This week Linn joins Matt to talk about the formation of the Solar System.

Upon a slight conjecture [on the origin of the solar system] I have ventured on a dangerous journey and I already behold the foothills of new lands. Those who have the courage to continue the search will set foot on them

Immanuel Kant

The Solar System

began about 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 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. 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, chemistry, geology, physics, and planetary science.

Since the dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, the model has been both challenged and refined to account for new observations.

The Solar System has evolved considerably since its initial formation.

MOONS

• Many moons have formed from circling discs of gas and dust around their parent planets,

• other moons are thought to have formed independently and later to have been captured by their planets.

• Still others, such as Earth's Moon, may be the result of giant collisions. Collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System.

• The positions of the planets might have shifted due to gravitational interactions. This planetary migration is now thought to have been responsible for much of the Solar System's early evolution.

In roughly 5 billion years, the Sun will cool and expand outward to many times its current diameter (becoming a red giant), before casting off its outer layers as a planetary nebula and leaving behind a stellar remnant known as a white dwarf. In the far distant future, the gravity of passing stars will gradually reduce the Sun's retinue of planets. Some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of tens of billions of years, it is likely that the Sun will be left with none of the original bodies in orbit around it

Presolar nebula

• gravitational collapse of a fragment of a giant molecular cloud.

• The cloud was about 20 parsec (65 light-years) across,

• 1 parsec (three and a quarter light-years) across.

• The further collapse of the fragments led to the formation of dense cores 0.01–0.1 parsec (2,000–20,000 AU) in size.

• One of these collapsing fragments (known as the presolar nebula) formed what became the Solar System.

• The composition of this region with a mass just over that of the Sun (M☉) was about the same as that of the Sun today, with hydrogen, helium, and trace amounts of lithium produced by Big Bang nucleosynthesis, forming about 98% of its mass. The remaining 2% of the mass consisted of heavier elements that were created by nucleosynthesis in earlier generations of stars. Late in the life of these stars, they ejected heavier elements into the interstellar medium

Orion Nebula, a light-years-wide "stellar nursery" probably very similar to the primordial nebula from which the Sun formed

• The oldest inclusions found in meteorites thought to trace the first solid material to form in the presolar nebula are 4568.2 million years old, which is one definition of the age of the Solar System.

• Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that only form in exploding, short-lived stars. This indicates that one or more supernovae occurred nearby.

• A shock wave from a supernova may have triggered the formation of the Sun by creating relatively dense regions within the cloud, causing these regions to collapse.

• Because only massive, short-lived stars produce supernovae, the Sun must have formed in a large star-forming region that produced massive stars, possibly similar to the Orion Nebula.

• Studies of the structure of the Kuiper belt and anomalous materials within it suggest that the Sun formed within a cluster of between 1,000 and 10,000 stars with a diameter of between 6.5 and 19.5 light-years and a collective mass of 3,000 M☉. This cluster began to break apart between 135 million and 535 million years after the formation

• Several simulations of our young Sun interacting with close-passing stars over the first 100 million years of its life produce anomalous orbits observed in the outer Solar System, such as detached objects.

Because of the conservation of angular momentum, the nebula spun faster as it collapsed. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, converting their kinetic energy into heat. The centre, where most of the mass was collected, became increasingly hotter than the surrounding disc.

Over about 100,000 years, the competing forces of gravity, gas pressure, magnetic fields, and rotation caused the contracting nebula to flatten into a spinning protoplanetary disc with a diameter of about 200 AU and form a hot, dense protostar (a star in which hydrogen fusion has not yet begun) at the centre.

At this point in its evolution, the Sun is thought to have been a T Tauri star. Studies of T Tauri stars show that they are often accompanied by discs of pre-planetary matter with masses of 0.001–0.1 M☉. These discs extend to several hundred AU—the Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter in star-forming regions such as the Orion Nebula and is rather cool, reaching a surface temperature of only about 1,000 K (730 °C; 1,340 °F) at their hottest.

Within 50 million years, the temperature and pressure at the core of the Sun became so great that its hydrogen began to fuse, creating an internal source of energy that countered gravitational contraction until hydrostatic equilibrium was achieved.

• This marked the Sun's entry into the prime phase of its life, known as the main sequence. Main-sequence stars derive energy from the fusion of hydrogen into helium in their cores.

• The Sun remains a main-sequence star today. As the early Solar System continued to evolve, it eventually drifted away from its siblings in the stellar nursery and continued orbiting the Milky Way's centre on its own.