the origin of the solar system Our Solar

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 (100 x 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 1000 s 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 sand seed 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 kinds of 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 location 2. there is a great difference in mass and density 3. there are craters everywhere! • all these are clues!

clue

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space debris asteroids, comets, meteoroids… • only three types: • 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 mudballs? ? ? • 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 of the 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 earthmasses 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

• 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! a

• 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? ü b/c solar nebulae do that (gravity & centrifugation) • sun and planets rotate and revolve in same direction? ü b/c all formed from same disk • all planets on same plane? ü b/c the solar nebula collapsed into a disk

• what about Venus & Uranus & Pluto? ü 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? ü 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

ü grow to ~15 earth-massee and you get to vacuum up gases in the solar-nebula (jovians) ü also, Jupiter’s gravity was probably responsible for preventing the formation of a planet between it and Mars ü those planetesimals are still there as the asteroid belt

ü comets are the icy planetesimals leftovers from the outer solar system ü the multitude of moons could have been formed from mini solar nebs around the planets, or… ü may have been captured space debris

ü the great distances of the jovians make it easier for them to hold onto their ring of debris ü and their ages all agree with a common birth date ü 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