Theme 7 – The Formation of the Solar System

ASTR 101Prof. Dave Hanes

First Impressions of the Solar System: It’s Highly Organized!

...and yet again!

Organized inThree Senses

1. Dynamical properties (motions) of the constituents

2. Physical properties

3. Spacing and position

1. Dynamical Properties

The planetary orbits are: - nearly circular- all in the same direction- all in nearly the same plane

Planetary spins (rotation) are - almost all in the same sense as the orbital motions- spin axes are (mostly) nearly perpendicular to the

orbital planeMajor moons

- orbit in the plane of the solar system- rotate and orbit in the same sense as the planets

Warning! Recognize What’s

SignificantAll orbiting objects must obey Kepler’s laws, whether

they date from the dawn of time or were launched into orbit around the Sun yesterday.

That obedience tells us only that the Sun’s gravity dominates the motions, but nothing about origins.

But is presumably significant that the planetary orbits are nearly circular. (The orbits could, in principle, be ellipses of any eccentricity you like.)

2. Physical Properties

There is a clear distinction between the:

Inner (terrestrial, rocky) planets: smaller, with solid surfaces and relatively thin atmospheres

and the

Outer (Jovian, gas giant) planets: much larger, with thick deep gaseous envelopes.

Planet Mass (Earth = 1)

Radius (Earth = 1)

Average density

Composition

Moons?

Rings?

Mercury

0.06 0.38 5.43 Rocks, metals

0 No

Venus 0.82 0.95 5.24 Rocks, metals

0 No

Earth 1.00 1.00 5.52 Rocks, metals

1 No

Mars 0.11 0.53 3.93 Rocks, metals

2 No

Planet Mass (Earth =

1)

Radius (Earth = 1)

Average density

Composition

Moons?

Rings?

Jupiter 318 11.2 1.33 H, He, hydrogen

compounds

>60 Yes

Saturn 95.2 9.4 0.70 H, He, hydrogen

compounds

>30 Yes

Uranus 14.5 4.0 1.32 H, He, hydrogen

compounds

>20 Yes

Neptune

17.1 3.9 1.64 H, He, hydrogen

compounds

>10 Yes

WarningAnother Irrelevance!

It is no surprise that the inner planets (Mercury, Venus,…) are currently warmer than the outer planets (Saturn, Uranus,…). This is merely a consequence of their proximity to the Sun.

But the warm surroundings may have had some influence on the formation process itself, in the distant past.

Why are the Rocky (Terrestrial)

Planets Nearest the Sun?Do the rocky planets settle closer to the sun, with

the low-density ones taking up orbits farther away?

NO!! Remember the astronauts in the Space Station. All bodies ‘fall’ equivalently under gravity

If you replaced Saturn (a low-density gaseous body) with a rocky object (say, the Earth), it would continue to orbit in exactly the same way. The rocky object would not ‘settle’ towards the sun.

This organizational feature has a different explanation.

3. Spacing and Position

The spacing looks regular, in some sense.

Bode’s Law (1772)[no need to memorize the numbers]

Here’s a procedure (found by trial and error):

Adopt a “starting number” = 4 For successive numbers, add 3, 6, 12, 24, 48, 96,

192, 384… in turn. Divide by 10

What do we get?

A Good Match to the Planets Known to Bode!But what about the ‘missing’ entries?

(0+4)/10 = .40 Mercury 0.39

(3+4)/10 = .70 Venus 0.72

(6+4)/10 = 1.00 Earth 1.00

(12+4)/10 = 1.6 Mars 1.52

(24+4)/10 = 2.8 ?? ??

(48+4)/10 = 5.2 Jupiter 5.20

(96+4)/10 = 10.0 Saturn 9.54

(192+4)/10 = 19.6

?? ??

(384+4)/10 = 38.8

?? ??

Subsequent Discoveries- two hits, then a miss

Uranus (1781)

Neptune (1846)

Ceres (1801)

Ceres: Not a Planet the first of millions of asteroids found

Now being orbited by a spacecraft!

Ceres is the Largest Asteroid

[but is still very small]

Bode’s Law: One Safe Conclusion

The orbits of the planets are not completely haphazard. The spacing, although not uniform, is regular in some sense.

To some degree, this is inevitable! If you had two planets in orbits that were quite close to one another, their mutual gravitational tugs would ‘tweak’ the orbits and, over time, lead to big changes. Planets can even migrate in this way.

For long-term stability, planetary orbits must be reasonably far apart.

But We Are Here

The Earth itself can not have ‘migrated’ much over the ~4.6 billion year life of the Solar System. (If it had, the great variations in temperature mean that we would not be here to discuss the issue!)

On the other hand, we may be ‘lucky survivors’ – many other planetary systems may be quite unstable. (We will return to this.)

