Solar System Formation

Age of the Solar System

The oldest rocks found on Earth are about 4.55 billion years old, not native but meteorites which fall from space.

The oldest native Earth rocks are 3.85 billion years old.

Radioactive dating

Unstable parent isotopes decay at a constant rate to stable daughter isotopes. By measuring how much of the parent isotope is still present, and how much of the daughter isotope there is, we can calculate the age of the rock.

Rubidium 187 Strontium 87 48800

Uranium 238 Lead 206 4470

Plutonium 244 Thorium 232 83

Iodine 129 Xenon 129 16

Manganese 53 Chromium 53 3.7

Aluminum 26 Magnesium 26 0.72

parent daughter ½ life (millions of years)

Discussion

The age of a rock given by radiometric dating is the time since the rock last solidified.

What happens to the lighter daughter elements when the rock is in a molten or gaseous phase?

The nebular hypothesis

The Sun and planets formed from the gravitational collapse (possibly triggered by nearby supernova) of a single, spherical, slowly rotating cloud of cold interstellar gas and dust.

Formation of the Solar System

Discussion

Why does the gas cloud need to be cold?

Consequence

Planet formation is a natural outcome of star formation.

Planetary systems should be common.

Dynamics of the Planets

1. The planets revolve counterclockwise around the Sun as viewed from above the Sun’s north pole, the same direction that the Sun rotates on its axis.

Discussion

Why can’t all the planets just orbit in any arbitrary direction? Why should they all go around the Sun in the same way?

2. The major planets have orbital planes that are only slightly inclined with the plane of the equator of the Sun’s rotation, i.e. the orbits are coplanar.

Orbital inclination

Orbital Inclination

Mercury 7.004Venus 3.394Earth 0Mars 1.85Jupiter 1.308Saturn 2.488Uranus 0.774Neptune 1.774

Discussion

What does this mean for the paths of the planets through the sky?

Discussion

If the solar nebula started as a spherical cloudwhy do all the planets lie in a plane above the Sun’s equator. Shouldn’t they be spherically distributed about the Sun?

Gravity can collapse a rotating cloud only along the axis of rotation.

Discussion

What two changes take place as the solar nebula collapses due to gravity?

3. The planets move in elliptical orbits that are very nearly circular.

Eccentricities

Mercury 0.206Venus .007Earth .017Mars .093Jupiter .048Saturn .056Uranus .046Neptune .010

Discussion

Why are all the orbits nearly circular? What happens to planets that formed with highly eccentric (very elliptical) orbits?

4. The planets rotate counterclockwise as viewed from above the north pole, the same direction as they revolve, except for Venus and Uranus.

Rotation of the Planets

Mercury 58.6 0.0Venus -243.0 177.4Earth 0.997 23.5Mars 1.026 25.2Jupiter 0.41 3.1Saturn 0.43 26.7Uranus -0.72 97.9Neptune 0.67 29.0

Period (days) Axis tilt

Discussion

Why do you think the planets rotate in the same direction and why is this direction in the same sense as the planets orbit the Sun?

Discussion

Unlike all the other planets Venus rotates backward. How would the diurnal and yearly motion of the Sun differ on Venus compared to that of the Earth?

Discussion

Is the solar day longer or shorter than the sidereal day on Venus?

Discussion

Which planet will have the most extreme seasons?

5. The Planets’ orbital distance from the Sun follows a regular

spacing.

Titius-Bode rule

Write down 0, 3, 6, 12, … each number, after the first, being double the previous value. Add 4 to each and divide by 10.

Titius-Bode Rule and Distance

Mercury 0.39 0.4Venus .72 0.7Earth 1.0 1.0Mars 1.52 1.6(Ceres) 2.77 2.8Jupiter 5.2 5.2Saturn 9.54 10.0Uranus 19.18 19.6Neptune 30.06 38.8

Distance AU T-B distance AU

A packed Solar System?

The solar system may be as densely packed as possible. There do not appear to be any orbits stable over the lifetime of the solar system between the current planets.

6. Most satellites revolve in the same direction as their parent

planet’s rotation and lie close to their parent planet’s equatorial

plane

An exception is Neptune’s Triton

Discussion

How would you explain this observation with our formation theory of the Solar System?

7. The Sun contains 99.8% of the solar systems mass but only 0.5%

of the angular momentum

Discussion

If the Sun formed from a single spherical rotating cloud, wouldn’t you expect that all the pieces would have the same angular momentum as the original cloud? How must the solar system have changed since the time of its formation that this is no longer the case?

Discussion

Either the Sun’s rotation rate has slowed over time, or the planet’s have been spun up in their orbits. How could we decide between these two possibilities?

The Sun rotates once every 33 days, but should rotate once in about 2 hours if angular momentum were distributed evenly.

This two hour rotation rate is common among other young solar mass stars elsewhere in the galaxy as well as higher mass stars.

Slowing the Sun’s rotation

Magnetic breaking – The Sun’s magnetic field might interact with the early solar nebula to slow the Sun’s rotation.

Strong solar winds early in the history of the Sun might have carried the extra angular momentum away.

8. Long period comets come from all directions and orbital inclinations in contrast to the

coplanar orbits of the planets.

Discussion

If the long period comets can have any inclination, what does this tell you about their distribution around the Sun?

2 types of planets

Terrestrial planets – iron-nickel cores and silicate mantles

Jovian planets – silicate/hydrogen compound (methane, ammonia, water) cores and mostly H and He mantles

Terrestrial & Jovian planets

Density

Mass of the planet divided by the volume of the planet.

Higher density implies a larger percentage of high density materials, such as iron and nickel, lower density implies more silicates.

Mean density of planets

Earth 5.5 g/cm3

Mercury 5.4 g/cm3

Venus 5.2 g/cm3

Mars 3.9 g/cm3

Moon 3.4 g/cm3

Neptune 1.6 g/cm3

Sun 1.4 g/cm3

Jupiter 1.3 g/cm3

Uranus 1.3 g/cm3

Saturn 0.7 g/cm3

9. Most solid planetary surfaces are heavily cratered

Meteor Crater (1.2 km)

Arizona from the Shuttle

10. All the Jovian planets have rings and a large number of

moons

11. All the Jovian planets have a core of icy/rocky material with

between 10-15 times the mass of the Earth

12. All planets are enriched with heavier elements in comparison

with the solar abundances

The Planetesimal Hypothesis

Fluffy dust grains condensing out of the solar nebula stick together as a result of low-speed collisions, building up to small bodies called planetesimals.

Protoplanets

As the protoplanets grow by accretion of planetesimals, their gravity increases spurring more accretion.

Simulation

Solar nebula composition

We expect that the solar nebula from which the Sun formed, had the same composition as the current solar surface.

98% hydrogen and helium1.4% hydrogen compounds – CH4, NH3, H2O0.4% silicate rocks0.2% metals

Discussion

If the planets and Sun all formed from the same nebula, why don’t all the planets and the Sun of the same chemical composition?

The outer planets have about the same composition as the Sun but the terrestrial planets do not. Why?