We
usually think of the solar system as consisting of nine
planets, including the Earth. Two main groups of planets, called the "inner" or
"terrestrial" planetsLast updated 30 March 2005 05:43 PM
Rather than make a tour of the planets one by one, we will focus on how we answer a few very fundamental general questions about the Solar System
We
usually think of the solar system as consisting of nine
planets, including the Earth. Two main groups of planets, called the "inner" or
"terrestrial" planets
Mercury: 0.4
AU from Sunand the "outer" or "Jovian" planets, which have much larger orbits.
Most
asteroids are in orbit between Mars and Jupiter, but a few have orbits that
cross the Earth's orbit. The largest ones are about 1000 km across. These represent a real danger to the Earth, as we
discuss later. All the asteroids add up to less than the Earth's Moon.
The
orbits of the planets are all in the same plane and they move around in the same
way. This is strong evidence that the whole solar system formed at one time. If
the planets had been "caught" by the Sun's gravity one would expect
that they would have randomly directed orbits
Except for Pluto the orbits are nearly circular: Pluto goes out to 40 AU, though sometimes close to the Sun than Neptune.
The periods of the planets vary from 88 days for Mercury to 250 years for Pluto
In
the last decade dozens more objects have been discovered with orbits beyond the
orbit of Neptune in the range 30 - 50 AU, and it is estimated that there may be
70,000 of them altogether. These are known as Kuiper Belt objects and are
important because they are probably similar to the primitive lumps of material
out of which the planets formed billions of years ago.
Pluto is really just the biggest of these, but for sentimental reasons astronomers have agreed to continue to call it a planet.
Beyond
the Kuiper Belt are the comets
| True Distance | Scaled Distance | ||
| Earth diameter | 13,000 km | 1.3 cm | Grape |
| Moon | 3,500 km | 0.35 cm | Papaya seed |
| Earth-Moon | 400,000 km | 40 cm | 16 inches |
| Sun diameter | 1.4 million km | 1.4 m | 5 feet |
| Sun-Earth distance | 150 million km | 150m | 1.5 football fields |
| Sun-Neptune | 30 A. U. | 5 km | Downtown |
| Nearby star | 4 light years | 40,000 km | Circumference of Earth |
| Nearby galaxy | 2 million LY | 140 AU | Farther than Neptune |
| Edge Universe | 15 Billion Ly | 15 Ly | Farther than nearest star |
Five (+Earth) are visible to the naked eye, and and hence have no discoverer.
Uranus discovered accidentally by William Herschel in 1781, while scanning the sky with a telescope. He saw something that was wider than a star image, and changed place each night.
Neptune discovered in 1846, after being predicted by two mathematicians who found that Uranus was not moving in a perfectly elliptical path. By calculating the gravitational tug necessary to pull Uranus in its orbit, they were able to tell astronomers where to point their telescopes to discover Neptune. This is an example of a "perturbation" of the kind we mentioned when we discussed Newton's Law of Gravity.
Pluto discovered in 1930 by examination of
photographic plates.
KBOs same way, but using computer searches of digital images
Comets are sometimes found by professional astronomers with large telescopes, but are also sometimes found by highly skilled amateurs.
Lot of evidence within the last five years. The problem is that planets are much fainter than stars, so most of the evidence is indirect. The most important method of searching for planets makes use of the so-called "Doppler Effect"
Doppler effect
Explorations in physics 1: Doppler shifts If we are dealing with sound waves the pitch of something coming towards us is higher than that of something moving away from us.
We use the Doppler shift to determine the speeds of galaxies that are billions of light years from the Earth
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Newton's laws tell us that a star and planet orbit around their center of
mass. Therefore even if we don't see the planet we should see the star
wobble to and fro. The side to side motion is too small to measure, but the
forward and backward Doppler shifts (of order a few tens of meters a second) can be measured.
This technique has led to the discovery of more than 100 planets around
ordinary stars.
The "solar systems" that have been found are
different from ours and from what was expected. Most of the planets are
roughly Jupiter's size, with orbits much smaller than that of the Earth. The
formation theory of our solar system does not explain these. It should be
noted that systems of this type are the ones that are easiest to detect.
