Chapters 16, 17, & 18 (!) – The Solar System
The basic shape of our solar system is a bit similar to that of the entire Milky Way. Instead of a
bulge, we have a star; instead of a disc, we have planets and asteroids in a disc, and instead of a
halo, we have the Oort cloud of comets. We see these disc-shapes when a revolving cloud
collapses in on itself – the cloud rotates faster as it gets more compact, and the disc tends to form
at the “equator” of the original revolving cloud. The Hubble Space Telescope has actually seen
these formative discs in protostars, which supports the solar nebula hypothesis. This also
suggests that planetary systems are quite common!
All planets form the same way. They start as diffuse rings in the protostar disc. Particles grow
like snowflakes in a process called condensation, pulling in one molecule at a time into an icyrocky structure. Once the particles are a little bigger, they can stick together (like snowflakes into
a snowball) in a process called accretion. Smaller planets stop at accretion. (Earth is still
gaining 100-200 tons of material each day – we’ll talk about “shooting stars” later.) Larger
planets have enough mass to pull gas in gravitationally, and continue growing by gravitational
collapse. To be called a planet, the object must be large enough that it’s own gravity makes it
spherical, and it must clear its orbit of remaining debris. (Pluto fails the second part… bigtime.)
Once the planet has formed, or while it’s forming, the decay of radioactive elements in the planet
heat it up - Jupiter and Saturn still get more energy from their own leftover radioactivity than they
do from the Sun! – so the planet “melts” and differentiates. This means the heavier material falls
to the center, and the lighter material comes up to the surface. The very lightest stuff – gas –
comes to the very surface, and forms the atmosphere. This is called outgassing.
Since (almost) all the matter in our solar system came from the same revolving cloud, we expect
most objects to lie in the same flat disc (the ecliptic), and to revolve and rotate in the same
direction. In fact, all eight planets orbit within seven degrees of Earth’s orbit, and except for
Venus (which rotates backwards) and Uranus (which rotates sideways), the planets rotate the
same direction they orbit.
Terrestrial planets: Mercury, Venus, Earth, and Mars.
Close to Sun (0.4 – 1.5 AU), small (0.06 – 1.00 Earth masses), no rings, basically no moons
(Earth’s moon is a weird story), carbon nitrogen oxygen atmospheres.
Jovian planets: Jupiter, Saturn, Uranus, and Neptune.
Far from Sun (5 – 30 AU), large (17 – 320 Earth masses), rings, moons, hydrogen helium
atmospheres.
There are two basic kinds of planets because, during planetary formation, the closer planets
were hotter than the farther ones. At higher temperatures (near the Sun), only heavy things
like metals and silicates could condense. At cooler temperatures (far from the Sun), lighter things
like water and methane can condense into ice. Lots more ices formed outside of the ice line,
which is somewhere between Mars and Jupiter, and this explains (we think) why farther planets
are larger, less dense (Saturn would float on water!), and have lighter elements in their
atmospheres. Rings and moons are a consequence of being so big.
A planet’s density is determined by (a) the density of its constituent material, and (b) its own
gravity compressing itself. If you just look at (a), then Mercury is the densest planet, followed by
Venus, then Earth, etc. This supports our solar nebula hypothesis of planetary formation.
The oddballs – Earth’s large and low-density moon, Venus’s backward rotation, Uranus’s
sideways rotation – are usually explained by catastrophic collisions.
The oddest oddball “planet” is the asteroid belt between Mars and Jupiter. It is the remains of a
failed planet. Every time a touch of protoplanet started to form, Jupiter would swing by and tear
it apart (through tidal forces). Most of the pieces of failed planet have been shot into the Sun or
flung into space by Jupiter, but the pieces that remain form the asteroid belt. (This is also how
some rings form around giant planets with moons.)
Using radioactive dating, we find that the all planets, comets, and asteroids are at least 4.3-4.56
billion years old, and we think the whole system is 4.6 billion years old. Older surfaces like the
Moon tend to show lots of impact craters, while younger surfaces like the Earth show few impact
craters. Lesson: collisions still occur, but they were more common in the early solar system.
(Erosion and life constantly renew the Earth’s surface – the Apollo astronauts’ footprints will
likely remain visible on the Moon for millions of years!)
Differentiation in terrestrial planets creates a small dense core, a thick mantle, and a thin lowdensity crust. (Earth’s core has a solid inner core and a liquid outer core, in case the diagrams
are confusing.) If it weren’t for plate tectonics, weather, and life, the Earth’s core would still look
heavily cratered from early bombardment… just like the Lunar highlands. (The Moon’s
smooth lowlands are due to post-bombardment lava flows.) The Moon’s small size prevented it
from keeping an atmosphere, and it cooled quickly, so the surface is inactive. Small Mercury
also lacks an atmosphere, has an inactive surface, and is heavily cratered. (Both the Moon and
Mercury are also in tidal lock with the things they orbit: the Moon in 1:1 and Mercury in 3:2.)
Greenhouse gases are gases that are clear to most solar radiation (UV and visible), but are
opaque to the infrared that Earth sends back out – they trap the radiation in. The greenhouse
effect warms Earth by ~70oF – global warming is necessary for us! If it weren’t for Earth’s life
and oceans, we’d have a lot more atmospheric carbon dioxide. On Venus (without life or
oceans), the greenhouse effect heats the atmosphere by several hundred degrees, to 900oF!
The Jovian planets formed outside the ice line, so they have lots of hydrogen and helium. Jupiter
and Saturn are mostly liquid hydrogen, and Uranus and Neptune are mostly liquid water and ice.
(The term “gas giant” is outdated, though the huge range of pressures in the Jovian planets mean
the “liquids” and “solids” are not necessarily what we’re used to.) The Jovian moons were
mostly formed during planetary formation, the same way planets formed around the Sun.
Tidal forces. I’ll go over this in class. (Sorry, I’m swamped with work this week and it’s 1am…
I need to sleep! Quick write-up.) Gravity gets weaker with distance, so if you were standing just
over the event horizon of a black hole, your feet would be pulled down much more strongly than
your head – you’d be ripped apart. This tearing effect of gravity is called a tidal force (remember,
it’s what makes our tides!). Tidal forces are what prevent most rings from forming into moons.
Anything inside the Roche limit of a celestial body cannot form into its own body – it will be
torn apart by tidal forces. Tidal forces also bend and flex moons, heating them up (energy for
life?), and because they cause friction, they slow rotation, eventually leading to tidal lock.
Read chapter 16
Read/skim chapter 17
Read/skim chapter 18
Homework
review questions on page 369: 2, 4, 12, 19
review questions on page 405: 2, 5
review questions on page 436: 1, 6, 14