The Solar System

Question 0

Define a planet (2003)?

Answer 0

Due to discovery of Eris 2003 - a planet is any object in orbit around the sun with a diameter greater than 2000km

Question 1

Define planets in the solar system (2006)?

Answer 1

2006 IAU - a planet is a celestial body that (a) is an orbit around the sun (b) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes hydrostatic equilibrium (c) has cleared the neighbourhood around its orbit
A dwarf planet - is a celestial body that (a) is in orbit around the sun (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium, (c) has NOT cleared the neighbourhood around its orbit (d) is not a satellite
All other objects - small solar system bodies

Question 2

What is trans-neptunium objects?

Answer 2

Kuiper belt - disc shaped region past the orbit of Neptune, containing many small icy bodies (30-50AU). Contains three dwarf planets (Pluto).
Oort Cloud - 1000x more distant and not flat. 10,000Au. Outer cosmographical boundary. Formed closer in the sun and was scattered by gravitational effects by giant planets. Outer Oort Cloud spherical - comets orbit have been randomised by passing star

Question 3

Overall structure of the solar system?

Answer 3

Central star/sun
Inner Rocky planets
Asteroid belt
Gas giants
Ice giants
Kuiper Belt
Oort Cloud

Question 4

How do meteorites give us timing information?

Answer 4

Stoney meteorites - chondrites are the oldest meteorite in the solar system
Inclusions (CAIs and chondrules) - very high rapid heating and cooling
Age - 4.6Ga (207Pb/206Pb)

Question 5

What are the two theories of the origin of our solar system?

Answer 5

Catastrophic - collision between the sun and a star (unique, accidental)
Non-catastrophic - nebula theory (non-unique, widespread)

Question 6

What is the Nebaula Theory and what evidence do we have?

Answer 6

A cloud of gas (H, He) and dust collapses under gravity
Proto-sun grown at centre when mass is concentrated
Compression of proto-sun causes heating
Nuclear fission of hydrogen ignites generating solar wind
Nebula reacts and flattens to a disk
Planet grows in disk

Evidence - The Allende Meteorite
Carbonaceous chondrite (CIA)
Fragment of least processed solar nebula

Question 7

5 Steps of how our solar system formed?

Answer 7

Step 1 - FORM A STAR FROM A SOLAR NEBULA
Giant molecular clouds exist throughout the our galaxy
Composition dominated by H & He lack of energy due to elements damaged so collapse explode and energy creates new elements
COLLAPSE THE NEBULA
Goes below the JEANS MASS (minimum mass to hold the molecular cloud in hydrostatic equilibrium)
Clouds on the edge of stability are DESTABILISED by nearby supernova or spiral arm (Supernova shockwave due to creation of heavier elements in red giants)
FREE FALL REGIME - Temperature and pressure both rise creating deuterium and helium at 10^7
STEP 2 - FORM A PLANETARY DISK
Angular momentum is conserved during cloud collapse. Rotation speeds up and cloud shrinks meaning material gets span out forming a disk. Enough energy was created to trigger nucleo-fusion creating our sun. The lighter gas was swepped into the solar system leaving heavier elements
STEP 3 - FORM PLANETESIMALS
SOLIDS in the disk form giant molecular clouds
Dust grains start to STICK TOGETHER when colliding (motion controlled by turbulent gas)
At around 1cm particles decouple from gas and start to orbit
PARTICLES GROW until they form a planetesimal (1km in size) - 2 body collision?
STEP 4 - FORM PLANETS
PLANETESIMALS COLLIDE with each other to grow - gravitational focusing helped speed things up. Mutual velocities must be low (around the escape velocity)
OLIGARCHIC GRWOTH - TERRESTRIAL PLANETS
The bigger the planetesimal accretes/accumulates all others in reach
Eventually it runs out of new material and reaches ISOLATION MASS
Migration and scattering of proto-planets to new locales provides new material
OLIGARCHIC - GAS GIANTS
Atmosphere accumulates when escape velocity is less than thermal speed
Captures planetesimals by GAS DRAG
Stops when disk dissipates or when gap is cleared in the disk
STEP 5 - CLEAN UP
Solar nebula dissipates - 10^7 years - STRONG SOLAR WIND moves gas, planetary positions are frozen, existing planets sweep up old debris
Differentiation model - ice giants
Saturns rings - rochee limit