05 Solar System

1. The inventory

Early Astronomers

  • Moon

  • Planets: Mercury, Venus, Mars, Jupiter, Saturn

  • Comets

  • Meteors - shooting stars

They didn't have any idea about the big picture...

Do you have the big picture in this century?

Galileo Galilei (17 cc)

  • He used a telescope (a very simple one)

  • Noticed phases of Venus

  • Discovered moons of Jupiter

  • Found something unusual around Saturn

End of 19 cc

  • Saturn's ring system (1659)

  • Uranus (1781)

  • Neptune (1846)

  • Many planetary moons

  • The first asteroid: Ceres (1801)

20 cc

  • Pluto

  • Ring systems around Jupiter, Uranus and Neptune

  • Dozens of moons

  • Thousands of asteroids


  • Non-optical astronomy (radio, infrared)

  • Spacecraft exploration

  • Exploration of the Moon

  • Unmanned probes to other planets

The Final Inventory

  • 1 Star

  • 8 Planets (4 Terrestrial, 4 Jovian)

  • 5 Dwarf Planet

    • Ceres - 1801 (asteroid belt)

    • Pluto - 1930

    • Haumea - 2004

    • Makemake - 2005

    • Eris - 2005

  • 194 Moons (and counting)

  • ~700 000 Asteroids in total (and counting)

    • 10 Large Asteroids

      • Ceres, Pallas, Juna, Vesta, Astraea, Hebe, Iris, Flora, Metis, Hygenia

    • 239 Asteroids (> 100 km in diameter)

  • 1 trillion Comets (estimated)

    • 6339 known Comets (as of 2018)

  • Countless Meteoroids

2. Measuring the Planets


  • Known by Kepler's Law

Orbital Period

  • Obtained by observation: repeated observation of its location on the sky


  • Known by its angular size


  • Known by Newton's Law: gravity of moon's orbits around the planets

Rotation Period

  • Obtained by observation: repeated observation of the disk of planets


  • Obtained by calculation: if radius and mass known

  • Mercury and Venus are difficult to determine. They produce small but measurable effects on each other's orbits as well as that of Earth i.e wobbles.

  • Ceres is the most difficult one because of its very weak gravity.

  • Now space probes are used to measure these parameters

  • Sun occupies 99.9% of the solar system!

3. The layout

  • Sun - Neptune: 30 AU (1 AU = 150 million km)

    • 1 500 000 x Earth's Radius

    • 15 000 x Earth - Moon distance

  • Even with these huge distances all planets lie close to the Sun

  • All the planets lie on the same plane (almost)

    • exception: Mercury: 7 degree (Pluto: 17 degree)

  • All paths of planets are ellipses (Sun is at one foci)

    • Most of them have low eccentricity (how much different than a circle)

    • exception: Mercury (and Pluto)

  • All the planets orbit the Sun in Counter Clockwise (CCW)

Titius - Bode Law (1766)

  • Orbits are not evenly spaced

  • They get farther and farther apart as distance increases

  • However, there exist some kind of regularity

  • This is experimental; No simple explanation exists!

4. Grouping

Terrestial Planets

  • Naming: earth land; Earth like

  • Inner: Mercury, Venus, Earth, Mars

  • Small

  • Dense

  • Rocky

Jovian Planets

  • Naming: jove; god Jupiter; Jupiter like

  • Outer: Jupiter Saturn Uranus, Neptune

  • Large

  • Low Density

  • Gaseous

Notes in Terrestrial Planets

  • All have atmospheres

  • All atmospheres are completely different

    • Mercury: near vacuum

    • Venus: dense inferno

    • Earth: Oxygen in the atmosphere, liquid water on the surface

  • Earth and Mars have similar spin rates (i.e 24 hours)

    • Spin rates of Mercury and Venus are in months

  • Venus rotates in Clockwise

  • Earth and Mars have moons

  • Earth and Mercury have measurable magnetic fields

    • Venus and Mars have none

Kuiper Belt

  • View of the solar system "beyond the Neptune" (including exploration history)

  • The belt is extended from Neptune's orbit (30 AU) to approximately 50 AU.

Oort Cloud

  • Cross-sectional view of the solar system (distances are in AU)

5. Interplanetary Matter

Relatively Large Bodies

  • Asteroids (in between Mars and Jupiter)

  • Kuiper Belt objects (beyond Neptune)

Small Bodies

  • Comets (at the end the solar system)

  • Meteoroids (everywhere in the solar system)

  • Dust: Created by continuous collision of large bodies

6. Main Structure of the Solar System

Terrestrial Planets

  • The four terrestrial planets (Mercury, Venus, Earth, Mars).

  • They are relatively small, have solid, rocky surfaces, and have an abundance of metals deep in their interiors.

