Embedded Files
Part A (Worlds & Light)
➤ Week-2: Observational Tools & The Architecture of the Solar System
➤ Topic-5: Structure and Inventory of the Solar System
1. The Inventory
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.
Q1
Does the list changed today?
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
And also...
Non-optical astronomy (radio, infrared)
Spacecraft exploration
Exploration of the Moon
Unmanned probes to other planets
Q2
Why did early astronomers believe the Solar System contained only a few objects, while modern astronomy lists millions?
The Final Inventory
1 Star
8 Planets (4 Terrestrial, 4 Jovian)
5 Dwarf Planet
1801 Ceres (asteroid belt)
1930 Pluto
2004 Haumea
2005 Makemake & Eris
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)
4076 cataloged comets (as of 2026)
516 of them are periodic
Countless Meteoroids
Sun occupies 99.9% of the solar system!
2. Measuring the Planets
2. Measuring the Planets
Properties of Solar Sytem Objects
Distance : Known by Kepler's Law (orbital semimajor axis).
Orbital Period : Obtained by repeated observation of its location on the sky.
Mass : Known by Newton's Law: gravity of moon's orbits around the planets.
Radius : Known by its angular size.
Rotation Period : Obtained by repeated observation of the disk of planets.
Density : Obtained by calculation (if radius and mass known).
Rotation Average
Object Distance Orbital Period Mass Radius # of Period Density
(AU) (Earth Years) (Earth Masses) (Earth Radii) Satellites (days) (g/cm3)
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Mercury 0.39 0.24 0.055 0.38 0 58.7 5.4
Venus 0.72 0.62 0.82 0.95 0 -243 5.2
Earth 1.0 1.0 1.0 1.0 1 1.0 5.5
Moon - - 0.012 0.27 - 27.3 3.3
Mars 1.52 1.88 0.11 0.53 2(?) 1.0 3.9
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Ceres 2.8 4.6 0.00016 0.074 0 0.38 2.2
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Jupiter 5.2 11.9 318 11.2 115 0.41 1.3
Saturn 9.5 29.5 95 9.5 293 0.44 0.7
Uranus 19.2 84 15 4.0 29 -0.72 1.3
Neptune 30.1 164.8 17 3.9 16 0.67 1.6
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Pluto 39.5 247.9 0.00218 0.187 5 -6.4 1.9
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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 and Pluto belong to "dwarf planet" group.
Ceres is the most difficult one because of its very weak gravity.
3. The Layout
3. The Layout
Sun - Neptune: 30 AU (1 AU = 150 million km)
= 1 500 000 ✖ Earth's Radius
= 15 000 ✖ 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!
Q3
Why do most planets orbit the Sun in nearly the same plane and the same direction?
4. The Solar System
4. 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 four jovian planets (Jupiter, Saturn, Uranus, Neptune).
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).
Property Terrestrial Planets Jovian Planets
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Naming Earth like Jupiter like
Planets Mercury, Venus Jupiter, Saturn
Earth, Mars Uranus, Neptune
Distance to the Sun Close Far
Orbital Distribution Closely Spaced Widely Spaced
Composition Rocky Gaseous
Mass Small Large
Density High Low
Radius Small Large
Surface Solid Not Definitive
Rotation Speed Slower Faster
Magnetic Field None-to-Weak Strong
Moons None-to-Few Many
Ring System None Many
Notes on Terrestrials:
All have atmospheres and all are completely different: Mercury is near vacuum; Venus is dense inferno; Earth has Oxygen in the atmosphere and liquid water on the surface.
Earth and Mars have similar spin rates (i.e 24 hours). Venus rotates in clockwise and spin rates of Mercury and Venus are in months.
Earth and Mercury have measurable magnetic fields, Venus and Mars have none
Q4
Why are the inner planets rocky while the outer planets are gas giants?
Relatively Large Bodies
Asteroid Belt objects (in between Mars and Jupiter)
Kuiper Belt objects (beyond Neptune)
Asteroid Eros. It is 35 km long.
Pluto. Largest Kuiper Belt object. Artist impression of Pluto-Charon system.
Small Bodies
Comets (at the end the solar system)
Meteoroids (everywhere in the solar system)
Dust (created by continuous collision of large bodies)
5. Asteroids
5. Asteroids
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. 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
The main asteroid belt, along with the orbits of Earth, Mars, and Jupiter. Note the Trojan asteroids at two locations in Jupiter’s orbit.
Top: Orbits of planets (blue lines) around the asteroid belt (white dots) and Jupiter trojans (green and red dots).
Right, upper panel: Apollo orbits (green shade).
Right, mid panel: Amor orbits (green shade) with Mars trojans (brown shades).
M: Mars, V: Venus, E: Earth, H: Mercury.
Right, bottom panel: Some asteroids, called Trojan asteroids, orbit at the L4 and L5 Lagrangian points of Jupiter’s orbit
Q5
Why are asteroids concentrated mainly between Mars and Jupiter?
(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.
(a) Kirkwood Gap. The distribution of asteroid semi-major axes shows some prominent gaps caused by resonances with Jupiter’s orbital motion (Mars is at 1.5 AU and Jupiter is at 5.2 AU). 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.
5. Comets
5. Comets
Comets move in highly eccentric paths that carry them far beyond the known planets.
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 yearsSometimes a comet’s nucleus can disintegrate violently, as comet LINEAR did.
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.
(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
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.
Q6
Why do comet tails always point away from the Sun, regardless of the comet’s direction of motion?
6. Meteors
6. 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).
7. Outer Parts
7. Outer Parts
(1) The Kuiper belt:
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.
The belt is extended from Neptune's orbit (30 AU) to approximately 50 AU.
See the view of the solar system "beyond the Neptune" (including exploration history):
(2) 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.
See the cross-sectional view of the solar system (distances are in AU):
Content Location Orbits
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Asteroids mostly rock Asteroid Belt direction & plane are the same as planets
Kuiper Belt Comets mostly ice from Neptune to 100 AU direction & plane are the same as planets
Oort Cloud Comets mostly ice upto 1/4 of way to nearby stars all directions & all inclinations
8. Formation of the Solar System
8. Formation of the Solar System
Summary of the system
Each planet is relatively isolated in space
The orbits of the planets are nearly circular
They all lie in nearly the same plane
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.
It is highly differentiated (inner, outer)
Asteroids are very old. Their properties differs very much from either inner or outer planets or moons.
Kuiper belt is a collection of asteroid-sized icy bodies orbiting beyond Neptune.
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
Nebular Theory
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 0.1 micron
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
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