A planet, such as Earth, is a large body that orbits a star. A moon is a smaller body that orbits a planet.
About 4.5 billion years ago, Earth came into being from matter floating in space around the young Sun. The newborn planet was covered with endless seas of molten lava until cooling down enough for a rocky surface.
The Solar System around Earth was crowded with debris and leftovers from its birth, and for hundreds of millions of years, giant rocks – many tens of kilometres wide – rained down from the sky, keeping the atmosphere hot. This period of early Earth is named Hadean, referring to the ancient Greek word for hell.
Once, a stone called Theia hit the infant planet. Theia was probably the size of Mars. Striking the surface, it blasted so much matter into space that our closest celestial neighbour, the Moon,¹ was born. Moreover, Theia’s blow probably tilted Earth to its angle of 23.5 degrees, causing the four seasons – winter, spring, summer, and autumn.
Around 4 billion years ago, the meteor shower significantly calmed down, leaving behind a cracked planet, still stormy but at least not hammered by rocky rain. The young Sun was dimmer than today, allowing Earth to cool down quickly. Water from volcanoes and icy comets condensed into permanent oceans² that soon covered the entire planet and eventually gave our home its beautiful blue look.
¹ Our Moon is a little more than one-quarter the size of Earth and currently an average of 384,400 kilometres from Earth. Its barren surface is filled with craters hit by asteroids and meteorites. There is no wind or liquid water. Nor air to carry even the slightest sound – the Moon is silent. For us, our closest celestial neighbour shows only one side of its face as it circles both Earth and itself in 27 days and a few hours. Therefore, its scorching days and freezing nights also last for two weeks each. Besides, as the Moon revolves around Earth, it pulls water toward itself, causing tides – long-lasting waves raising and lowering sea levels twice a day. But the Moon is slowly drifting away. And as it departs a few centimetres per year, a day on Earth will lengthen by 19 hours every 4.5 billion years.
² The first seas probably came into being some 4.4 billion years ago, but since the high temperatures of early Earth, water may have been nearly or entirely lost more than once during the first few hundred million years. Once the Earth began to cool, oceans covered the entire planet before the first landmasses started to rise from the depths (4 BYA – 2.5 BYA), formed by erupting volcanoes and movements of tectonic plates.
Bibliography
Rutherford, A. 2014. Creation The Origin of Life. London, United Kingdom: Penguin Books. 272 p. ISBN 9781780229072. Pages 54-61.
Panciroli, E. 2022. The Earth a Biography of Life The Story of Life On Our Planet Through 47 Incredible Organisms. London, United Kingdom: An Hachette UK company. 255 p. ISBN 9781529413984. Pages 15-20.
Boyle, R. 2024. Our Moon a Human History. London, United Kingdom: Hodder & Stoughton. 313 p. ISBN 9781529342789. Pages 7, 10-13, 16, 18, 34, 36.
Cohen, A. & Cox, B. 2023. The Universe. Dublin, Ireland: HarperCollinsPublishers. 272 p. ISBN 9780008389352. Pages 110-111.
DK. 2021. Science the Definitive Visual Guide. London, United Kingdom: Dorling Kindersley. 520 p. ISBN 9780241446331. Page 434.
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3.8 BYA
Life begins on Earth
Once oceans had formed on Earth, molecules began to merge into increasingly sophisticated structures deep in the water. They grouped into countless combinations until one, by sheer chance, formed a cell, the smallest unit of life that could grow and reproduce.
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4.5 BYA
Solar System forms
Around 4.5 billion years ago, a dense cloud of stardust began to form our planetary system in the outer regions of the Milky Way Galaxy. First came into being our star, the Sun, which amassed more than 99 per cent of the available matter. The remnants clumped into the eight planets, multiple dwarf planets, dozens of moons, and millions of asteroids, comets, and meteoroids.