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 187Strontium Uranium 238Lead Plutonium 244Thorium Iodine 129Xenon Manganese 53Chromium Aluminum 26Magnesium parentdaughter½ 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 Mercury7.004 Venus3.394 Earth0 Mars1.85 Jupiter1.308 Saturn2.488 Uranus0.774 Neptune1.774

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

Discussion If the solar nebula started as a spherical cloud why 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 Mercury0.206 Venus.007 Earth.017 Mars.093 Jupiter.048 Saturn.056 Uranus.046 Neptune.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 Venus Earth Mars Jupiter Saturn Uranus Neptune 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 Venus Earth1.0 Mars (Ceres) Jupiter5.2 Saturn Uranus Neptune Distance AUT-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/cm 3 Mercury 5.4 g/cm 3 Venus 5.2 g/cm 3 Mars 3.9 g/cm 3 Moon 3.4 g/cm 3 Neptune 1.6 g/cm 3 Sun 1.4 g/cm 3 Jupiter 1.3 g/cm 3 Uranus 1.3 g/cm 3 Saturn 0.7 g/cm 3

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 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 helium 1.4% hydrogen compounds – CH 4, NH 3, H 2 O 0.4% silicate rocks 0.2% 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?