the origin of the solar

system

Our Solar System consists of…

• the sun at its center

• eight planets,circling around the sun

• moons• asteroids• and comets.

a review of the origin of matter

• the mass of our planet (and you and me) was made w/i about 3 minutes of the Creation Event

• then atoms were only H and He• w/i about a billion years stars and

galaxies come to be (but no rocky planets at all)

• the stars is where all the rest of the elements bigger than H and He was made ≤Fe

• the elements >Fe were made in supernovae (which is why those elements are rare)

• how they all got into a planet and into you is what this lecture is all about

the origin of planets

• there are two processes to consider here: catastrophes and

evolution• catastrophes are sudden “life

changing” events (e.g. supernovae, impacts, marriage)

• evolution here is just change over a period of time, nothing sudden about it

• we’ll see here that the origin and life of planets is not either/or, but a combo of both

• e.g. possibly 2 local supernovae (c) started the condensation of local gas and dust (e) to form a disk of gas and dust called a…

• solar nebula which we see all over the place…

CONDENSATION/ACCRETION

• a cloud of dust and gas collapses due to gravity

• it forms a “pizza dough” disk

• the new sun blows away excess debris

• stuff that has had time to accrete remains as planets

Condensation/Accretion• In the beginning, our Solar

System was a huge disc of dirt, rocks, dust, gas, ice etc.

• In the middle of this disc, the Sun formed due to gravitational pressure until fusion commenced and it began to glow.

• At a distance from the center, the planets accreted from these rings of dirt, rock, dust and gas.

• The rocky terrestrial planets coalesced closer to the sun out of denser/heavier materials.

• The gaseous Jovian giants coalesced further from the sun where it was cold enough for the light elements like hydrogen, helium, and methane to condense.

planets orbiting other stars

• if solar nebula theory is good, there’d better be extrasolar planets out there

• but finding them is a pain: they’re too close to their star, ultra-dim, small, far…

• several evidences that they exist: dust and wobbles and light drops…

• beta Pictoris is the most well known of the dust disk stars

• it has a huge cold disk (100x bigger than our solar system) and a hole in the middle, just like our solar system has

• planets may have already formed in the hole

• and here young stars are surrounded by disks of gas and dust

• are there new planets being formed here???

• in Orion we see protostars with dust but no center clearing yet

• are planets forming here now?

• and then there’s wobble• a planet orbiting a star can cause it to

wobble just slightly

• first seen in 51 Pegasus 10+ years ago• have seen 1000s now, but• most are quite large and close to parent

star

• and we have ever-so-slight dimming of star light implying tiny planets are moving in front of a star

• and we can see tiny Doppler effects from a star’s spectrum implying something small and reflecting star light is moving about up there

A survey of the solar system

• here we look at the significant characteristics, and clues to how it formed

• and we’ll begin to see what a miracle this place is…

• our solar system is mostly empty space (good)

• if our sun were a ping pong ball in this room, Pluto would be a speck of dust on the football field

• all the other planets would be sandseed sized at various distances between

• = almost entirely empty

Size & Distance• Imagine the Solar System being a football

field (about 100 m long).• The sun would be a glowing orange in the

center.• Pluto would encircle the sun at the edge

of the soccer ground, having the size of a dust particle.

• The Earth would be 1.3 m away from the “orange“, having the size of a sesame seed.

Scaled Down…

revolution and rotation

• overall we’re pretty darn flat; Mercury is tipped (7˚), Pluto even more (>17˚) (= clues)

• we all go around the sun in the same direction (= clue)

• and all spin in the same direction as well (= clue), except…

• Venus is spinning backwards(!), Uranus & Pluto on their side

• all this helps us understand how they all formed

two kindsof planets

• here is a big clue to how it all came about

• there are inner:

earthlike - terrestrial - planets…

• and the more distant outer:

jupiterlike - jovian - planets

• notice three things following:1. the two kinds of planets are

distinguished by their location2. there is a great difference in

mass and density3. there are craters everywhere!• all these are clues!

