4.6 billion years ago, hydrogen and other elements along with ice and other molecules in the nebula that was the birthplace of our sun began to collapse under the force of gravity to form a large disc with the young sun in its centre. The disc likely contained rings, a little like those seen around Saturn today. Dust and ice in these rings began to join, or in cosmic parlance accrete, into larger objects, with these pebble-sized meteoroids combining and colliding until small-planet-sized rocks developed. The collisions and the actions of gravity increased the size and temperature of these rocks and they combined into a few larger small planets called planetesimals orbiting the sun. These planetesimals steadily accreted more material from the disc, becoming planets. Eventually, at a distance of 150 million kilometres from the sun, a molten rocky mass formed that was the baby Earth. By 4.543 billion years ago the Earth existed. Back then, it was not prime real estate, and any form of life would have quickly met a grizzly end.
Similar processes happened at different distances from the sun. Our solar system has four rocky planets – Mercury, Venus, Earth and Mars – that are closest to our star. Further out, there are the four gas giants Jupiter, Saturn, Uranus and Neptune, and these are all beyond the frost line, a distance from the sun beyond which water forms ice and gaseous compounds condense into solids, creating the gas giants, while within it only heavier compounds can be accreted, forming rocky planets. Planets that formed close to the sun are rocky because they have a greater proportion of heavier elements than those that formed further out. It is a key reason why each of the planets that form the solar system are so different from one another.
The early Earth was about 40 million years old when it collided with another planet named Theia. The collision heated the slowly cooling Earth, creating a surface of molten rock, and it would have thrown a large amount of dust and debris into the atmosphere and beyond. Some of this debris fell back to Earth, but the remainder accreted to form the moon. The young moon orbited the Earth at a much closer distance than it does today, probably at between 25,000 and 30,000 kilometres. Tidal forces for the first oceans when they formed must have been colossal with the moon close by, yet with each orbit it has slowly moved further away, and today it is currently 384,400 kilometres distant.
Collisions between celestial objects were more common between 3.9 and 4.5 billion years ago than they are now. Many of these collisions were between the young planets and the myriad meteors that littered the solar system. Collisions with meteorites were particularly important for Earth, as this is where most of the water in our oceans came from. Although geologists have found evidence of past meteorite and space-rock impacts on Earth, including the site of the impact that ushered in the end of the Cretaceous geological epoch and the extinction of the dinosaurs, much of the evidence of the frequency of past impacts comes from studying the surface of the moon and Mars. Scars from past meteorite and space-rock impacts disappear on Earth at a much faster rate than they do on the moon and Mars. The reasons for this are that the moon and Mars do not have plate tectonics caused by shifting slabs of rock, surface water that can erode rocks, or plant roots and bacteria that over millennia can crumble and break rocks apart. The scars on these other celestial objects reveal impacts between planets, moons, asteroids and meteoroids are frequent on astronomical timescales. This history is hard to read on Earth because it is steadily eroded over time.
Jupiter, the fifth and largest planet in the solar system, formed at about the same time as Earth. Computer simulations of the youthful solar system suggest that shortly after it formed it began a journey towards the sun, only ceasing it at about the distance that Mars is now from our star. It stopped its journey towards a fiery collision course with the sun because Saturn had developed an orbit that resulted in a complicated dance between the two large gas giants, resulting in Jupiter and Saturn establishing orbits close to where they are now found. Jupiter is about three and a half times further away from the sun than the Earth is, while Saturn is even further distant. This account of early planetary meanderings, which took millions of years to complete, is called the Grand Tack hypothesis.
As Jupiter Grand Tacked towards the sun and back, its gravity prevented another planet forming. The asteroid belt, which lies between Mars’s and Jupiter’s current positions, would have accreted into another planet had Jupiter not trekked towards the sun and back. Jupiter’s journey to the inner solar system also resulted in dust, gas and debris either being pushed further out into the solar system or colliding with the gas giant itself, resulting in Mars being significantly smaller than it would have been otherwise. Without Jupiter’s wanderings, Mars would be larger than Earth, yet it is much smaller. In some star systems, astronomers have observed large, rocky planets that orbit so close to their star they are at risk of being consumed by it. It is possible that Jupiter’s Grand Tack prevented Earth realizing this fate, by stopping it from accreting more material and moving closer to the sun.
We tend to think of our solar system as being fairly static in its behaviour, and certainly it is on the span of a human life, or even over a few million years. On longer timescales our solar system is dynamic and the behaviour of one planet can influence the others. Without Jupiter’s wanderings early in its life our planet might not have settled to an orbit in the habitable zone – the range of distances from a star where water exists as a liquid and not as steam or ice. Without young Jupiter’s exotic orbit, it is possible that Earth would not have remained in the habitable zone and life would not have emerged.
The Earth’s orbit around the sun takes 365.24 days, an Earth year. The planets closer to the sun take less time to make one of their orbits, while those further away take longer. Neptune, the most distant planet, takes nearly 165 Earth years to orbit the sun. Pluto, which was a planet when I was a child but got downgraded to a dwarf planet when scientists learned it had failed to accrete other objects in its immediate neighbourhood, takes even longer: 248 years pass while it makes one orbit of the sun. Not only do the planets furthest from the sun have the greatest distance to travel to complete an orbit, they also travel more slowly. Mercury moves through space at a little under 110,000 mph, while Neptune moves nine times more slowly. Earth, by comparison, hurtles through space at 66,615 mph, or 0.006 the speed of light.
The distance from the sun to the Earth, nearly 93 million miles, is defined by scientists as one astronomical unit. Mercury, the closest planet, orbits the sun at 0.387 astronomical units, while Neptune is just over thirty astronomical units from our star. Life exists on Earth, but is that because it is just the right distance from the sun, or could there be life on any of the other planets or their moons? Astronomers have long been interested in this question, and they initially addressed it through estimating the range of astronomical units at which water would exist as a liquid.
Early estimates suggested that the habitable zone was narrow, and Earth was rather lucky to sit within it. More recently the width of the habitable zone has been extended, as the probes we have launched to explore our solar system have revealed evidence that water may exist as a liquid on both Europa, a moon of Jupiter, and Enceladus, a moon of Saturn that is 9.5 times further away from the sun than Earth. These findings complicate estimates of the size of the habitable zone around a star because it depends not only on how far a celestial object is from the star, but also on the atmosphere and the internal geology of the object, as well as the gravitational pull of other celestial objects. Mars has long been thought to be in the habitable zone, but it has a very thin atmosphere and liquid water is not found on its surface. In contrast, Europa is covered in a layer of ice 10–15 miles thick. Under this, liquid water almost certainly exists in an ocean 40–100 miles deep. The liquid water doesn’t freeze, due to the complex gravitational pull of Jupiter and two of its other moons, Io and Ganymede. All three of these moons have eccentric orbits around their mother planet. These orbits result in a tidal pull that heats the oceans so they do not freeze. Europa may also experience volcanic geothermal activity, injecting further heat into its oceans. The compelling evidence for the existence of liquid water on Europa and Enceladus has led to scientists speculating that these may be the most promising homes for life beyond Earth in our solar system. Who knows, perhaps alien fish swim in these oceans?