The solar system may be defined as consisting of those objects that are governed by the Sun's gravitational field. It extends out to about two light years (a light year is the distance light travels in one year and therefore equals about 9.5 x 1015 m or 63,000 Astronomical Units where 1 AU is the average Earth-Sun separation; the closest star, Proxima Centauri, is just over 4 light years away).
However, the most distant known objects, the long-period comets, appear to originate from between 1,000 and 30,000 AU away although we never actually detect them until they enter the inner regions of the solar system. Pluto's orbit extends almost 50 AU from the Sun at its most distant.
The solar system is composed of a star, 9 planets, various satellites of these planets, small bodies (comets, asteroids & meteorites) and an interplanetary medium consisting of dust, gas and high-energy particles. Note that about 99.85% of the mass of the solar system is actually accounted for by the Sun.
| Planet | Mass | Diameter | Density | Rotation | Distance | Revolution | Eccentricity | Inclination | Axis tilt |
| (x MEarth) | (km) | (g/cm3) | (A.U.) | (deg) | (deg) | ||||
| Mercury | 0.0553 | 4880 | 5.43 | 58.81d | 0.387 | 87.97d | 0.2056 | 7.0 | 0.1 |
| Venus | 0.815 | 12,104 | 5.20 | 243.69d | 0.723 | 224.70d | 0.0068 | 3.4 | 177.3 |
| Earth | 1.000 | 12,742 | 5.52 | 23.9345h | 1.000 | 365.26d | 0.0167 | 0.00 | 23.45 |
| Mars | 0.107 | 6780 | 3.93 | 24.623h | 1.524 | 686.98d | 0.0934 | 1.85 | 25.19 |
| Jupiter | 317.83 | 139,822 | 1.33 | 9.925h | 5.203 | 11.86y | 0.04845 | 1.305 | 3.12 |
| Saturn | 95.162 | 116,464 | 0.687 | 10.50h | 9.539 | 29.46y | 0.05565 | 2.489 | 26.73 |
| Uranus | 14.536 | 50,724 | 1.32 | 17.24h | 19.182 | 84.01y | 0.0472 | 0.773 | 97.86 |
| Neptune | 17.147 | 49,248 | 1.64 | 16.11h | 30.06 | 164.79y | 0.00858 | 1.773 | 29.56 |
| Pluto | 0.0021 | 2274 | 2.05 | 6.405d | 39.53 | 247.68y | 0.2482 | 17.15 | 122.46 |
|
|
| The relative sizes of the Planets |
|
| Orbital planes |
The major bodies travel around the Sun in almost circular orbits in virtually the same plane, see figure above. Only the orbits of Mercury and Pluto deviate from the ecliptic by more than a few degrees (7° and 17° respectively).
These planets also have the most elliptical orbits (although still far less elliptical than comets for example). Note Pluto was closer to the Sun than Neptune between 1979 and 1999. All the planets also spin on their own axes, usually prograde (i.e. in the same direction as their orbital rotation) other than Venus and Pluto. These spin axes are usually less than 30° away from perpendicular to their orbital planes (this angle is called the obliquity. Note that Pluto and Uranus have obliquities of over 90°!). To get a feel for the motions of the planets, have a look at the Electric Orrery.
Most minor bodies such as asteroids, short-period comets, dust grains generally follow the same rules. Note that long-period comets have randomly oriented orbits.
In the main, therefore, the solar system forms an essentially empty disc with the massive, bright Sun at its centre.
As the total mass of the planets and satellites is small in comparison to that of the Sun we can, to first approximation, reduce to a two-body problem. This is referred to as the Keplerian approximation - very useful because the N-body problem is not exactly soluble and must be integrated numerically.
