March 30, 2011, jeb
Outline
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Planetary orbits generally lie in a plane (ecliptic plane)
- largest deviation: Mercury (7o)
| Terrestrial planets | Jovian planets |
| close to the Sun | far from the Sun |
| closely spaced orbits | widely spaced orbits |
| small masses | large masses |
| low escape velocity | high escape velocity |
| small radii | large radii |
| predominantly rocky | predominant gaseous |
| high density | low density |
| slower rotation | faster rotation |
| weak magnetic fields | strong magnetic fields |
| few moons | many moons |
| no rings | rings |
Large densities (= mass/volume) of inner planets
=> iron-nickel cores
Planetary atmospheres depend on escape velocity
=> which depends on gravity (or total mass of planet)
Table of Solar System Properties
- Asteroids & meteoroids
- Comets
- icy, with some rocky material
- ancient material
- typical diameters of 1-10 km
-
trans-Neptunian Objects
- largest is Pluto, the dwarf planet
-
Kuiper Belt
- range in size from fraction of km to 1000 km
- Interplanetary dust
- Sun 99.80%
- Jupiter 0.10%
- Comets 0.05%
- Other 7 planets 0.04%
- Total of Sun + Planets + Comets = 99.99%
- 4.6 billion years
(4,600,000,000 years)
- Derived primarily from studies of rocks from:
- Meteorites
- Moon
- Earth
using
radioactive dating
Rocks from meteorites and the Moon as old as 4.6 x 109 years,
but oldest rock on Earth 3.9 x 109 years
- Five planets were know to ancient peoples, due to
their wandering nature
The word planet comes from the Greek word
&pi&lambda&alpha&nu&eta&tau&eta&sigmaf
meaning "wanderer"
- Mercury
- Venus
- Mars
- Jupiter
- Saturn
- Moons of Jupiter - 1609/1610 (Galileo,
first telescope)
- Saturn's rings - 1659
- Uranus - 1758
- Neptune - 1846
- Ceres (largest asteroid) - 1801
- Pluto - 1930
- The Moon was explored by spacecraft beginning with
the lunar passage of Soviet Luna 1 continuing through the 1960's,
until the Apollo mission landed on the Moon six times between 1969 and 1972.
- All eight planets have been visited by U.S. or Soviet craft;
a mission to the dwarf planet Pluto
(
New Horizons) was launched in 2006 and will arrive there in July 2015
- Mercury
- The surfaces of Venus and Mars
Venus
- 1970 - Soviet
Venera 7 lands
- 1978 - US Pioneer Venus drops 5 packages
- 1990 - US
Magellan - detailed map from orbit
- Outer Planets
- Theory must explain observed details
- details we have now introduced and will
study in more detail in this course
-
Condensation Theory
- favored by most astronomers
- evolutionary theory
- Solar System developed by gradual and natural steps
Any model must explain what we observe:
- Each planet is
relatively isolated
- each planet is about twice as far from Sun as its inner neighbor
(table)
- Planetary orbits slightly elliptical -
nearly circular
- All the planets' orbits
lie roughly in same plane
- Sun's rotational equator lies nearly in this plane
- Planets orbit the Sun in the same direction as the Sun rotates
- Sun and most planets rotate in same direction
- Tilt of rotation axis with respect to orbit usually small
- Most moons revolve around their parent planets in the same direction as the
planets rotate on their axes
- Planets differ in composition
-
Asteroid belt
- concentration of bodies which differ in chemical and geologic properties from planets and moons
- primitive, unevolved material
-
Oort cloud of comets
- icy fragments not contained in ecliptic plane
- Additional facts
- planets contain about 90% of solar system's angular momentum
- planet and asteroid rotation rates are similar (5-15 hours) unless tides slow them down.
- despite their differences
the bodies of the solar system seem to form a common family.
- seem to have originated at the same time
- few indications exist of later captures from other stars or interstellar space
- Early attempts to explain the origin of this system
- the
nebular theory
- French philosopher Rene Descartes
- German philosopher Immanuel Kant
- French astronomer/mathematician Pierre Simon de Laplace
- the nebular theory
- a cloud of gas broke into rings
- condensed to form planets.
- an evolutionary theory
- with a series of gradual & natural steps
- doubts about the stability of such rings
- scientists then considered catastrophic theories
- accidental or unlikely celestial events such as close encounter of the sun with a star?
- Such encounters are quite rare, and the hot, tidally disrupted gases would dissipate rather than condense to planets
- So - > is there a way to fix the nebular theory?
- condensation theory - based on evolutionary processes
- the preferred scientific model for the formation of the Solar System
- add effects of interstellar dust to the nebular theory
- dust help cool nebula and acts as condensation nuclei
- -> explains the observables
Steps in the evolutionary process of the condensation theory
-
Gas cloud with dust collapses.
- As it collapses its slight
rotation increases
- Note - dust is a key element
- speeds cooling, allowing collapse
- nuclei for condensation, growing into planetismals
- Centrifugal effects
- Gas molecules and dust grains in
circular orbits
- those on noncircular orbits collide with particles and eventually dampen noncircular motion
- large scale motion is parallel, circular orbits
- Collapsing gas and dust heats through collisions
- around 3000 K so everything in gaseous form
- Hydrogen (about 90%) and Helium (about 10%) make up most of nebula
- silicates and Iron compounds about 1%
- Nebula cools
- outer parts cooling off more than inner parts (that are close to hot proto-sun).
- metal stuff can condense (freeze) at high temperatures while volatile stuff condenses at lower temps
-
local temperature and density depends on distance from proto-sun.
- at Jupiter temperature cool enough to freeze water
- further out ammonia and methane freezing out
- chondrules of material with highest freezing temp form and become incorporated in material of lower freezing temperature
- planets will also differentiate later on:
- heavy metals in core - lighter near surface
- Gas and dust particles in
parallel, circular orbits
- small eddies collide at low velocities
- stick together by gravity and electrostatic forces
- coalescing particles form bodies rotating in same direction as revolution with similar rot. rates
- gravity tends to divide nebula into ring-shaped zones (later form planets)
- Massive planetesimals
pull in nearby nebula.
- some can form mini-solar nebulae to form moons
- Jupiter and Saturn have a lot of water ice mass
- -> sweep up a lot of Hydrogen and Helium
- Uranus and Neptune less so
- Icy planetesimals interact over period of hundreds of millions of
years with Jupiter, Saturn, Uranus, and Neptune
- Early Sun has magnetic field and
spews out ions
- ions dragged along by rotating magnetic field
- dragging ions brake the Sun
- also accretion disks like solar nebula
- tend to transfer angular momentum outward.
- Proto-sun core gets to about 10 million degrees Kelvin
Candidate newborn solar system
