The Solar System Today

When we make a list of the characteristics of the solar system as we see it today, we find the following facts:

• The planet orbits have the following characteristics
• They all go around the Sun in the same direction (right hand rule). (The right hand rule is this: point your thumb in the direction of the north pole, and the object rotates or revolves in the direction that your fingers curl.)
• The Sun rotates in this same, right-hand-rule direction.
• All planetary orbits lie in nearly the same plane.
• All planetary orbits are nearly circular (eccentricity near zero).
• The planets spin on axes that are tilted randomly from this direction, but most obey right hand rule.
• Almost all moons orbit their planet in the same, right-hand-rule sense, and near the planet's equatorial plane.
• The inner planets are "terrestrial" planets, like Earth, with the following characteristics
• They are small, and made of rock and metal
• Their density (mass/volume) is high (4000-5000 kg/m³)
• They have few moons
• None have rings
• The outer planets are "jovian" planets, like Jupiter, with the following characteristics
• They are large, and made of hydrogen, helium, and hydrogen compounds
• Their density is low (less than 2000 kg/m³)
• They have many moons
• They all have rings
• Pluto is an exception to many of these rules--it is an outer planet that is small, orbits in an eccentric, inclined orbit, and has a low density.
• The asteroids orbit mostly in the asteroid belt between Mars and Jupiter.  They obey most of the rules for terrestrial planets.  They are rocky or metallic objects.
• Comets orbit in all directions, with highly inclined, eccentric orbits.  They are icy objects, with hydrogen compounds.  They appear to come from a spherical cloud called the Oort Cloud.
• There are recently discovered Kuiper Belt Objects (KBO's) that are like Pluto (some are called Plutinos), icy, small, and apparently numerous!  All orbit close to the ecliptic plane, beyond Neptune.

Lecture Question #1

If we want to understand how the Earth and planets came about, we need a framework (a scientific model) that explains all of these characteristics.  We discussed the difference between a hypothesis and a theory.  What was once called the Nebular Hypothesis for the origin of the solar system has now reached the status of a theory, both by explaining known facts and also predicting other facts that have since been verified by observations.  We will discuss these verifying observations later.  We can now call it the Nebular Theory of Solar System Formation.  Recently there are observations of other solar systems that do not seem to be explained by the nebular theory, so the theory is in need of adjustment.  We will discuss this later, too.

For a good interactive introduction to this theory, select the Formation of the Solar System tutorial on Astronomy Place web site and go through it.

When you are finished with this tutorial, you should know that

• the solar system formed from a cloud, or nebula
• as the cloud collapsed due to gravity it
• heated up by converting gravitational potential energy to heat (a form of kinetic energy)
• spun faster and faster due to conservation of angular momentum
• flattened into a disk (also due to conservation of angular momentum)
• the disk was hotter in the center and cooler in the outer parts which
• beyond the "rock line" (1500 K), allowed only rocky and metallic flakes to condense beyond about 0.3 AU
• beyond the "frost line" (150 K), allowed rock, metal AND ice flakes to condense beyond about 3.5 AU
• the flakes stuck together to form planetesimals, with the following characteristics
• the inner part had small, rocky planetesimals
• the outer part had much larger solid planetesimals due to both rock and ice flakes
• the planetesimals joined to form the planets, with the following characteristics
• the inner planets had only rock and metal, so did not get very big and could not keep light gases
• the outer planets had rock, metal, and ice, and grew so large that they could gravitationally keep light gases bound to them.  They could grow to be giant planets, with rock, metal, ice, AND gases
• eventually, the Sun became a star and the young stellar wind blew the gases away, stopping the planet formation process.
• Pluto never grew large enough to attract gases, so it never became a giant planet.  It and other KBO's are left over remnants, along with asteroids and comets--pieces that never became parts of planets.