The paths of the planets are determined by the gravitational influence of the Sun, proportional to the inverse square of the separation and directed along the line connecting the planet to the Sun. This feature leads to a path which is a conic section (ellipse, hyperbola or parabola) with the Sun at one focus. Kepler's three laws of planetary motion are:
|
where r is the heliocentric distance, a is the semi-major axis of the ellipse, e is its eccentricity (the ratio of the distance between the two foci to the length of the major axis 2a ) and θ is the angle of the planet measured relative to the point on the orbit closest to the Sun (at perihelion, θ = 0 ). The most distant point is the aphelion at θ = π.
|
| Two equal areas swept out in equal time. |
From Newtonian mechanics we have:
|
where a is the semi-major axis of the orbital ellipse,
P is the period, M1 and
M2 are the masses of the two objects (eg the Sun
and a planet) and G=6.67 x 10-11 N m2
kg-2. Since the mass of the Sun is much larger than the
mass of any planet, the sum MSun +
Mplanet ~ MSun. Therefore:
|
If P is measured in years and
a in astronomical units then this can be simplified to
|
| |
| A visible light image of the solar disc (a few sunspots are visible). | |
This is our nearest star - if the Sun and the Earth were 1 metre apart then Proxima Centauri would be over 250 km away. It is important because it is the source of all our energy, it affects us on a daily basis and it is a nearby example of one of the primary constituents of the universe.
As we shall see, the Sun is a fairly typical star in terms of
size, mass, temperature and chemical composition.
| |
| A simplified diagram showing some of the features of the interior of the Sun | |
A very hot central core (15 million degrees or so) produces the Sun's energy by the fusion of hydrogen into helium. The overlying weight of the Sun's mass must be balanced at the core by the gas pressure otherwise the Sun would collapse. An ideal adiabatic gas has pressure proportional to temperature which means the temperature must be very high. At these pressures, densities and temperatures, fusion occurs via the proton-proton reaction chain. Here, 4 protons (hydrogen nuclei) are fused into one alpha particle (helium nucleus consisting of two protons and two neutrons). One alpha particle is 0.7% less massive than 4 protons and this mass is the source of energy given by E = mc2. Each second the Sun converts about 700 million tons of hydrogen into helium with 4 or 5 million tons of this being released as energy. In the 4 billion years or so since the Sun formed it has consumed about 50% of the hydrogen in its core.
The energy released via nuclear fusion is carried outwards by radiation (photons undergoing random walks of repeated emission and absorption) through the inner 70% of the Sun's radius. At this point the temperature has dropped to about 1.5 million degrees and the material is very opaque to radiation. The Sun then becomes unstable and energy is carried outwards by convection in eddies where hot material rises upwards and deposits energy at larger radii. At the photosphere near the surface, energy escapes as photons which we detect on Earth.
The fairly new science of helioseismology is now telling us a great deal about the internal structure of the Sun. This involves analysing the passage of sound waves through the solar interior which produce motions at the surface with doppler shifts of 400 and 500 ms-1
| |
| Granulation on the Solar surface | |
The visible surface of the Sun, the photosphere, shows a great amount of detail. The whole surface appears as a pattern of bright ~ 1 arcsec (1000 km) patches termed granulation. Each granule marks a convective cell rising at up to 500 ms-1 from the Sun's interior before spreading out and falling back in the darker cooler inter-granule regions.
The interaction of convective motion with rotation probably amplifies deep, weak magnetic fields that then rise to the surface and create magnetically active regions and dark Sunspots. (See image of visible disc above) Each sunspot lasts a few weeks and their overall numbers rise and fall in a cycle with a period of about 11 years. You can select sunspot data for about 150 years or observations from this page.
Historical records indicate that this cycle was interrupted during the Maunder Minimum from 1640 to 1710 which appeared to coincide with colder than average temperatures on the Earth. Sunspots appear dark because they are cooler than their surroundings (although if one were placed alone in the night sky it would appear as a reddish star 10 times brighter than the full moon).
| |
|
One of the first "butterfly diagrams".
Time goes along the X-axis, solar latitude up the Y-axis. | |
Sunspots also exhibit a butterfly diagram showing that at the beginning of a cycle the first spots form at high latitudes, near the end they form near the equator.