There may well be other systems that do resemble our own solar system that
we haven't found yet.
We also find a number of stars with disks of dust around them. Matter in
the form of dust is much easier to detect than planets (think how much smoke
you can get from a single firework) . We can see the dust either by
scattered light or by the heat it gives off.
In one case we have seen an "occultation" which is like an
eclipse.
Study of planets is a combination of observations made with telescopes on Earth (or in orbit abound the Earth), and space probes sent out to the planets.
Only
the Moon, in 1969-1972. Apollo Program. Six landings. Can actually get
hold of handfuls of moon rocks and do chemical analysis. We will come back
to this later. There is discussion about whether we should send a manned mission
to Mars, or even return to the Moon.
We can only land on things with solid surfaces. This excludes giant planets like Jupiter
Two
Russian robotic spacecraft landed on Venus in 1970. Lasted less than 1 hour each
because of the high temperature. US has never tried to land on Venus.
US
landed two Viking spacecraft on Mars in 1976, plus a smaller one called
Pathfinder in 1997. Two Mars Rovers landed in January 2004.
For the best images and up-to-date information go to the NASA's Mars website
NASA's Mars Global Surveyor website
Can see details the size of a vehicle.
NASA also landed on an asteroid, Eros
We've very recently landed on Titan, Saturn's moon
We've orbited everything we've landed on, plus Jupiter and Saturn. Orbiters allow detailed mapping of the whole planet.
Venus
has such thick clouds that we cannot take photos of its surface, even
through clouds.
Size of Venus is very similar to Earth: the most important difference is the atmosphere, which we will be discussing later.
Magellan spacecraft orbited Venus in the 1980s for several months and used RADAR to map through the clouds and provide maps of the surface. The color in the picture is only a guess.
Jupiter was orbited by the Galileo spacecraft for several years,
though the main point of the mission was to study the moons of Jupiter rather
than Jupiter itself. The spacecraft was programmed to crash into Jupiter
in September 2003, so as not to contaminate any of the Moons.
What we see are clouds that have colors due to nitrogen and sulfur
compounds.
Saturn is currently being orbited by a NASA spacecraft called Cassini.
Saturn is like Jupiter, but a bit smaller, and has the very obvious pattern of rings. The rings are lumps of ice, existing in a layer only about 1 km thick
In both cases the spacecraft spent more time looking at the moons of the planets than at the moons themselves. This is because the surfaces of the moons are solid, and therefore have much more information to impart.
Images of all four of Jupiter's Moons.
Images
of Io showing volcanos.
It is much easier to fly past a planet than to slow down enough to go into orbit. Fly bys take only a few hours. Preprogrammed photos radioed back to Earth. Mars, Venus and Jupiter had fly bys before there was a spacecraft landing or orbiting.
Mercury
had a single spacecraft fly past it in 1974. It looks very like the Moon,
and is not much larger, with a cratered surface. It is too hot to
have any atmosphere.
Uranus and Neptune were visited by the Voyager spacecraft in the 1980s.
Neptune
Besides sending pictures back to Earth spacecraft can measure things like magnetic fields of planets, their radiation and even sample the gases in their atmosphere.
Telescopes on Earth can make more precise analysis of the spectrum, can study changes that are not apparent in the brief period of a flyby, and search for new objects, including moons of planets. Because ground-based telescopes can be bigger than spacecraft, they can see fainter (and thus smaller) objects in distant orbits.
Earth and Venus are the largest of the terrestrial planets. Mercury, Mars (and the Moon) are smaller
The
greatest contrast with the Earth is the planet Jupiter. 11 times the diameter, 300 times
the mass of Earth
Sun is 10 times radius Jupiter, 1000 times mass of Jupiter
So Jupiter is sort of half-way between a terrestrial planet and a star.
We
can tell what the planets are made of by using the same arguments that we used
to show that the Sun was a gas. We find that the terrestrial planets have large
densities, consistent with rock or iron, and the Giant planets have low
densities, consistent with being balls of gas. We also look at the
spectrum lines of the light from the planets, and confirm that Jupiter is mainly
hydrogen and helium.