  • They have few moons, if any, and none have rings.

  • We often count our Moon as a fifth terrestrial world, because it shares these general characteristics, although it's not technically a planet.

Jovian Planets

  • The Jovian planets have little in common with the terrestrial planets.

  • They are much larger in size and lower in average density than the terrestrial planets, and each has rings and numerous moons.

  • Their composition is very different from that of the terrestrial worlds.

  • The Jovian planets are made mostly of hydrogen, helium, and hydrogen compounds–compounds containing hydrogen, such as water (H2O), ammonia (NH3), and methane (CH4).

Asteroids and Comets

  • Asteroids are small, rocky bodies that orbit the Sun much like planets, but they are much smaller than planets.

  • Even the largest of the asteroids have radii of only a few hundred kilometers.

  • Most asteroids are found within the relatively wide gap between the orbits of Mars and Jupiter that marks the asteroid belt.

  • They orbit the Sun in the same direction and nearly in the same plane as the planets.

  • More than 10,000 asteroids have been identified and cataloged, but these are probably only the largest among a much greater number of small asteroids.

7. Asteroids

  • The main asteroid belt, along with the orbits of Earth, Mars, and Jupiter.

  • Note the Trojan asteroids at two locations in Jupiter’s orbit.

  • Apollo orbits: Earth-crossing.

  • Amor orbits: Mars-crossing.

  • The largest:

    • Ceres - 940 km

    • Pallas - 580 km

    • Vesta - 540 km (showing volcanism)

Asteroid types:

  • C-type: Carbonaceous, dark

  • S-type: Silicate (rocky)

  • M-type: Metallic; iron and nickel

(a) The S-type asteroid Gaspra, as seen from a distance of 1600 km by the space probe Galileo on its way to Jupiter.

(b) The S-type asteroid Ida, photographed by Galileo from a distance of 3400 km. (Ida’s moon, Dactyl, is visible at right.)

Right Panel:

A mosaic of detailed images of the asteroid Eros, as seen by the NEAR spacecraft (which actually landed on this asteroid). Craters of all sizes, ranging from 50 m (the resolution of the image) to 5 km, pit the surface. The inset shows a close-up image of a “young” section of the surface, where loose material from recent impacts has apparently filled in and erased all trace of older craters.

Kirkwood Gap

Some asteroids, called Trojan asteroids, orbit at the L4 and L5 Lagrangian points of Jupiter’s orbit

(a) The distribution of asteroid semi-major axes shows some prominent gaps caused by resonances with Jupiter’s orbital motion. Note, for example, the prominent gap at 3.3 AU, which corresponds to the 2:1 resonance—the orbital period is 5.9 years, exactly half that of Jupiter.

(b) An asteroid in a 2:1 resonance with Jupiter receives a strong gravitational tug from the planet each time they are closest together (as in panels 1 and 3). Because the asteroid’s period is precisely half that of Jupiter, the tugs come at exactly the same point in every other orbit, and their effects reinforce each other.

8. Comets

Comets move in highly eccentric paths that carry them far beyond the known planets.

Halley’s comet has a smaller orbital path and a shorter period than most comets, but its orbital orientation is not typical of a short-period comet. Sometime in the past, this comet must have encountered a Jovian planet (probably Jupiter itself), which threw it into a tighter orbit that extends not to the Oort cloud, but merely a little beyond Neptune. Edmund Halley applied Newton’s law of gravity to predict this comet’s return

  • Typical cometary mass: 1012 to 1016 kg

  • Each trip close to the Sun removes some material; Halley’s comet, for example, is expected to last about another 40,000 years

  • Sometimes a comet’s nucleus can disintegrate violently, as comet LINEAR did.

(a) Diagram of a typical comet, showing the nucleus, coma, hydrogen envelope, and tail. The tail is not a sudden streak in time across the sky, as in the case of meteors or fireworks. Instead, it travels along with the rest of the comet (as long as the comet is sufficiently close to the Sun for the tail to exist). Note how the invisible hydrogen envelope is usually larger than the visible extent of the comet; it is often even much larger than drawn here.

(b) Halley’s Comet in 1986, about one month before it rounded the Sun at perihelion.

(a) A comet with a primarily ion tail. Called comet Giacobini–Zinner and seen here in 1959, its coma measured 70,000 km across; its tail was well over 500,000 km long.

(b) Photograph of a comet having both an ion tail (dark blue) and a dust tail (white blue), both marked in the inset, showing the gentle curvature and inherent fuzziness of the dust. This is comet Hale–Bopp in 1997

(a) Halley’s comet as it appeared in 1910. Top, on May 10, with a 30° tail, bottom, on May 12, with a 40° tail. (b) Halley on its return and photographed with higher resolution on March 14, 1986.

