The Solar System - I
Alexei Gilchrist
[The Story of the Solar System]
Some resources
• Section 13.3 of Voyages (references and links at end)
• References noted in these slides
• The Story of the Solar System, M Garlick,
(Cambridge Uni. Press, 2002)
Constraints
o Rather like a crime scene –
we are left with a set of clues
o The clues – where, what and when (motion, composition, and age)
o Each clue gives a constraint – any explanation has to fit within this
constraint
o what makes a good constraint?
... look for patterns, general features
o We don’t yet have enough
constraints – many models could
have led to the Solar System
- This lecture: the crime scene
- Next lecture: we accuse someone
Caveat
o We only have one solar system ... science needs to test
o We can look for internal consistency between the sciences and
the models (test parts of the model, e.g. fusion mechanisms),
... and we can look elsewhere in the universe
o Starting to discover other planets ... they don’t fit the models !
o current observation
techniques give a very
biased sample though
like estimating
heights in a maze
o May turn out that our
solar system happened
by a very improbable
mechanism and not the way we think at all
(so some constraints may be bogus)
o Great time to watch
science in action!
Caveat
On to the crime scene
You’ve sat through 9 weeks of lectures ...
What constraints would you put on a
model of the origin of the solar system?
e.g. All the planets end up moving pretty much in circles
(any model would have to end up with this feature)
Motional Constraints
o Planetary orbits are nearly circular
Orbit Eccentricity
Mercury
0.21
Venus
0.01
Earth
0.02
Mars
0.09
Jupiter
0.05
Saturn
0.06
Uranus
0.05
Neptune
0.01
http://www.nineplanets.org/data.html
[The Story of the Solar System]
Motional Constraints
o Planetary orbits in same plane
The Ecliptic is the mean plane
of the Earth's orbit around
the sun.
http://www.allaboutspace.com/subjects/astronomy/solarsystem/ecliptic.shtml
[The Story of the Solar System]
Degrees of Inclination
Mercury
7.0
Venus
3.4
Earth
0
Mars
1.9
Jupiter
1.3
Saturn
2.5
Uranus
0.8
Neptune
1.8
http://www.nineplanets.org/data.html
Motional Constraints
o Asteroids mainly occupy 2 locations:
- Asteroid belt
between Mars
and Jupiter
(Roughly disk shaped)
- Trojan asteroids
in same orbit
as Jupiter
(At L3 & L4 points)
http://en.wikipedia.org/wiki/Trojan_asteroid
Motional Constraints
o Comets seem to occupy 2 locations:
NB distiction between
asteroids and comets
not that clear cut
- Oort cloud – indirect evidence ... suspect trillions of objects
spherically distributed
- Kuiper belt - past Neptune (trans-Neptunian objects) ... may
contain 100s of millions of objects
Kuiper belt
o (1980) Realised most short period comets couldn’t originate
from captured long period comets – had to come from closer.
Suggested trans-Neptunian belt of icy comets
o (1992) First trans-Neptunian object found (QB1); ~250km
diameter
o Now know of >400 such objects; largest KX76 is 1300 km across
o Exact extent of Kuiper belt not known; the objects are dark,
small and a long way away – very difficult to detect
Oort
cloud
http://en.wikipedia.org/wiki/90377_Sedna
[The Story of the Solar System]
Inner solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_inner
Inner solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_inner
Outer solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_outer
Outer solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_outer
Distant solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_distant
Distant solar system as of 1 Oct 2008
http://ssd.jpl.nasa.gov/?ss_distant
Motional Constraints
o All the planets revolve around in same direction
er
pt
u
to
Ne
P lu
ne
Ur
an
us
J
it
up
Sa
tu
rn
M
ar
s
anticlockwise
Viewed from ‘above’ (N)
Motional Constraints
o most planets (and sun) also rotate in same direction
Mercury 0°
Jupiter 3°
Venus 177°
Saturn 27°
Earth 23°
Mars 25°
Uranus 98°
Neptune 30°
http://www.lpi.usra.edu/education/resources/s_system/solar_sys_formation.ppt
http://www.nineplanets.org/data2.html
Motional Constraints
o The planetary regular satellites
- move in nearly circular orbits
- have the same orbital direction
as the planets spins
o The planetary irregular satellites
-
Tend to be smaller
large orbits
possibly significant eccentricity
possibly significant inclination
Motional Constraints
o The sun is spinning way too slow
Sun has 99.9% of mass of solar system
but only 1% or less of the angular momentum
Angular momentum is conserved
before
after
Motional Constraints
o Evidence of frequent and massive impacts
Earth
Venus
Mercury
Moon
Mars
Composition Constraints
o 3 classes of planets
Terrestrial
o small
o rocky & metalic
(not to scale)
Gas giants
o largest
o mostly H & He
(same as sun)
Ice Giants
o intermediate size
o ices
Composition
Constraints
http://www.mso.anu.edu.au/~josan/filesfordown/PPV-2005.pdf
http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html
[The Story of the Solar System]
An obvious constraint?
o The planets are close to spheres
Composition Constraints
o Comets tend to be icy
dirty
snowball
model
o Asteroids tend to be rocky and carbon rich
Composition Constraints
Composition Constraints
o Particular chemical abundances
http://en.wikipedia.org/wiki/Image:SolarSystemAbundances.jpg
Age Constraints
o Earliest fossils are around 3.5 billion years old
o Some rocks on earths surface 3.8 billion years old
o primitive meteorites are about 4.5 billion years old