This explains their immense size. Jupiter, the largest planet in the solar system, has a diameter of about 143 000 km (89 000 miles), eleven times that of the Earth. To fill its great volume would require more than a thousand Earths.
Vast as they are, the outer planets are extremely light in density only 1.4 grams per cu cm, about a quarter the density of typical material from the Earth. Among these remote giants is an oddball that doesn’t quite fit the pattern. Pluto, the Guardian of the Dark, is smaller than our Moon and extremely cold. It is believed to be completely covered by frozen methane, a gas that turns to ice at -182.5°C (-296.5°F). Astronomers puzzle over how this midget got out there among the giants. One theory is that it was once a moon of Neptune, and during an extraordinary close encounter between that planet and Triton (Neptune’s largest moon) escaped to take up its own lonely orbit far out in space.
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