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Astronomy Picture of the Day

Wonder - A Scientific Oratorio

Planetary disc
Artist's impression of formation of planets, asteroids and comets in a protoplanetary disc surrounding a young star.
Credit: NASA/JPL-Caltech.

Theme 6 - The birth of planets

The planets formed in the outer parts of the collapsing cloud at the centre of which the Sun was forming about 4.6 billion years ago. They were built by accretion - the sticking together of small particles and dust grains, building larger and larger objects attracted together by their gravity. Eventualy colliding in massive impacts which form the planets. One impact between the Earth and a Mars-sized planet probably formed our Moon.

The young Sun would have formed at the centre of a collapsing cloud of gas and dust. The rotation of this cloud causes it to become flattened, creating a disc within which the planets formed.

We think the planets formed initially by collisions of tiny particles within the protoplanetary disc. Dust grains would stick together and, gradually over millions of years, form pebble-sized objects and eventually mountain-sized objects called planetismals. These would be similar to modern-day asteroids, maybe 10 km across. The gravity of these planetismals is sufficiently strong to draw them together to form Moon-size bodies called protoplanets. The larger bodies accrete the smaller bodies. Eventually over about 100 million years or so, titanic collisions between these protoplanets results in the present-day planets like Venus, Earth and Mars.

The Earth is about 4.6 billion years old. The first eight hundred million years or so of the Earth's history are called the Hadean eon. The Earth would have been a hellish place to be. It was still subject to massive impacts as it swept up the remnants of the protoplanetary disc, it was extremely hot with widespread volcanism. It is thought that during this time a collision between the young Earth and a Mars-sized planet ripped out a chunk of the Earth to form our Moon.

Relics of the early history of the Solar System are provided by rocky/metallic meteorites and icy comets. Their study provides important clues to understanding the origin of our solar system.

Go on to Theme 7 - Earth: the cradle of life.

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