In 1908, Hale discovered Zeeman splitting of spectral lines in
the radiation from sunspots indicating they possess strong magnetic
fields. In addition, they form in pairs of opposite polarity.
|
| Near-simultaneous visible (left) and magnetogram (right) images of the Sun. |
The magnetogram shown to the right of the figure above, is a map of magnetic field strength made by measuring the splitting of spectral lines by magnetic fields (the Zeeman effect). Here colours (dark blue and yellow) are used to indicate regions of opposite magnetic polarity. Note that the polarities of east-west pairs in the southern hemisphere are reversed from those in the north. At the left is a visible-light photograph of the Sun taken at the same time, so that the correspondence between the surface magnetic field and the visible sunspots can easily be seen.
The sense of this polarity swaps every 11 years, making an
underlying 22 year magnetic cycle. Sunspot pairs represent loops of
magnetic field breaking through the surface.
|
| Images of 1999 Solar Eclipse (by Pavel and Roman Cagas) |
The very outer layers of the Sun can be seen visibly during
eclipse.
Here you can see the red chromosphere and beyond the white
corona extending millions of miles into space.
The chromosphere emits
light mainly in the hydrogen-alpha line (656 nm) and therefore appears
red. Its temperature rises from about 4,500 K to 8,500 K at the
top. Above this the gas cannot cool efficiently, the hydrogen is
ionized beyond the Balmer series, and the temperature rises rapidly
through the transition zone into the million degree corona. The
ionized gas is bonded to the magnetic field lines. This is thought to
be the source of coronal heating via magnetohydrodynamic waves and is
also the source of energy for the massive prominences and solar flares
seen every so often erupting from the surface of the Sun into space.
The solar wind consists of a hot (about 100,000 K) plasma - an electrically neutral mixture of ions (mainly protons) and electrons - flowing radially outwards from the Sun at typically 450 km s-1. It extends throughout the solar system and probably beyond.
| |
| Hα image of the Sun showing a dramatic prominence | |
The solar wind arises in so-called coronal holes (seen as dark, cooler areas in X-ray images of the solar corona) where rising loops of magnetic field break open and gas is accelerated outwards. These are particularly strong near the Sun's poles. A few days after a coronal hole rotates past the centre of the solar disc, earth-orbiting spacecraft detect a stream of high-velocity particles with a density of 10 to 100 cm-3. The typical density is about 5. The solar wind interacts with the magnetic fields of planets to produce, in the case of the Earth, interesting phenomena like the van Allen belts and aurorae.
The interplanetary medium also contains dust, cosmic rays
(high-energy charged particles of extraterrestrial origin) and neutral
gas of interstellar origin.
The planets may be divided into two classes (with the exception
of Pluto): the terrestrial planets, like the Earth, including Mercury,
Venus and Mars; and the giant planets, Jupiter, Saturn, Uranus &
Neptune. This classification is based on their structure, surfaces and
atmospheres. The terrestrials are generally small, high density and
have rocky surfaces.
|
| The Terrestrial planets (not to scale!) |
Three fundamental processes determine the surface of terrestrial planets - impact cratering, volcanism and tectonics. All terrestrial planets are cratered, for the first 600 million years or so of their history this was an incessant process. On the Earth the effects have been wiped out to a large extent by the influence of the atmosphere, oceans and geological activity. Volcanism appears to be a consequence of radioactive decay within the interiors. Basaltic plains represent the upwelling of lava over the surface (e.g. the dark lunar maria). Tectonic activity is the movement of areas of crust seen on the terrestrial planets.
The atmospheres have been formed and modified by various
processes: accretion of planetisimals early in the history; outgassing
from volcanoes; exospheric escape; chemical reactions; condensation;
weathering etc. These processes vary in importance from planet to
planet so atmospheres can be markedly different although the physical
processes are the same.
|
The gas giants (sometimes called Jovian planets) are much larger than the terrestrials, have no solid surface and have much lower densities (Saturn would float in water!).
The two larger planets, Jupiter and Saturn, have small, solid cores surrounded by a layer of liquid metallic hydrogen. Uranus and Neptune do not have liquid metallic hydrogen zones because the pressure is not sufficient, but as on Jupiter and Saturn, differentiation has caused virtually all their heavier elements to sink into the core. The cloud bands seen on Jupiter are thought to be layers of varying molecular composition lying at different heights within the atmospheres.