We can look out at other stars, with the tremendous instruments available today, and see many of the features of this theory in progress.  We can also look at left-over pieces of the solar system that fall to Earth (meteorites).  Let's look at a few:

• Stellar nursery
• Orion nebula
• Stellar disks
• More disks

As long as we had only one solar system to explain, we could adjust the nebular theory to be a great fit.  But what happens when we discover other solar systems.  Do they match the scenario?  In just the last 5 or 6 years, new techniques have been developed to detect planets around other stars.  We mentioned them earlier, in the first lecture (see list).  The technique uses the "wobble" of a star due to the gravitational influence of an orbiting planet, detected by a periodic doppler shift of spectral lines.  Select the Detecting Extrasolar Planets tutorial on Astronomy Place web site and go through it.

When you are finished, you should know that

• stars travel in tiny orbits (the "wobble") to balance the pull of an orbiting planet
• massive and/or close-in planets are the easiest to detect
• at least one star is eclipsed by the planet that causes the wobble
• tiny and distant planets are hard to detect

The planets so far detected are all larger than Jupiter--some are much larger.  Here is an older overview, (latest one) showing only about half of the planetary systems now detected.  Here is an artist's conception of 51 Pegasi.

We have detected many stars with planets, but the planets are gas giant (jovian) type planets, some very close to the star.  In what ways does this violate the nebular theory model?  No one yet knows how such planets could form, but here are a couple of possibilities--

• If the nebula starts out with more than one center, then the larger one could end up as a star while a smaller one collapses in place to form a planet.  The planet would not have to form from the bottom up, starting with flakes.
• A planetary collision between two very large "planetesimals" could cause one to move in close to the star while the other is ejected from the newly forming solar system.  One might expect these planets to have a very eccentric orbit.

Why do we see so many of these solar systems that are so different from ours.  Is ours the odd-ball?  Maybe not.  The method for finding planets can (so far) only detect large, close-in planets, so it is not surprising that those are the kind we see.  We could not detect solar systems like ours with present techniques.  But we are exploring ways to do better, and we hope to detect Earth-like planets some day.  One thing is certain--this is an exciting time to be alive, to discover more about how we came to be here, and what other possible solar system types there are.

Lecture Question #2

Remnants of the Solar System

Remnants of Formation

The small bodies of the solar system are the leftovers in the process of solar system formation.  These are the asteroids, comets, and Kuiper Belt objects (KBOs).  We saw that during the formation, small flakes of ices, rock, and metal gradually combined to make planetesimals, which in turn combined to form the planets.  Some of these small planetesimals, the building blocks of planets, survived in the form of asteroids, comets, and other bodies.  They bring up several questions, for which we have to look at the following clues:

• Why didn't they end up as parts of planets?
  • clue: orbital properties
• Have they changed at all since they were formed 4.6 billion years ago?
  • clue: their structure
• What can they tell us about the early formation of the solar system?
  • clue: what they are made of

Orbital Properties

These remnants of the solar system come in several distinct types, with the following properties:

• Asteroids -- in nearly circular orbits, at low inclination (near the plane of the ecliptic), mainly between Mars and Jupiter (asteroid belt).  Plot of semi-major axis vs. eccentricity.  Plot of semi-major axis vs. inclination.
• Comets -- in all kinds of orbits, many with large eccentricity and high inclinations (source is the Oort Cloud, perhaps as far away at 50,000 AU).  Another likely source of comets is the Kuiper Belt.
• Kuiper Belt Objects -- in moderately elliptical, moderately inclined orbits beyond the orbit of Neptune (the Kuiper Belt).

How did these orbits come to be?  Both asteroids and KBOs were formed where they are now, but they continue to undergo collisions and gravitational perturbations that can "scatter" some of them.  This has the effect of flinging them about the solar system, so that some pass nearby the Earth, and sometimes even hit us!

Comets from the Oort Cloud have a different origin.  We think that they were initially formed in the region between Jupiter and Neptune (the gas giant planets), and were flung far out into space due to near-miss collisions with those planets.  There may be as many as one trillion comets in the Oort Cloud!