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space debris• only three types: asteroids, comets, meteoroids…

• these little guys are some of the remnants of the earliest days

• asteroids (minor planets) are found mostly between Mars and Jupiter

• >20,000 known, but perhaps billions more (can’t be seen)

• about 1000 have orbits that take them to inner solar system

• many found in other special orbits

• have actually flown by a few and seen that even they have tiny impact craters in them

• were the ones b/t M and J (@ 2.8 AU) the debris of a broken up planet or a planet that never formed?

• comets are the best known and prettiest of space junk

• small nucleus just km across can produce a tail >AU!

• as the rocky ice ball comes close to the sun, its ices boil away and dust is released

• the solar winds push the gases and dust into an enormous tail away from the sun

• are they just icy mud-balls???

• one way or another they are made of the earliest stuff of the solar system and are a clue to it all

• here is dust from comets which help give us meteor showers

• meteors are here and gone (aka shooting stars)

• they are just ~sand-sized bits of stuff which vaporize when they hit our atmosphere, lighting up the air as they do

• we put on about 40,000 tons a year from this space debris

• meteor is the streak of light in the atmosphere

• meteoroid is the object itself

• meteorites are those big enough to make it to the ground

• almost never does it make it through

• but those that do can tell us how old the solar system is!

the age ofthe solar

system• we use half-life to

help us here• radioactive elements

found on Earth, Moon, and meteorites can be examined to see the ratios of parent to daughter

• this helps us determine how long these things have been solidified

• earliest rocks on Earth are ~4.4 billion years old

• but Earth is constantly recycling so oldest ones may never be found!!! :(

• the Apollo landings found Moon rocks that dated to 4.48 billion years old

• Martian rocks that have made it here date to ~4.5 billion years ago

• meteorites have a wide range as well but the oldest are ~4.6 billion years old

• we kind of cheat with the sun’s age @ 5 billion years

• we assume it is 5 billion because no rocks date older than that, but!

• computer models agree with that age

• here’s all the evidence so far…

the story of planet building

• here astronomers have to take the evidence and solar nebula theory to find out how it all may have happened…

the chemical composition of

the solar nebula• the composition of the early times

can still be seen on the sun’s surface:73% H, 25% He, 2% heavy elements

• we think the planets started by sticking tiny bits of elements and molecules into bigger bits

• only when a planet gets to about 15 earth-masses does it have enough gravity to steal H and He from the solar nebula

• gravitational collapse occurs increased density, pressure…

• J and S did this quickly, U and N more slowly (and U and N were at the outskirts which may mean some of their building blocks were blown away by the sun’s brothers and sisters)

• This process actually heats the planet up! (J & S still hot)

• so what happened with the terrestrials?

• they were too small to hold onto lightweight H and He :(

• they are dense b/c they formed from the heavier elements

• but how did it all happen? a two step process…

the condensation of solids

• why dense inners, and light outers?• all has to do with the way gases condense

in those regions…• near the new hot sun only heavier

elements like silicates and metals could condense at all

• further out where it’s cooler, lighter elements could condense

• way out there water ice, methane, and ammonia could be used as bldg blocks

• all this is called a condensation sequence

• it’s probably even more complicated than this (e.g. the nebula probably cooled as time went by) but however it happened the process was perfect

the formation of planetesimals

• three processes at work here:1. grains of stuff accumulate to

bigger things (cms kms)2. bigger clumps (kms) called

planetesimals collect into planets

3. the solar nebula clears up

• the little guys grow in two ways: condensation and accretion

• condensation is like the formation of a snowflake, small stuff runs into bigger stuff and sticks to make bigger stuff

• bigger things could stick to each other by electrostatic attraction, or polar molecules, or chemical bonding, or other forces such as gravity!