Jupiter depends on gas pressure to hold itself up, in the same way that the Sun does, but because it is lighter than the Sun, the pressure in the center is not so great, and is not enough to start nuclear reactions. This is why Jupiter is not a star.
Uranus and Neptune are also largely gas, but also contain a thick
layer of ice deep down.
The crucial factor is the escape velocity. To keep an atmosphere the average speed of the gas molecules (vmol) must be less than one tenth of the escape velocity (vescape).
Earlier in the course we had a formula for the escape velocity:
From this we can calculate the escape velocities for various objects:
One can show from elementary physics that the molecular velocity depends on the temperature and molecular weight of the gas:
,
where T is the temperature and m is the mass of a single molecule. The mass is proportional to the molecular weight:
We can calculate the molecular velocities of hydrogen and oxygen gas at the orbit of the Earth/Moon (300K) and at the orbit of Jupiter (140 K)
| Earth (300K) | Moon (300K) | Jupiter (140K) | |
| Escape Velocity (km/sec) | 11 | 2.3 | 60 |
| 0.1 x Escape Velocity (km/sec) | 1.1 | 0.23 | 6 |
| O2 speed (km/sec) | 0.5 (keep) | 0.5 (lose) | 0.3 (keep) |
| H2 speed (km/sec) | 2.0 (lose) | 2.0 (lose) | 1.3 (keep) |
For Jupiter, the average molecular velocity for both hydrogen and oxygen is less than 1/10 of the escape velocity, so that all the gases are retained. Helium also.
For the Moon, the molecular velocity is more than 1/10 of the escape velocity for all gases, so they escape and the Moon has no atmosphere
For the Earth, the molecular velocity for oxygen is less than 1/10 of the escape velocity (11 km per second) so we keep oxygen in our atmosphere. But the hydrogen moves faster and escapes.
So the bottom line is, big cool planets can keep their hydrogen, but small warm ones can't. Since most of the Universe is in the form of hydrogen and helium, a planet that keeps its hydrogen is likely to be much much bigger than one which loses it.
To determine the age of rocks we can make use of the technique of radioactive dating.
Certain isotopes of certain elements are radioactive. This means that the nucleus spontaneously ejects a particle such as a helium nucleus and turns into another element. This is called radioactive decay. This happens completely randomly, but each radioactive isotope has a characteristic half-life. The half life is the time for half of the atoms to decay. Half lives can be a fraction of a second, or more than a billion years.
We will take the example of 40K (Potassium-40). At any time
there is a small probability that an atom of 40K will
spontaneously turn itself into an atom of Argon-40 (40Ar)
Potassium-40 has 19 protons and 21 neutrons
Argon-40 has 18 protons and 22 neutrons
In effect, one of the protons spontaneously turns itself into a neutron.
Start
off with 1000 atoms of potassium-40
After 1.3 billion years we have 500 left, plus 500 argon atoms
After another 1.3 billion years we have 250 left plus 750 argon atoms
After another 1.3 billion years (ie 3.9 in total) we have 125 left plus 875
argon atoms
etc
Mathematically
this is called an exponential decay
Argon is a gas that does not combine chemically with any other element. If there is any argon in the liquid lava it will bubble away into space. Thus when rock solidifies it will contain no argon at all. Over time some of the potassium-40 atoms in the crystals in the rock decay into argon, but since the rock is solid the argon atoms cannot escape and are trapped in the rock. A skilled chemist can analyze the rock and determine the ratio of potassium to argon. This ration tells us the age of the rock. A high ratio of argon to potassium implies an old rock. This method (and others using other radioactive substances) are now extremely reliable. The same method is used for dating human artifacts by archaeologists: they use carbon rather than potassium.
The results of the measurements are:
The oldest rocks on Earth are 3.9 billion years. Youngest are brand new.
The oldest on Moon 4.48 Billion (the highlands)
Almost all meteorites: 4.6 Billion.
As we shall see later, the 4.6 billion years corresponds to the formation of the
solar system.
On Earth the erosion and plate tectonics leads to the destruction and remelting of old rocks, so that the Earth could easily be older than 3.9 billion years
Note that these ages are in direct contradiction to fundamentalist theories of creation.
This is one of the three methods that indicate that the Universe is billions of years old. The others are the expansion of the Universe from the Big Bang, and the ages of stars.
