Date: February 16, 2010

Title: How Did the Earth Form?


Podcaster: Stuart Clark

Description: How did the Earth form? Are we really just a piece of cosmic flotsam?

Bio: Dr Stuart Clark is an award-winning astronomy author and journalist. His books include The Sun Kings, and the highly illustrated Deep Space, and Galaxy. His next book is Big Questions: Universe, from which this podcast is adapted. Stuart is a Fellow of the Royal Astronomical Society, a Visiting Fellow of the University of Hertfordshire, UK, and senior editor for space science at the European Space Agency. He is also a frequent contributor to newspapers, magazines, radio and television programmes. His website is and his Twitter account is @DrStuClark

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored anonymously.



Hello I’m Dr Stuart Clark, astronomy author and journalist. Today I’d like to explore the question: how did the Earth form?

With its profusion of landscapes and climates, animals and people, Earth certainly seems to be something special. Yet, to planetary astronomers, the Earth and the other planets of the Solar System are nothing more than cosmic flotsam, debris left over from the formation of the Sun.

Stars and planets account for 85 percent of all the atoms in the Galaxy, leaving around 15 percent to form gas clouds in space where new stars and planets can form. This material creates a tenuous mist of gas and dust throughout the Galaxy, of which hydrogen is the most abundant constituent, and the ‘dust’ consists of particles of heavier elements given out by previous generations of dying stars.

The hydrogen atoms tend to pair up into molecules, and the molecules gather together into enormous clouds. Astronomers estimate that there are about 4000 of these giant molecular clouds in our Galaxy. Within each there are a multitude of slightly denser clumps, the seeds from which stars grow − or, more accurately, shrink because a clump generates gravity and pulls itself together in a slow process that takes anything from a million to ten million years.

When astronomers look into such dusty cocoons using infrared telescopes they see something remarkable: each collapsing clump transforms itself from a nearly spherical shape into a disc surrounding the nascent star. The discs are flat because of centrifugal force, which is created by a spinning object; the faster something rotates, the greater the centrifugal force it generates. The flat discs can be anything from 100 to 1000 times larger than the Earth’s orbit, and it is in these that planets form.

The Earth and the rest of the Solar System formed this way during a hellish maelstrom around 4.6 billion years ago. In the inner Solar System, the planets are like the Earth – small, rocky, and with thin atmospheres – whereas in the outer Solar System they are more like Jupiter – large, gaseous, with thick atmospheres.

As the newly forming Sun shrank to its modern size and density, it released energy. This heat determined which chemicals could condense into solids throughout the disc. Near the Sun, in what would eventually be Mercury’s orbit, the temperature would have been several thousands of degrees and only metallic and silicate atoms could condense into dust; other chemicals would remain as gas. Further out, the lower temperatures enabled other elements to form dust.

At five times the Earth’s distance from the Sun, where Jupiter orbits today, we find the snow line. When the planetary material was condensing the temperature here would have been low enough for molecules such as water, ammonia and methane to form ice. The larger size of the outer planets can therefore be explained by the much greater reservoir of matter from which they could feed. At 40 times the Earth’s distance from the Sun, roughly where Pluto orbits, the temperature would have been so low that almost every chemical element could condense; the only exceptions were hydrogen and helium, which remain in a gaseous state.

In the inner Solar System, the dust gradually accumulated into vast numbers of objects called ‘planetesimals’, these were the building blocks of the rocky planets. To build Mercury, Venus, Earth and Mars would have required ten billion or more planetesimals of 10 kilometres in diameter. Sometimes an impact was just enough to melt the planetesimals together; at other times it broke them into fragments, but the pieces remained together and continued to orbit as, essentially, a pile of rubble in space.

Such close encounters were repeated many times until eventually some larger planetesimals began generating enough gravity to pull smaller ones into them. Throughout the disc, these major planetesimals began to outpace their lesser companions and, the bigger they became, the more efficient they grew at drawing in smaller bodies. They became oligarchs, each containing between the mass of the Moon and Mars. Computer simulations show that 20 to 30 must have ultimately smashed together to build the Solar System’s four terrestrial planets of today.

The same is thought to have happened in the outer solar system too. Once the forming Jupiter and Saturn reached between three and five times the mass of the Earth, they generated so much gravity that they began pulling in gas from their surroundings and this gave them thick atmospheres. Uranus and Neptune formed in a similar fashion although, being less massive, they were not so good at attracting the hydrogen and helium.

Out where Pluto formed, the density of matter orbiting the young Sun was smaller, so the bodies were naturally smaller. With the recent discovery of a number of other Pluto-like objects in the outer reaches of the Solar System, including Haumea and Eris, the International Astronomical Union voted in 2006 to term them dwarf planets and controversially downgraded Pluto into this same category.

By 4.6 billion years ago the Solar System looked almost as it does now; the familiar planets and their moons had formed. The space between the planets, however, remained home to countless leftovers. These tiny objects ranged from pebbles to planetesimals that had so far escaped the planets’ gravitational clutches. Jupiter’s gravity trapped many of them between itself and Mars to form the asteroid belt, but most flew freely through the Solar System.

During the next 700 million years these celestial vagabonds collided with the planets and the moons, blasting out craters of all sizes. Old planetary surfaces are easily identified today because of their heavily pockmarked appearance: our Moon is the classic example. On Earth, the early craters have been eroded away; today, less than 200 craters are known, and all of them are from comparatively recent impacts.

The heat from the young Sun would have vaporized any water molecules that formed on Earth; so how come we have oceans? The bombardment suggests a way; planetesimals that formed in the outer Solar System, incorporating water and other ices, rained down on Earth and the other planets of the inner Solar System, supplying them with water and other volatile substances. On Earth, this material was swiftly transformed into life. After the bombardment petered out, the formation of the Earth and the rest of the Solar System, was complete.

During the bombardment period, many of the planetesimal remnants would have been ejected from the solar vicinity by near misses with Jupiter. Indeed, according to computer simulations, Jupiter may have thrown away whole planets too. An entire second Solar System’s worth of them may be lurking at thousands of times the distance of Earth from the Sun. But these have yet to be found.

Smaller planetesimals were thrown even further but occasionally one or another will return, as a comet. Its ice begins to melt as it approaches the Sun, leaving a tail of gases, and it releases dust, littering interplanetary space. If the dust falls into the Earth’s atmosphere it burns up and creates meteor showers, or shooting stars.

Orbiting spacecraft have even shown that comets the size of houses regularly approach the Earth, but fortunately they break up in the atmosphere and present no danger. However, sometimes a large meteorite does hit the ground: notably the strike on the Tunguska region of Siberia in 1908, which demolished an area of uninhabited forest the size of London.

Inevitably, at some point in the future, an asteroid will be discovered to be on a collision course with Earth. It seems ironic that the very objects that brought water to Earth and made life possible may threaten to destroy it. When it comes to safeguarding the Earth against dangerous impacts, it is literally a case of ‘watch this space’.

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