Chapter 7: The Birth and Evolution of Planetary Systems

Where did the solar system come from? How was it made? Up until the mid 1990’s the only planets known were those in our solar system. As a result, the theories we developed to explain the formation of a solar system fit our system. Since the 1990’s we have discovered hundreds of extra- solar planets. How does our theory match these newly discovered worlds?

“Facts” that must be accounted for in any theory of solar system formation All the major planets orbit in almost the same plane All the planets orbit in the same direction Almost all the planets rotate in the same direction as they orbit The inner planets are rocky bodies while the outer planets are gaseous and/or icy bodies 99% of the mass of the solar system is in the Sun Most of the angular momentum of the solar system is in the planets, not the Sun Look at ClassAction Solar System Properties Explorer in the Solar System Characteristics module

We start with a cold cloud of gas and dust Since most of the mass is in the Sun and it is almost entirely hydrogen and helium, we need to look for places that have lots of hydrogen and helium

The cloud starts to collapse due to gravity As it shrinks in size, angular momentum causes the cloud’s initial slow rotation to spin faster and flatten out into a thick disk.

Angular momentum is what causes a skater to “spin-up” It is what causes the pizza dough to flatten out when tossed or an ice skater to spin-up as she pulls her arms and legs in close to her body. Watch Ice Skater Spin-up and Pizza Toss videos

The “Spin-up” causes the cloud to flatten out Angular momentum keeps stuff from falling straight in. Instead, it spirals down onto a disk. This is the pizza toss effect

At this point we have something that looks like a star surrounded by a disk of gas and dust The protostellar Sun is getting its energy from gravitational collapse, not from fusion like “normal” stars.

The temperature in the protoplanetary disk falls off as you get farther from the protosun Check out planet Formation Temperature Plot on ClassAction website Solar System Characteristics module

The solar nebula is composed mostly of hydrogen and helium Since hydrogen is the most common element in the disk, the most common things to condense will be hydrides of carbon (CH 4 …methane), nitrogen (NH 3 …ammonia) and oxygen (H 2 O…water). These condense at fairly low temperatures. Elements like silicon and iron condense at higher temperatures. Hydrogen and helium will never condense, they always remain a gas.

What is found at different distances from the protosun depends on temperature and abundance

Condensation begins to form dust grains The dust grains are tiny: about the size of particles in smoke. They are also charged with static electricity

The dust grains quickly start sticking together Close to the protosun the grains are exclusively silicon, iron and other heavy elements: “rocky” materials. Farther out there are more grains of “icy” materials than rocky ones. Static electricity plays an important part in making the grains stick together

Accretion is a snowball effect that builds larger and larger objects

Eventually Planetesimals are formed Close to the Sun the planetesimals look like asteroids Far from the Sun the planetesimals are a mix of ice and rock

Planetesimals merge to form protoplanets The larger the planetesimal, the stronger its gravity is. The stronger its gravity, the more it attracts stuff and the more violent the collisions become.

The gas giants form a large core of ice and rock and then grow by sweeping up large amounts of gas There is still lots of hydrogen and helium around but because they are such light weight gasses, only the most massive objects have enough gravity to hold on to them.

When the gasses get blown away, the condensation phase ends Once the star at the center ignites, strong winds will blow away any remaining gas and the condensation process ends.

The Solar Nebula Stage Condensation starts and planetesimals begin growing. The object at the center is still shrinking in size and gaining mass. “Rocky” materials will begin to condense everywhere while “icy” materials will only condense far from the new protostar.

The Accretion Stage Planetesimals grow bigger by collisions. There may be hundreds of moon sized protoplanets form in the inner solar system. The outer planets have grabbed up the last of the gas. The protostar at the center is beginning to start fusion in its center. Violent winds from the new star blow away any remaining gas.

The accretion stage was a violent period with planet smashing collisions The final stages of accretion would have seen tremendous collisions between planet sized objects.

Finally, we have a new star and new planets The new planets at this stage are nothing like the planets we see today. They will evolve over time to become the eight planets we see now. The new star, too, is not like the Sun of today. It is larger and more violent with huge sunspots and solar flares blasting the inner solar system with radiation.

Finding extra-solar planets Our theory was designed to explain the formation of our solar system. How does it match up with other planetary systems around other stars?

We have seen lots of disks around forming stars confirming some of the nebular theory Watch Orion Nebula Fly-through video

Actually seeing a planet has only recently been done Planets are tiny objects which shine by the reflected light from their star. This makes them extremely difficult to see.

Newton’s 3 rd Law applies to the Sun and planets If the Sun pulls on Jupiter, then Jupiter pulls on the Sun. The two actually orbit a common point just outside the surface of the Sun. The Doppler technique uses spectroscopy to detect the tiny motion of a star caused by an orbiting planet. Watch ClassAction Extrasolar Planet module Influence of Planets on the Sun animation

The Doppler Effect technique detects the motion of a star caused by a planet The planets orbit has to be aligned properly for the Doppler technique to work. The orbit needs to be almost edge on for the Doppler technique to work. Watch ClassAction Extrasolar Planet module Radial Velocity Graph animation

The transit method measures a planet directly if it passes in front of its star The planet will be a dark spot passing across the face of the star. The dimming of the light from the star may be tiny but it is measurable if the planet is large enough.

OGLE detects gravitational microlensing caused by a planet According to Einstein’s theory of gravity, if a planet passes directly between us and some distant star (not its host star), the light from the distant star will brighten and fade in a very particular way.

The Doppler method is the most prolific but it finds large mass planets close to their star Visit The transit method is starting to catch up with the Doppler technique in finding planets because of the Kepler mission but the Doppler technique still has a slight lead.

So what do we do about our solar nebula model? The modification is to the accretion phase. We call it Migration theory: things move, sometimes a lot. Our model predicted small rocky planets close to the star. We are finding large gas giants close to their star! Obviously, the theory needs to be modified.