Most
planets rotate in the same direction as they orbit the Sun. The major moons of
the outer planets also go in the same direction,.
There are only a couple of exceptions to this:
Surface
of the Moon is dominated by craters which are due to meteorite impacts in the
past. We can tell that the craters are meteorite impacts from their shape
The
craters that are formed are much larger than the meteorite itself; it is the
explosion that makes the crater large.
The best picture we have envisages several stages:
Form the solar system out of an interstellar cloud. The cloud collapses because of its own gravity. The smaller it becomes, the stronger the gravity, by Newton's law. The cloud is made of a mixture of gas (mainly hydrogen and helium) and dust particles.
Even before it starts to collapse the cloud has some rotation. As it gets
smaller the rotation speed
increases because of the Law of Conservation of Angular Momentum. As it
rotates its outer regions get spun out into a disk. This is how we explain
the rotation of the solar system.
Main point is to compare the atmospheres of the terrestrial planets
Earth's
atmosphere is about 76% nitrogen, 23% oxygen 1% argon.
There are also rarer gases (less than 1% concentration) which are very important
We will see later that the Earth's atmosphere is quite different from the other planets.
No atmosphere
The
atmosphere of Mars is mainly CO2, (like that of Venus) but the
pressure is only about 1% that of Earth. There are clouds of water vapor
sometimes visible. The atmosphere condenses at the poles to form ice caps.
The atmosphere is very different from the Earth:
Why is Venus so very hot? The answer is the greenhouse effect, a process that also explains why the interiors of cars get so hot in the sunshine.
In
a greenhouse light can travel through glass and heat the plants inside. The
plants radiate infrared radiation that balances the heat they receive. But glass
is opaque to infrared radiation, so it gets trapped inside the
greenhouse Thus the temperature rises.
On Venus carbon dioxide takes the place of glass. The more carbon dioxide in the atmosphere the more heating there will be. The Earth has much less carbon dioxide than Venus, but the amount is rising because of the burning of fuel containing carbon, including wood, coal, oil. This is the danger of global warming that there is a lot of publicity about now.
The atmospheres of the terrestrial planets are so different that we must ask why. The answer is still controversial, but the most likely explanation is that CO2 is widely formed by volcanoes. Earth is the only planet with liquid water. Water can dissolve carbon dioxide (think of Coca Cola) in its oceans, then turn it into rocks such as limestone. Thus Earth can get rid of CO2 but Venus cannot. Nitrogen (which is the second most common gas on Venus) then becomes the most common gas on Earth, because it does not react with anything. After life evolves on Earth some more of the carbon dioxide is turned in to Oxygen, though the details of how this happened are still uncertain.
On Mars much of the carbon dioxide must have frozen out.
There is now much evidence that there once was water on Mars, We know that there is lots of ice on the moons of Jupiter, and it is possible that there is water under the icy surface of Europa.
What are the dangers from objects hitting the Earth. First let's review the kinds of small object in the Solar System.
Both are meteoroids, which are lumps of stuff that hit the Earth.
Thousands of tons of meteoroids hit the Earth each day; very little damage is done.
Small specks of matter burn up in the atmosphere giving rise to shooting stars or meteors.
I will explain about meteor showers a little later
Large lumps of matter can pass right through the atmosphere and land on
Earth. These are meteorites. Meteorites can
be made of stone or of iron, the latter are easy to recognize if you find them.
The biggest meteorites are the same as the smallest asteroids.
Most meteorites have the same age, 4.6 billion years, and are believed to
date from the formation of the solar system. By studying their crystals
scientists can determine a lot about how the early planets were put together and
about the history of the solar system. For example, we can deduce that iron meteorites
came from a fairly large object that got broken up.
A few meteorites came from the Moon or from Mars.
Meteorites and Meteors have larger cousins such as:
First one, Ceres, discovered in 1801, is about 1/3 the size of the earth's moon. Most much smaller. About 200 now known, with new ones being found each year. Some can only be seen when their orbit brings them near to the Earth. Total mass of all the asteroids is much less than any other planet. We know about most of them which are larger than a mile across, but it is hard to see the fainter ones.