Solar System FormationThree Possible Scenarios

1. Haphazard accumulation: The Sun forms first, then later somehow collects a ‘family’ of planets and other things

2. Uniquely catastrophic event: a near-collision between two stars sparks the formation of planets. This suggests that planetary systems might be rare.

3. Routine formation: planets and so on form along with the Sun. This suggests that planetary systems should be found around many stars.

1. Haphazard Accumulation? NO!

There is too much order and regularity in the Solar System.

There is also no mechanism for the sun to capture the planets. (As they fall towards it, they pick up speed, then just race on by.)

Anyway, it doesn’t really address the question. Where did the planets come from before capture by

the Sun?

2. Near-Catastrophic Formation[a popular theory ~1900]

Invoked the close passage of two stars, plus tidal effects.

On the Positive Side

It would explain:

Why planets orbit in the same plane

Why planets orbit in the same direction

Maybe even why the biggest planets are in the middle of the distribution

But Near-Collisions of Stars are Rare!

Stars move at modest speeds, in random directions, and are separated by huge voids.

We expect very few ‘close passages’ in the entire Galaxy of 100 billion stars.

One Clear Implication

If that’s how Solar Systems form, there will be very few of them around! We might be unique in the whole galaxy.

Conversely: if we find evidence of lots of planetary systems, they must form in some other more routine way!

Anyway: Two Fatal Flaws

1. Gas pulled off a regular star would be too hot to condense! – it would just evaporate into the vacuum of space

2. There are angular momentum problems: somehow you have to form rapidly spinning big planets in huge orbits, but with a Sun that is rotating very slowly. This is hard to explain.

3. The Nebular Hypothesis[dates back to the mid-1700s]

The Solar System formed as a unit from a once-distended cloud, or ‘nebula,’ of gas and dust.

(Dust = small particles, grains, flakes, and chips of common elements, ices and minerals. Don’t visualize household dust!)

Summarized in Words

An originally distended cloud of interstellar gas starts to shrink under its own gravitation spins faster (by the conservation of

angular momentum!) flattens out as it does so gets hotter at the centre as atoms collide

where the Sun is forming condenses into small pebbles to start with these merge into larger pieces and planets

Summarized in Pictures

…and Again

Note the timescale: from gas cloud to planets in about 100 million years. This is about 2% of the age of the Solar System, so it all happens relatively fast!

Issues to Address1. We still see gas clouds in space. What starts the

collapse to produce a planetary system? Why hasn’t all the available gas in the galaxy turned into stars and planets?

2. Why are there planets of different sizes and compositions in different locations? (i.e. why the organization of physical properties in the SS?)

3. Why are the motions, directions of rotation, etc so organized? (i.e. why the organization of dynamical properties in the SS?)

4. Can we see other Solar Systems in formation?

Gas Clouds in Space

Why don’t the atoms all rush together under gravity? (Analogous question: why doesn’t all the air on Earth settle right down to the ground?)

Answer: the gas is warm, and the random motions of particles provides a sustaining pressure.

To Trigger a Collapse

We need to cool the gas, so the supporting pressure is reduced; or compress the cloud a bit. This could be the result of a shock wave from a nearby supernova, as a massive star dies in a huge explosion; once the atoms are closer together, the force of gravity between them is stronger.

These things happen from time to time, so there are continuing cycles of star formation – some going on right now.

Composition

The pre-solar-system gas cloud was

big (millions of times the volume of the present solar system)

like the universe overall, made of Hydrogen (2/3) and Helium (1/3), with mere traces (a few %) of other material

What Happens as it Gets Denser?

Atoms, dust grains and so on get packed closer together and start to merge and condense.

A‘Fog’ – but not just water droplets. Atoms of all types bind together to form complex

minerals and grains, depending on the ambient temperature.

The Temperature Dependence

Near the proto-Sun, where it’s getting quite hot, only the relatively rare heavy elements can condense. This yields small rocky planets.

Farther out, everything condenses, yielding giant hydrogen-rich planets.

Continued Growth

Pebbles Planetesimals Planets

A steady buildup to a final few larger planets whose gravity tends to dominate their local zones

Physical Organization Explained!

The inner planets are small and rocky because 90%+ of the original material never condenses that close to the sun. (Only a fraction of the available building material is used!)

The outer planets are big and gaseous because all the material condenses there, and is also captured by the gravity of these huge objects -- and it’s mostly H and He. (All the building material gets used up!)

Dynamical Behaviour Explained

Grains, pebbles, rocks and planetesimals moving originally in oblique directions suffer many collisions and get ‘caught up in traffic.’

In the end, the whole system – planets and moons – tends to

have same sense of revolution around the sun, the same senses

of rotation (spin), to be in same plane, etc. (Of course, there will

be continued gravitational interactions, with the smallest pieces

most affected. Not everything moves in circles or in theflattened plane!)