• like building a snowman from the snowflakes

• called accretion

a• big accreted guys could be brought to a growing thin disk and concentrate there; gas and tiny dust particles couldn’t

• speeds up planet-making!

• see in this meteorite all the tiny grains of stuff all stuck together

• see in this meteorite all the bigger bits of stuff all stuck together

• new computer models show that this disk is unstable, that little eddies would have formed

• this further concentrates stuff; now we have particles growing from cm’s tiny planets…

the growth of protoplanets

• thankfully, all the clumps of stuff is moving in the same direction so they don’t annihilate each other

• in fact, this helps them grow bigger!• chemical adhesive forces and

electrostatic cling still let this stuff hold on to each other, to be sure, but -

• gravity plays a huge role in holding it all together

• and now even small impacts might help:

• by getting smacked, a little layer of dust might form

• the layer of dust could act like a trap

• and getting bigger means more gravity means better able to hold onto a very thin atmosphere

• now can be considered a protoplanet

• now this big homogeneous ball begins to heat up (b/c of gravitational pressure and radioactive decay)

• as it melts, the heavy stuff (Ni & Fe) falls toward center; lighter silicates float to top

• = differentiation• the heat causes outgassing;

the driving out of gases trapped in rocks

• makes early atmosphere!!!

• a slight variation says this:• as the solar nebula cooled, the

metals condensed first• as the solar nebula cooled more,

the silicates could condense onto the metal-based planets

• an even better theory says the planets formed quickly and that as they condensed the heat of condensation helped differentiate the planetary layers as they formed

• which actually happened? we don’t know.

• way out there in the extreme cold the jovians quickly swept up gases as part of their makeup

• they probably formed pretty quickly (< 10 million years)

• [recent computer models show jovian types can form in just a couple revolutions! only hundreds of years!]

• the terrestrials took ~30 million years

• all over within 100 million years

explaining the characteristics of the solar system

• let’s look at the clues and see how they are explained…

• whole solar system disk shaped?b/c solar nebulae do that (gravity &

centrifugation)• sun and planets rotate and revolve in same

direction?b/c all formed from same disk• all planets on same plane?b/c the solar nebula collapsed into a disk

• what about Venus & Uranus

& Pluto?catastrophes! planet collision, though rare, are

possible and probably

knocked Venus and Uranus

around• even we were probably hit by another planet

(--> moon)

• why terrestrial/jovian difference?the condensation process (inside

and hot condense only metals/silicates; outside and cold can condense water and other gases to build planets)

• and more material means more planet

grow to ~15 earth-massee and you get to vacuum up gases in the solar-nebula (jovians)

also, Jupiter’s gravity was probably responsible for preventing the formation of a planet between it and Mars

those planetesimals are still there as the asteroid belt

comets are the icy planetesimals left-overs from the outer solar system

the multitude of moons could have been formed from mini solar nebs around the planets, or…

may have been captured space debris

the great distances of the jovians make it easier for them to hold onto their ring of debris

and their ages all agree with a common birth date

and the asteroids, comets, and meteorites are explained as left-overs from the solar nebula

clearing the nebula

• look up in the night sky and it’s pretty clear!

• what happened to the solar nebula?

• it vanished when sun was young; but how?

• radiation pressure!• when the sun got luminous

enough, its radiation’s interaction with matter could literally push tiny specks and atoms right out of solar system

• solar wind!!!

• the flow of ionized stuff from the sun, the sun’s breeze, pushes the dust and gas out, as well

• vacuum effect:• the planets, as they grew, swept up the

bigger stuff with their gravity• look at the surfaces of the planets and

satellites and asteroids• there are millions of impacts

• and most of them date from ~4 billion years ago

• this was the time of heavy bombardment

• we still get hit but not like the rain of old

• ejection• little stuff can whip around big

guys and be tossed from the solar system

• jovians were very effective here (and still are)

• the wonderfully orchestrated set of events that led from the Creation Event to the formation of the solar system can be fairly well explained