They cannot be seen at all without a telescope. No detail seen from Earth,
but a few have been seen by spacecraft
including Gaspra (right)
The NEAR mission put a spacecraft in orbit around the asteroid EROS and actually landed on it in 2001.
Most asteroids are in
orbit between
Mars and Jupiter, but a few have orbits that cross the Earth's orbit. These
represent a real danger to the Earth, as we will discuss below.
From their colors we deduce that there are different types, some rocky and
some metallic. Asteroids are either the debris of a planet(s) that broke up,
presumably as a result of a collision, or else they are the bits of a planet
that never finished being formed.
Because asteroids are small they have low gravity and no atmosphere. Also, they can have irregular shapes since weak gravity is not good as pulling planets into round shapes.
Comets are quite different from asteroids. By studying their spectra we deduce they are made mainly of ice rather than rock or metal.
Comets travel in
elliptical or parabolic
orbits, reaching to the outer edge of the Solar System and beyond. Halley is 76
year orbit, reaching to beyond the planet Neptune. It passed close to us in
1986. Next time 2061. It is called Halley's comet because Halley was the first
person to understand that comets were in orbit round the Sun, and the first
person to predict the return of any comet.
Within the last few years we have had comets
Hale-Bopp (left). Neither of these will return
within our lifetime
The nucleus of a comet is small. That of Halley's comet is only 10 km across. One can think of it as an iceberg, or dirty snowball. For most of the life of a comet that is all there is to a comet, and a comet can survive for billions of years "deep frozen in space" In this form they are impossible to see from Earth.
As they
approach the Sun the nucleus heats
up and gases and water vapor are emitted. The nucleus of Halley's comet was
photographed as it was emitting gases by the European Giotto spacecraft.
The tail
is formed which is blown away from the Sun. The tail consists of thin gas and
dust; the Earth can pass through the tail of a comet with no ill effects at all.
The dust grains that make meteors come from comets. On certain nights of the
year you can see excessive numbers of meteors (A meteor shower). These
are the nights that the earth crosses the orbit of a comet, and encountering the
debris that had been blown off the comet earlier. Occasionally you can see
bright fireballs.
Comets burn themselves out and eventually die; alternative fates are to crash into the Sun or into a planets.
Comet Shoemaker-Levy broke up and hit Jupiter in 1994
Animation of impact
Hubble Space Telescope picture of
Jupiter showing scars from impact
There is believed to be a vast cloud of comets well outside the orbit of Pluto. The comet nuclei live there for billions of years. Every so often one of these nuclei will suffer some slightly different gravitational force which will send it on a trip to the inner solar system and make it visible.
Large meteorites hitting the earth can cause disasters.
The
Baringer crater in Arizona
was formed 50,000 years ago by the impact of a meteorite about 50 meters
across hitting the Earth at about 25,000 mph.
There was a mysterious explosion in
Siberia
in about 1908 that might have been due to a small comet exploding in the
atmosphere.
There are are now serious attempts to devise ways of protecting the Earth from major meteorite impact. A major study is the "Spaceguard" survey which proposes a major program of telescope surveys
The most ambitious plan to protect the Earth from "killer" asteroids is UH's "Pan-Starrs" project to build 4 smallish telescopes with very wide-field cameras. This will be placed either on Mauna Kea or on Haleakala.
There
is a relationship between size of object and probability of a
collision.
| Impactor Diameter (meters) | Yield (megatons) | Interval (years) | Consequences |
|---|---|---|---|
| < 50 | < 10 | < 1 | meteors in upper atmosphere most don't reach surface |
| 75 | 10 - 100 | 1000 | irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city |
| 160 | 100 - 1000 | 5000 | irons,stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo) |
| 350 | 1000 - 10,000 | 15,000 | land impacts destroy area size of small state; ocean impact produces mild tsunamis |
| 700 | 10,000 - 100,000 | 63,000 | land impacts destroy area size of moderate state (Virginia) ocean impact makes big tsunamis |
| 1700 | 100,000 - 1,000,000 | 250,000 | land impact raises dust with global implication; destroys area size of large state (California, France) |
.
is the change in
wavelength, and 
is the
velocity of the star.
