The Origin of the Solar System I

Preliminary announcement: We’ll try it again next week: Jupiter, Saturn, Vesta, Comet Linear.

Picture of the day: from Callisto: http://photojournal-b.jpl.nasa.gov/outdir/PIA03455.10323.jpeg

Here it is, just like in the title of the course! By this point in the semester, we have discussed a lot of what we know about the solar system. Now we ask, how did it get here? Here we are, sitting on the large terrestrial planet Earth. Next time you are on an airplane, look out the window and think of this huge sphere of silicon, oxygen, and iron orbiting the Sun at a speed of 30kilometers per second. The Earth seems so huge and permanent, it is difficult to think of it forming in some way. And then we can think that the Earth is a runt as solar system objects go.

So now we start describing what we know about the physics, chemistry, and astronomy of the formation events of the solar system.

We immediately run into a problem. Already at this point we know that the solar system objects such as the Earth, Moon, Mars, and meteors were in existence 4.5 billion years ago, so we are trying to piece together a story that is 4.5 Gyr old. This presents a challenge. As the Nobel Prize winner Hannes Alfven said, to trace the origin of the solar system is archaeology, not physics.

While Alfven’s statement is catchy, it’s not quite true. As will be seen over the next couple of lectures, we know a lot about the overall processes which occurred, although it is true that some very important aspects are still missing. In piecing this picture together, we get help from both physics and astronomy.

The plan of the lectures will be to bring up pieces of things that we know. Sometimes these pieces will give us an important hint in themselves. In other cases, we will be able to induce an important result from several of them. Here goes.

How Long Ago? This is an important aspect of the formation of the solar system that we already know. > Question for the august assembly.

Let’s look at an interesting table from a book by William Hartmann, which gives the age of formation of several objects in the solar system

>>>>>>>> Table 5-1 of Hartmann.

The amazing fact about this table is that not only does it give us a precise number for the age of the solar system, it indicates that things happened fast (geologically speaking). Different objects in different parts of the solar system were completely formed at the same time. The moral of the story is that 4.5 to 4.6 Gyr ago it happened, and happened fast in a big part of space. It was not a gradual process.

Hints from Astronomy When we look out, there are hints in the sky. We can make a list of these hints. They are contained in Table 5-2 of Hartmann’s book Moons and Planets. Hartmann refers to them as “characteristics to be explained” like homework problems, rather than “hints”. Nonetheless, regardless of terminology, the fact remains that there are characteristics of the solar system that contain information on the phenomena that occurred in the formation epoch, 4.5 billion years ago. Let’s look at some of these.

All the planets’ orbits lie in the same plane (a fact I have beat on all semester).
The Sun’s rotational equator lies nearly in this plane.
The major planets all revolve in the same west-to-east direction, called prograde motion. This is the same sense as the rotation of the Sun.
Planetary orbits are nearly circular (the big guys).
Planets differ in composition, correlating with distance from the Sun. The terrestrial planets are dense, heavy-element rich, and have small masses. The Jovian planets are of low density, composed of hydrogen, and massive. This is a strong observation that has to be explained.
Wherever observations have been possible, it is clear that solar system objects have been impacts by objects with diameters up to hundreds of kilometers.

Help from Stellar Astronomy The comparison between origins of the solar system and archaeology is defective in one important respect. In astronomy, we have stellar astronomy. If you are interested in figuring out some aspect of Old Kingdom Egypt, you are stuck with fossils essentially. You can’t turn on some special television and see the Egyptians going about their daily lives.

We can however, do that in astronomy, by looking out at other stars. There are regions where stars are forming right now. In the Milky Way galaxy there are 2000-3000 major star formation regions. The two nearest and best examples are visible in the eastern sky early in the evenings now. These are the Taurus-Auriga association, spread over the constellations that bear its name, and Orion star formation region, represented by the famous Orion Nebula.

Nebula = Latin for “fog” or “mist”

Observations of these star formation regions give us a lot of information. The first is that stars form out of big clouds of gas in interstellar space.

The Horsehead nebula http://antwrp.gsfc.nasa.gov/apod/ap001229.html

And smaller clouds that will probably form individual stars:

http://antwrp.gsfc.nasa.gov/apod/ap990924.html

These observations indicate that the process of star and planet formation involves the contraction of a gas cloud which involves a big decrease in volume, and a big increase in the density of the gas.

Disk Formation: If the cloud is rotating even slowly at the start, it will rotate more and more rapidly as it contracts. This is one of the most basic rules of physics, and is referred to as the conservation of angular momentum. So as the cloud contracts to the size of the solar system, it would be spinning like a top.

However, this gas cloud is not a rigid object like a top, it is a fluid, and spreads out in its equatorial plane. The net result is that a contracting cloud will end up as a rotating pancake with a central condensation. All of this is a prediction of theoretical physics, but it is on good ground. A picture of the result when the protostellar cloud has shrunk to the size of the solar system is given on p 288 of your textbook.

But we can do even better than this, using some of the hints stellar astronomy gives us. We can see star systems in the process of forming right now and “check our answers”. One of the most famous such examples is the star Beta Pictoris, which we are viewing in the plane of its disk.

http://antwrp.gsfc.nasa.gov/apod/ap971128.html

Question for the august assembly: Which of the attributes in the above table are we able to explain with these results of fundamental physics and stellar astronomy?

The Formation of Solid Matter: So far all of this is fine and good, but the young evolving star doesn’t have anything that looks like planets. After all, we are on the surface of a big solid object made of rocks. Jupiter and Saturn are isolated, massive objects, not sections of a hydrogen pancake. How do we get from a hydrogen pancake to planets.

This is an involved process, and not all aspects are completely understood at present. Nonetheless, we do understand some aspects of the planet formation in the early solar system. An important piece of the picture is condensation of particles from gas.

The temperature in this gaseous pancake varied from one place to another, and it almost certainly was hotter closer to the Sun than further out. The temperature may also have decreased with time during this early phase. Theoretical “models” of this nebula indicate that the temperature may have been as high as 1500K in the inner part of the disk (near Mercury) to as low as 200K out where the Jovian planets formed.

This gaseous pancake was primarily hydrogen and helium, but it had a sprinkling of everything else in the periodic table of the elements. These elements existed by themselves in the pure elemental state (carbon, iron, etc), and they could react chemically to form molecules (water, methane, silica). Depending on the temperature at a given point in the gaseous disk, certain of these substances could condense, or form bits of solid or liquid matter from the vapor phase.

Table 13.10 of your textbook shows how various substances would have formed out of the protoplanetary gaseous disk at various temperatures.

Question for the august assembly: In what part of the solar system would these various materials have precipitated? How about metal oxides? How about hydrated minerals?

The process of condensation means that the originally gaseous disk would start forming tiny bits of liquid and solid matter. We are beginning to see how solid matter formed in the early solar system.

A Common Example: The process of formation of small liquid or solid droplets from the vapor phase is common to us here in Iowa. When moist air is cooled, water condenses from the vapor phase and forms raindrops and snowflakes and ice crystals. What happened in the early solar system is that the same process was occurring, but involving a number of substances, and different substances in different parts of the nebula.

A Less Common Example: We can simulate the early days by introducing ammonium hydroxide and hydrochloric acid in a box. Both of the these evaporate and form vapor ammonium hydroxide and hydrochloric acid. The vapors chemically react to form a new substance, ammonium chloride. At the pressure and temperature in this room, forms a solid substance, which condenses in the form of small particles, which settle to the bottom of the box. This box gives an illustration of what was happening in the first years of the solar system. These small particles formed the beginnings of what became the planets, and everybody in this room.