Astronomy - Formation Of The Planets


Formation Of The Planets 


The formation and evolution of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud.

Most of the collapsing mass collected in the center, 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.

This model, known as the nebular hypothesis was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace.

Its subsequent development has interwoven a variety of scientific disciplines including astronomy, physics, geology, and planetary science.


          Emanuel Swedenborg                             Immanuel Kant                              Pierre-Simon Laplace

Self-accretion of cosmic dust accelerates the growth of the particles into boulder-sized planetesimals. The more massive planetesimals accrete some smaller ones, while others shatter in collisions.

Collisions and gravitational interactions between planetesimals combine to produce Moon-size planetary embryos (protoplanets)

Finally, the planetary embryos collide to form planets.

In the formation of terrestrial planets or planetary cores, several stages can be considered. First, when gas and dust grains collide, they agglomerate by microphysical processes forming micrometre-sized particles.

Planetesimal formation in the centimetre-to-meter range is not well understood, and no convincing explanation is offered as to why such grains would accumulate rather than simply rebound.

This problem is known as the "meter size barrier

A number of mechanisms have been proposed for crossing the 'meter-sized' barrier. Local concentrations of pebbles may form, which then gravitationally collapse into planetesimals the size of large asteroids.

Or the particles may take an active role in their concentration via a feedback mechanism referred to as a streaming instability.

In a streaming instability the interaction between the solids and the gas in the protoplanetary disk results in the growth of local concentrations, as new particles accumulate in the wake of small concentrations, causing them to grow into massive filaments.

Alternatively, if the grains that form due to the agglomeration of dust are highly porous their growth may continue until they become large enough to collapse due to their own gravity.

The planets were originally thought to have formed in or near their current orbits. From that a minimum mass of the nebula i.e. the protoplanetary disc, was derived which was necessary to form the planets – the minimum mass solar nebula. It was derived that the nebula mass must have exceeded 3585 times that of the Earth.

However, this has been questioned during the last 20 years. Currently, many planetary scientists think that the Solar System might have looked very different after its initial formation.

Cores of the Rocky Planets

The cores of the rocky planets were initially characterized by analysing data from spacecraft, such as NASA's Mariner 10 that flew by Mercury and Venus to observe their surface characteristics.

The cores of other planets cannot be measured using seismometers on their surface, so instead they have to be inferred based on calculations from these fly-by observation. Mass and size can provide a first-order calculation of the components that make up the interior of a planetary body.


All of the rocky inner planets, as well as the moon, have an iron-dominant core. Venus and Mars have an additional major element in the core.

Venus’ core is believed to be iron-nickel, similarly to Earth. Mars, on the other hand, is believed to have an iron-sulphur core and is separated into an outer liquid layer around an inner solid core.

Mercury has an observed magnetic field, which is believed to be generated within its metallic core. Mercury's core occupies 85% of the planet's radius, making it the largest core relative to the size of the planet in the Solar System.



Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulphide outer core layer, a deeper liquid core layer, and a solid inner core.




The inner core of Venus is mostly made up of iron and nickel.

The similarity in size and density between Venus and Earth suggests they share a similar internal structure: a core, mantle, and crust.

Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.

The slightly smaller size of Venus means pressures are 24% lower in its deep interior than Earth's.



The outer core of the Earth is a liquid layer about 2,260 kilometres thick. It is made of iron and nickel. This is above the Earth's solid inner core and below the mantle.

Its outer boundary is 2,890 km (1,800 mi) beneath the Earth's surface. The transition between the inner core and outer core is approximately 5,000 km (3,100 mi) beneath the Earth's surface.

Without the outer core, life on Earth would be very different. Convection of liquid metals in the outer core creates the Earth's magnetic field. This magnetic field extends outward from the Earth for several thousand kilometres, and creates a protective magnetosphere around the Earth that deflects the Sun's solar wind.

Without this field, the solar wind would directly strike the Earth's atmosphere.This might have removed the Earth's atmosphere, making the planet nearly lifeless. It may have happened to Mars.



Mars possibly hosted a core-generated magnetic field in the past. The dynamo ceased within 0.5 billion years of the planet's formation.

Like Earth, Mars is a differentiated planet, meaning that it has a central core made up of metallic iron and nickel surrounded by a less dense, silicate mantle and crust.

The planet's distinctive red colour is due to the oxidation of iron on its surface.

Outer Gas and Ice Giants

Current understanding of the outer planets in the solar system, the ice and gas giants, theorizes small cores of rock surrounded by a layer of ice, and in Jupiter and Saturn models suggest a large region of liquid metallic hydrogen and helium.

Jupiter and Saturn appear to release a lot more energy than they should be radiating just from the sun, which is attributed to heat released by the hydrogen and helium layer. Uranus does not appear to have a significant heat source, but Neptune has a heat source that is attributed to a “hot” formation.

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

The ices that formed the Jovian planets were more abundant than the metals and silicates that formed the terrestrial planets, allowing the giant planets to grow massive enough to capture hydrogen and helium, the lightest and most abundant elements.




Jupiter has a rock and/or ice core 10–30 times the mass of the Earth, and this core is likely soluble in the gas envelope above, and so primordial in composition.

Jupiter has an observed magnetic field generated within its core, indicating some metallic substance is present. Its magnetic field is the strongest in the Solar System after the Sun's.



Despite consisting mostly of hydrogen and helium, most of Saturn's mass is not in the gas phase, because hydrogen becomes a liquid at high density.

The temperature, pressure, and density inside Saturn all rise steadily toward the core, which causes hydrogen to be a metal in the deeper layers.

Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles.



Uranus's mass is roughly 14.5 times that of Earth, making it the least massive of the giant planets. Its diameter is slightly larger than Neptune's at roughly four times that of Earth. A resulting density of 1.27 g/cm3 makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane.

The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope.



Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core.

The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane.

The conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones. Scientists also believe that this kind of diamond rain occurs on Jupiter, Saturn, and Uranus.

The core of Neptune is likely composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of Earth.

The pressure at the centre is about twice as high as that at the centre of Earth.

Currently, many planetary scientists think that the Solar System might have looked very different.

Several objects at least as massive as Mercury were present in the inner Solar System, the outer Solar System was much more compact than it is now, and the Kuiper belt was much closer to the Sun.

Models show that density and temperature variations in the disk governed this rate of migration, but the net trend was for the inner planets to migrate inward as the disk dissipated, leaving the planets in their current orbits.