We now know that all four gas giants possess ring systems of
varying extent. They are composed of small bodies ranging in size from
dust (size ~ microns) to rocks (size ~ m).
They lie within the Roche limit
given by
|
where RP and ρP are the radius and density of the planet and ρO is the density of an orbiting object.
Within this limit a body cannot hold itself together under the
stress of differential gravitational forces (tidal
forces). Conversely, bodies could not form by accreting from component
parts within this limit.
A disc is formed because interactions between
the particles cause the distribution of orbits to flatten and slowly
spread inwards and outwards. However, the spreading is halted by
"shepherding" moons. The pictures below show the narrow F ring of
Saturn and the two tiny moons (Pandora and Prometheus) that "shepherd"
it.
|
| These figures illustrate (top) how a pair of small moons can keep particles trapped in orbit between them. The inner shepherd satellite overtakes the particles, accelerating them if they fall inward and thus moving them out. Similarly, the outer shepherd slows particles that move too far out, so that they fall back in. Below is an observation of two "shepherds". |
|
Asteroids, sometimes known as minor planets, are rocky objects up to several hundred km in diameter. They mainly orbit in the asteroid belt between Mars and Jupiter. It is thought that this represents material that never got the chance to accrete together to form a planet due to the gravitational influence of Jupiter. There are however several "families" including the Apollos, some of which have Earth-crossing orbits. Project SPACEGUARD aims to detect as many such asteroids as possible. In total, there are around 16000 catalogued asteroids, but their total mass is less than that of the Moon. There are thought to be around 1 million 1km-sized bodies, however, these are very difficult to spot.
Comets are thought to be 'dirty snowballs' - gravel or
boulders embedded in frozen gases. We see periodic comets like Halley
when they come near the Sun on their highly elliptical orbits.
|
| The Comet Mrkos over four days in August 1957 |
At this point they are heated and a dust tail (curving to the right in the picture above of Comet Mrkos from 1957) and ionized gas tail (pointing more vertically) is pushed out away from the Sun by the influence of the solar wind and radiation pressure. As well as the nucleus and plasma and dust tails, comets are observed to possess a Coma comprising neutral molecules sublimed off the nucleus, and a neutral hydrogen cloud over a million km in diameter in some cases. The dust tail may be up to 10 million km long, and the plasma tail ten times this length.
Over 1000 comets are now catalogued, about 1/5 of which are periodic. As with asteroids, some comets have Earth-crossing orbits.
The long-period comets with periods of millions of years and
randomly-oriented orbits are thought to originate in the
Oort cloud of comets, a spherical shell lying up to 100,000 AU
from the Sun. The short-period comets have orbits close to the
ecliptic and are thought to arise in the Kuiper belt, a disc-like
distribution some 30 to 50 AU in radius. The Kuiper belt is thought to
contain about an Earth's mass of comets, whereas the Oort cloud may
contain about 1011 objects totalling over 100 Earth
masses. Orbits of comets within these clouds are disturbed by objects
such as passing stars or other comets and they then plummet in towards
the inner solar system.
|
| Diagrams showing the relatives sizes of the Kuiper Belt and Oort cloud. |
Meteorites are pieces of solid material which enter the Earth's atmosphere and reach the ground intact. When they burn up in the atmosphere they are called meteors, fireballs, bolides or shooting stars. A famous example of one that exploded over Siberia is the Tunguska event of 1908. The most famous and reliable meteor shower is the Perseids (mid-August). The most newsworthy meteorites recently have been the Mars Meteorites with their contentious "microfossils".
The Solar system forms as a by-product of the formation of the Sun.
In essence, the current theory states:
There is some controversy surrounding this simple theory since many of the extra-solar planets found so far are large (ie Jupiter-sized) planets very close to their parent stars. However, this could be a "selection effect" since these are the planets that are easiest to detect. The problem of trying to understand the formation of planetary systems when we only have one decent example (our own!) is a major one. However, the increasing sophisitication of extra-solar planets searches is likely to make the job much easier in the coming decades.