Introduction to Astrophysics

Lecture 7: Formation of the Solar System

How did the Solar System reach its present form?

Useful facts about the solar system

There are several useful things we should bear in mind when considering how the solar

system might have formed.

The planets orbit in a plane

The most acutely angled planet is Pluto, which orbits at an angle of 17 degrees to the central plane of the Solar System (defined by the Sun’s rotation). But Pluto is odd in many ways.

After Pluto, Mercury is at an angle of 7 degrees, and the rest are all within three degrees of the plane.

Further, the planets all orbit in the same direction.

Most objects rotate and revolve the same way

With some exceptions, objects tend to rotate about their axes in the same sense as their revolution about the Sun. That is to say, their rotational angular momentum adds to rather than opposes their orbital angular momentum.

The majority of the angular momentum in the Solar System resides in Jupiter, as it is so massive.

Exceptions include the rotation of Venus (which bizarrely has a longer rotation period than orbital period), and the revolution of some of the giant planet moons (including Triton and some of the outer moons of Jupiter). Uranus rotates on its side.

Most objects are regularly peltedSolar system bodies take a repeated and persistent beating from various asteroids and comets.

Mercury and the Moon are geologically dead and are heavily cratered. The extent of cratering on other planets depends on surface geology.

Major impacts are not rare! For example in 1994 comet Shoemaker-Levy 9 smashed into Jupiter.

More useful information comes from the chemical composition of the Solar System

Inner planets: primarily made of metallic compounds, rocks (silicates) and other heavy elements. Their density is about 4 times that of water.

Giant planets: primarily made of gas, e.g. Jupiter is about 77% hydrogen, 22% helium and only trace amounts of other materials. This composition is very similar to that of the Sun.

Saturn is so light that it would float in a bucket of water, should you be able to find one big enough.

From radioactive dating we have a good knowledge of the necessary timescales.

a) There are rocks on the surface of the Earthwhich are at least 3.8 billion years old.

b) There are lunar rock samples 4.4 billionyears old.

c) Primitive meteorites have ages of 4.5 billion years.

The Age of the Solar System

What questions should we ask?

1. When did Solar System objects form?

A. The planets formed first and the Sun later.

B. The Sun formed first and the planets later.

C. Sun and planets formed at the same time.

2. Where did Solar System objects form?

A. In the same place.

B. In different places and then assembled later.

The Solar Nebula hypothesis

The best chance of explaining these observations is to assume that everything formed in the same place at the same time, about 4.5 billion years ago.

It is now widely believed that the solar system was formed in the gravitational collapse of a giant cloud of gas, known as the solar nebula.

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Initial stages

The Solar System is thought to have started out as a giant gas cloud, composed mainly of hydrogen and helium, but also containing some heavier elements including ice and metals.

Under its own gravitational attraction, it collapses.

Disk formation

As a gas cloud collapses, it begins to spin more quickly due to conservation of angular momentum.

The centrifugal force of this rotation flattens it out into a rotating disk with a protostar at the centre.

The nebula is hot because of gravitational energy released by the collapse, and when the protostar ignites at the centre it produces more heat. So there is a strong temperature gradient across the disk.

Planetesimals

The temperature determines whether materials are solid or gaseous. Close in only the metals are solid, while further out ice can also condense.

These form chunks called `planetesimals’, perhaps ten kilometres across.

The planetesimals merge together quickly to form objects of perhaps the size of the moon in the inner solar system, and 10 times more massive in the outer solar system where ice is available.

Giant planet formation

The more massive chunks in the outer solar system are dense enough that they can attract the hydrogen and helium gas from the solar nebula.

The original chunks form the cores of the giant planets, and the remainder of the planets is made up from the accreted hydrogen and helium.

This explains why the giant planets have roughly the same chemical composition as the Sun, since they are largely formed from the same material.

Terrestrial planet formation

In the inner Solar System, the planetesimals are not massive enough to accrete gas.

Initially it is thought there may be as many as one hundred moon-sized planetesimals jostling for position in the inner Solar System. During the first few million years collisions would be frequent, sometimes merging and sometimes smashing.

Eventually the four terrestrial planets would be left, all with similar chemical compositions.

The Big Clear Out

The young energetic Sun emits lots of energy in its early stages in the form of a strong solar wind.

This ejects all the stray remaining nebular material out of the Solar System, but the planets are dense and strong enough to survive.

It is thought that no more than 20% of the original disk material remains in the Solar System.

Water!

In the picture as described so far, there would be no water on Earth, as it would have been too hot for it to condense. Many other crucial organic elements would be absent too.

The water has to come later. Throughout history, the Earth has been bombarded by comets and meteorites, many of which originate in the outer solar system where ice has condensed.

These bring water to the inner planets. Depending on conditions there, the water can remain.

The Origin of the Moon

The large size of the moon poses a problem for planetary formation scenarios. Some ideas are

a) The Earth and Moon formed together.

b) The Earth captured the Moon.

c) The Moon broke off the Earth.

d) The Moon was formed in a giant impact of the Earth with another large body.

The asteroids

What are the asteroids?

a) The broken up remnants of a destroyed planet?

b) Material that never managed to form into a planet?

The current balance of opinion favours the latter, due to the existence of stable orbits between Mars and Jupiter.

Pluto

Pluto is basically pretty weird.

a) Strange orbit.

b) Suspiciously large moon.

c) Rather similar to Neptune’s moon Triton.

Pluto

Many scenarios have been considered:

1. Interaction with Neptunian system, but in fact they have never been close.

2. Gravitational perturbations from Neptune are what allowed Pluto to form.

3. Pluto originally rotated so fast that it broke up to form Charon.

It may even be that Pluto gets reclassified as non-planetary!

Conclusions

The Solar Nebula hypothesis is the best explanation we currently have for the origin of the Solar System.

It explains why the planets orbit in a plane, and why there is a preferred orientation of angular momentum.

There is a plausible explanation of the chemical composition of the planets.

A full explanation requires many interactions after the Solar System forms, for example existence of water on Earth, and that some objects have retrograde motions.

Every object has a complicated story of its own!