Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
Look around our solar system and you could be forgiven for thinking its eight planets drifted in from completely different parts of the cosmos. Yet they all formed from the same cloud of gas and dust that surrounded the sun more than 4.5 billion years ago. As gravity pulled this cloud together with the sun at its centre, dust grains collided and stuck to each other, growing in size and generating ever larger gravitational fields. These clumps collided and merged, building the planets we know today.
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That鈥檚 the big picture, but the details of what happened in the early stages of Earth鈥檚 life remain a mystery. Solving it is fundamental to understanding why Earth is so suitable for life. We know that its distance from the sun provides the right amount of heat and light to make the planet habitable, but that alone is not enough. Without the unique mix of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur that makes up living things, and without liquid water on the planet鈥檚 surface, life as we know it could not have evolved. Chemically speaking, Earth is simply better set up for life than its neighbours. So how come we got all the good stuff?
What we do know is that different elements would have condensed from the cloud at different temperatures, which would depend on their distance from the sun. We cannot know exactly what happened next, though, because Earth rocks have been compressed, melted and weathered too many times to retain any clues about how they formed. And, since most of the planets in the solar system are out of reach, meteorites are our best hope. They formed at the same time as the planets, and since then have remained largely undisturbed. But to study them, we have to wait for one to fall from space.
A class of meteorite called chondrites match many aspects of Earth鈥檚 composition, which suggests they may have formed from the same raw materials. However, there are subtle differences that are proving tough to explain. For example, the mix of oxygen isotopes in chondritic meteorites does not match those found on Earth. So far no one knows why, but since oxygen is the most abundant element in the Earth鈥檚 crust, making up nearly half of its mass, it is a mystery that cannot be ignored.
Another big unknown is how Earth acquired its life-giving water supply. Being so close to the sun, it was probably too hot for water to simply condense out of the gas cloud as the planet formed, and any that did collect would have evaporated away during the titanic collision that formed the moon (see 鈥淲hat happened during Earth鈥檚 dark ages鈥�). The most popular explanation is that the water arrived later, in the form of icy comets from the outer solar system that rained down in the period known as the 鈥淟ate Heavy Bombardment鈥�. As yet, though, there is no firm evidence to confirm this as the source of Earth鈥檚 water.
Clearly we need new insights into how planets form. The European Space Agency鈥檚 , which takes to the heavens later this year, could provide some of the answers. With a mirror that is almost one-and-a-half times the size of the Hubble Space Telescope鈥檚, it will peer deep into space and use its infrared detectors to give us an unprecedented look at the dusty clouds where new stars and planets are forming, and where brand new planets may be striking it as lucky as Earth did.
Read all seven of the biggest mysteries about Earth
Explore an interactive map of our Unknown Earth
