Cosmic timeline 17

How the planets formed
The formation and evolution of the Solar System is estimated to have begun 4.55 to 4.56 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

The various planets are thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun’s formation. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar.

The inner Solar System was too warm for water and methane to condense, so the planetesimals, as the forming planets are called, could only form from compounds with high melting points, such as metals like iron, nickel, and aluminium and rocky silicates. These rocky bodies would become the terrestrial planets, Mercury, Venus, Earth, and Mars. These compounds are quite rare in the universe, so the terrestrial planets could not grow very large. These forming planets grew to about 0.05 Earth masses and ceased accumulating matter about 100,000 years after the formation of the Sun. It was the period involving subsequent collisions and mergers between these planet-sized bodies that allowed these terrestrial planets to grow to their present sizes. When the terrestrial planets were forming, they remained immersed in a disk of gas and dust.

At the end of the planetary formation epoch the inner Solar System was populated by 50–100 Moon- to Mars-sized planetary objects.

Further growth was possible only because these bodies collided and merged in a period which took less than 100 million years. These objects would have gravitationally interacted with one another, tugging at each other’s orbits until they collided, growing larger until the four terrestrial planets we know today took shape. One such giant collision is believed to have formed the Moon, while another removed the outer envelope of the young Mercury.

The gas giant planets, Jupiter, Saturn, Uranus, and Neptune formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where the material is cool enough for volatile icy compounds to remain solid.

The Jovian planets

The Jovian planets are called Jovian because they are named after the planet Jupiter. The planet Jupiter was itself named by the Romans who named it after Jupiter, the principal god of Roman mythology.

The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the Jovian planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements. Planetesimals beyond the frost line accumulated up to four Earth masses within about 3 million years. Today, the four gas giants comprise just under 99% of all the mass orbiting the Sun.

Theorists believe it is no accident that Jupiter lies just beyond the frost line. Because the frost line accumulated large amounts of water via evaporation from infalling icy material, it created a region of lower pressure that increased the speed of orbiting dust particles and halted their motion toward the Sun.

In effect, the frost line acted as a barrier that caused material to accumulate rapidly at this distance from the Sun. This excess material coalesced into a body of about 10 Earth masses, which then began to grow rapidly by swallowing hydrogen from the surrounding disc, reaching 150 Earth masses in only another 1000 years and finally topping out at 318 Earth masses.

Saturn may owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume.

Asteroid belt
The outer edge of the terrestrial region is called the asteroid belt. The asteroid belt initially contained more than enough matter to form 2–3 Earth-like planets, and, indeed, a large number of planetesimals formed there. As with the terrestrials, planetesimals in this region later coalesced and formed 20–30 Moon- to Mars-sized planetary embryos, however, the proximity of Jupiter meant that after this planet formed, 3 million years after the Sun, the region’s history changed dramatically.

Orbital resonances
An orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, and these gravitational influences of both Jupiter and Saturn are particularly strong in the asteroid belt. These gravitational interactions with more the more massive forming planetary material scattered many planetesimals into those resonances. Jupiter’s gravity increased the velocity of objects within these resonances, causing them to shatter upon collision with other bodies, breaking up into smaller objects rather than combining to make larger objects.

As Jupiter migrated inward, following its formation, resonances would have swept across the asteroid belt. The effects of the giant planets left the asteroid belt with a total mass equivalent to less than 1% that of the Earth, composed mainly of small planetesimals.

A second period that brought the asteroid belt down close to its present mass is believed to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance. The inner Solar System’s period of giant impacts probably played a role in the Earth acquiring its current water content from the early asteroid belt. Water is too volatile to have been present at Earth’s formation and must have been subsequently delivered from outer, colder parts of the Solar System. The water was probably delivered by small planetesimals thrown out of the asteroid belt by Jupiter.

Planetary movements of the outer planets
According to a theory called the nebular hypothesis, the outer two planets are in the “wrong place”. The ice giants Uranus and Neptune exist in a region where there was hardly any material of the solar nebula from which they could form and their longer orbital times also suggests that their formation in this part of the solar system where we see them now does not make sense. An explanation could be that the two ice giants formed in orbits near Jupiter and Saturn, where more material was available, but then moved outward to their current positions over hundreds of millions of years.

After the formation of the Solar System, the orbits of all the giant planets continued to change slowly, influenced by their interaction with large number of remaining planetesimals. After 500–600 million years (about 4 billion years ago) Jupiter and Saturn fell into a 2:1 resonance; Saturn orbited the Sun once for every two Jupiter orbits. This resonance created a gravitational push against the outer planets, causing Neptune to surge past Uranus and plough into the ancient Kuiper belt.

Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter.

The planets scattered the majority of the small icy bodies inwards, while themselves moving outwards. These planetesimals then scattered off the next planet they encountered in a similar manner, moving the planets’ orbits outwards while they moved inwards. This process continued until the planetesimals interacted with Jupiter, whose immense gravity sent them into highly elliptical orbits or even ejected them outright from the Solar System. This caused Jupiter to move slightly inward. Those objects scattered by Jupiter into highly elliptical orbits formed the Oort cloud; those objects scattered to a lesser degree by the changing orbit of Neptune formed the current Kuiper belt and scattered disc.

This explains the Kuiper belt’s and scattered disc’s present low mass. Some of the scattered objects, including Pluto, became gravitationally tied to Neptune’s orbit, forcing them into mean-motion resonances. Eventually, friction within the planetesimal disc made the orbits of Uranus and Neptune circular again.

The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of several wide-field survey telescopes such as Pan-STARRS and the future LSST, which should reveal many currently unknown KBOs. These surveys will provide data that will help determine answers to these questions.

The Kuiper belt is believed to consist of planetesimals; fragments from the original protoplanetary disc around the Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than 3,000 km (1,864 mi) in diameter.

Stable orbits of the inner planets
In contrast to the outer planets, the inner planets are not believed to have migrated significantly over the age of the Solar System, because their orbits have remained stable following the period of giant impacts.

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