As it approaches the Sun, a comet develops an ion tail, which is always directed away from the Sun. Closer in, a curved dust tail, also directed generally away from the Sun, may appear. Notice that the ion tail always points directly away from the Sun on both the inbound and the outgoing portions of the orbit. The dust tail has a marked curvature and tends to lag behind the ion tail.

Origins of Comets:

They come from two distinct regions of space.

  1. The first region, called the Kuiper belt extends roughly from the orbit of Neptune to about three times Neptune's distance from the Sun (that is, from about 30 to 100 AU).

    • The Kuiper belt comets orbit the Sun in the same direction as the planets, and their orbits generally lie close to the plane of planetary orbits.

    • In fact, some of the known Kuiper belt comets are nearly as large as Pluto, leading scientists to suspect that Pluto may simply be an unusually large member of this group.

  2. The second and much larger region, called the Oort cloud, may extend more than one-fourth of the way to the nearest stars.

    • Comets in this region have orbits around the Sun that are inclined at all angles to the plane of planetary orbits and go in every possible direction around the Sun.

    • Thus, the Oort cloud would look roughly spherical in shape if we could see it.

    • It is so vast, however, that even with a trillion comets each comet is typically separated from the next by more than a billion kilometers.

Summary of the group

9. Meteors

A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heating the air to incandescence.

(a) A small meteor photographed against stars and the Northern Lights provide a stunning background for a bright meteor trail.

(b) These meteors (one with a red smoke trail) streaked across the sky during the height of the Leonid meteor storm of November 2001

A meteoroid swarm associated with a given comet intersects Earth’s orbit at specific locations, giving rise to meteor showers at certain fixed times of the year.

  • A portion of the comet breaks up as it rounds the Sun, at the point marked 1.

  • Fragments continue along the comet’s orbit, gradually spreading out (points 2 and 3).

  • The rate at which the debris disperses around the orbit is much slower than depicted here. It actually takes many orbits for the material to disperse as shown, but eventually the fragments extend all around the orbit, more or less uniformly.

  • If the orbit happens to intersect Earth’s, the result is a meteor shower each time Earth passes through the intersection (point 4).

Larger meteoroids are usually loners from the asteroid belt and have produced most of the visible craters in the Solar System.

The Earth has about 100 craters more than 0.1 km in diameter; erosion has made most of them hard to discern. This one which is one of the largest, is in Canada.

10. Summary of the Solar System

11. Formation of Solar System

Summary of the system

  1. Each planet is relatively isolated in space

  2. The orbits of the planets are nearly circular

  3. They all lie in nearly the same plane

  4. They circle the sun in CCW direction which is the same as the sun itself.

    • This is true for a planet and moon of a planet.

  5. It is highly differentiated (inner, outer)

  6. Asteroids are very old. Their properties differs very much from either inner or outer planets or moons.

  7. Kuiper belt is a collection of asteroid-sized icy bodies orbiting beyond Neptune.

  8. Oort cloud comets are primitive icy fragments. They don't orbit on the ecliptic plane and their reside at large distances from the sun.

Nebular Contraction

  • The idea dates back to Descartes (17 cc)

  • A large cloud of interstellar gas began to collapse under the influence of its own gravity

    • Getting smaller ➤ Getting denser ➤ Getting hotter ➤ STAR

    • Outer cooler parts: Planets, Moons i.e. debris

  • This swirling mass is called the solar nebula

  • Laplace improved this idea

  • Solar nebula must spin faster as it contracts

    • decrease in size must be balanced with increase in rotation speed

    • increase in rotation speed ➤ changes nebula's shape

      • cloud ➤ bulge ➤ disk

  • As the contraction continues

    • it leaves behind a series of concentric rings each of which orbiting a central proto-sun

    • each ring then clumped into a proto-planet

  • Therefore the idea that planets formed from a disk is called nebular theory

Condensation Theory

  • Today, a disk of warm gas would not form clumps of matter that would subsequently evolve into planets. In fact, just the opposite is predicted.

    • Condensation Theory = old Nebular Theory + interstellar chemistry

  • The key is interstellar dust in the solar nebula.

    • Interstellar space is littered with microscopic dust grains.

    • They are chunks of icy and rocky matter having typical sizes of about 1E-5 m

  • Dust helps to cool warm matter by efficiently radiating its heat away in Infrared

    • Cooling of matter ➤ reduces the pressure

    • Reduced pressure ➤ allows the gas to collapse more easily by gravity

    • Furthermore ➤ they speed up the process of collecting enough atoms to form a planet

    • Condensation Nuclei: microscopic platforms to which other atoms can attach, forming larger and larger balls of matter ➤ Planets