Lecture Question #3

Structure
 

Asteroids
Several spacecraft have flown past asteroids on the way to the outer solar system, and one (the NEAR spacecraft) actually orbited one for over 1 year, and eventually landed on it!  Here are some close-up images of a few asteroids:

• Galileo (Ida, Gaspra)
• NEAR (Mathilde, Eros)

Not all asteroids look like this, however.  The larger ones are so large that their gravity forces them into a spherical shape.  Any object with a radius of more than 500 km will be spherical.
Near Earth Asteroids, Potentially Hazardous Asteroids and Collision Probability

NEAT and NEOs

Comets
Comets can best be described as "dirty snowballs" typically of a few km in size, although they can range in size all the way up to Pluto.  Comets spend most of their time far from the Sun (recall that objects move slowest when at aphelion) where they remain inert icy snowballs.  As they approach the Sun, they move ever faster, and spend only a few weeks in the hotter part of the solar system.  There, the ices of volatile compounds (water ice, methane ice, frozen carbon dioxide, frozen ammonia, etc.) start to sublimate into gases.  These gases jet out from the interior, and sometimes break off chunks of the rocky parts.  Comets are fragile!  Several have been seen to break up, and some have disappeared completely, as we will see below.

Two Tails
Comets have two tails, as shown in the image below -- a ionized gas tail (the blue, straight tail) and a dust tail (the white, curved tail).  They start out very small, and grow larger and longer as the comet nears the Sun.

• The ionized gas tail is pulled away from the Sun by the solar wind.
• The dust tail is pushed away from the Sun by light pressure.
• Both tails point away from the Sun, but
  • the ionized gas is much lighter and faster, and so points directly away from the Sun.
  • the heavier, slower moving dust curves more along the direction of the comet's orbit.

Comets and Meteor Showers
The dust from a comet stretches all along its orbit, so when Earth crosses the orbit of a comet, these dust and fine grained particles hit the Earth's atmosphere and burn up as meteors.  This is what causes meteor showers.  The Earth crosses the orbits of several comets each year, at the same time of year each year.  That is how we know when meteor showers will occur. 

Images of some famous comets

• Halley (nucleus)
• Comet Borrelly
• Shoemaker-Levy 9 (Collision!)
• Hale-Bopp (nucleus/coma) (coma and tail)
• Hyakutake (nucleus/coma) (coma and tail)
• Linear (HST images) (Fade to black)
• Chiron--KBO or comet, or what?
• Macholtz

KBOs
We already talked about Kuiper Belt objects, and what they look like.  We have found a number of larger objects, which are spherical in shape.  Smaller objects, similar to the asteroids, also no doubt exist but are too small and dim to see from Earth.

Constitution

We have many ways to learn what these objects are made of.  We can look at the reflected light from them, which can be matched roughly to rock, metal, and ices.  We can also measure the IR radiation that they radiate.  But the best way to know what asteroids are made of is to find pieces of them that fall to Earth as meteorites.  We find that some are made of undifferentiated rock and metal flakes, and were never part of a larger object.  Others are differentiated (separated into pure rocks and pure metals), and so were part of the larger, spherical asteroids such as Vesta.  Here is an idea of the history of Vesta:

Broken chunks of the interiors of asteroids could be a source of valuable metals and minerals, possibly making mining of asteroids economical in the future. Were the asteroids all part of a single planet that broke up? We do not think so, because even if all of the asteroids were gathered into a single "planet," it would still only be 1/10th the mass of our Moon, which itself is smaller than the smallest planet. In addition, we do not know how such a body could break up entirely. Even a collision between two large objects would not scatter the pieces in the way we see them, but would leave the pieces in a similar orbit that eventually would recombine. Instead, we believe that the asteroids are a planet that did not finish forming before the young Sun's wind blew the remaining solar nebula out into interstellar space.

It is even easier to determine what comets are made of, because we can see the volatile gases that come from them when they heat up as they near the Sun.  We can also study the light reflected from the dust of comets.   We already mentioned that comets are made of the ices of volatile compounds (water ice, methane ice, frozen carbon dioxide, frozen ammonia), things that we expect to be present in the solar nebula from which the solar system formed.

So we see that we can explain the orbital dynamics, structure, and chemical makeup of the small bodies of the solar system in terms of the Nebular Theory of the formation of the Solar System.  But if so, there are a lot of clues out there that need to fit our understanding.  So astronomers and space scientists are very interested in studying these primordial pieces of the solar nebula up close.  Missions to collect comet dust, or return samples of comets and asteroids, are now being planned.

Lecture Question #4