1 chapter 12 the origin of the solar system -architecture of the solar system - planets -asteroids...

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Chapter 12The Origin of the Solar System

-Architecture of the solar system- Planets-Asteroids-Comets

-Meteors and meteoroids

ASTA01 @ UTSC – Lecture 12

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A Survey of the Planets

• To explore consequences of the solar nebula theory, astronomers search the present solar system for evidence of its past.• You should begin with the most general view

of the solar system. • It is almost entirely empty space.

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• Imagine that you reduce the solar system until Earth is the size of a grain of table salt – about 0.3 mm in diameter. • The Sun is the size of a small plum 4 m from

Earth.• Jupiter is an apple seed 20 m from the Sun. • Neptune, at the edge of the solar system, is a

large grain of sand located 120 m from the central plum.

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• You can see from the rescaled models of the solar system that planets are tiny specks of matter scattered around the Sun, the last significant remains of the solar nebula.

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Revolution and Rotation

• The planets revolve around the Sun in orbits that lie close to a common plane.• The orbit around the Sun of Mercury, the

planet closest to the Sun, is tipped 7.0°to Earth’s orbit.• The rest of the planets’ orbital planes are inclined

by no more than 3.4°.• The solar system is basically ‘flat’ and

disk-shaped.

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• The rotation of the Sun and planets on their axes also seems related to the same overall direction of motion. • The Sun rotates with its equator inclined 7.2° to ecliptic• Most planets’ equators are tipped less than 30°.

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• However, the rotations of Venus and Uranus are peculiar. • Compared with the other planets, Venus rotates backward.• Uranus rotates on its sides, with the equator almost

perpendicular to its orbit.

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• The preferred direction of motion in the solar system (counterclockwise as seen from the north) is related to the rotation of a disk of material that became the planets.• All the planets revolve around the Sun in that

direction.

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• Furthermore, nearly all the moons in the solar system, including Earth’s moon, orbit around their planets counterclockwise.• With only a few exceptions, most of which are

understood, revolution and rotation in the solar system follow a common theme.

Triton

Neptune & Triton

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Two Kinds of Planets

1. Four small Earth-like worlds, called terrestrial planets

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2. Four giant Jupiter-like worlds.

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• The difference is so dramatic that you are led to say,

• “Aha, this must

mean something!”

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• There are three important points to note about these categories.

Terrestrial Jovian

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• One, they are distinguished by their location.• The four inner planets are

quite different from the outer four.

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• Two, almost every solid surface in the solar system is covered with craters:

• Terrestrial planets and moons have solid surfaces, while giant planets don’t have a visible surface (in fact, they don’t have surfaces)

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• Three, the planets are distinguished by properties such as composition, rings, and moons.

Rocky Gaseous

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• The division of the planets into two families is a clue to how our solar system formed.• The present properties of individual planets,

however, don’t reveal everything you need to know about their origins. • The planets have all evolved since they formed. • For further clues, you can look at smaller objects

that have remained largely unchanged since the birth of the solar system.

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Asteroids

• The first asteroid (gr.: asteroidos = star-like) was Ceres, ~500 km diameter body discovered in 1801

• It is covered by ice and clay, and may have

liquid water between the rocky core and icy

crust

Ceres filled an empty slot for m=3 in the so-called Titius-Bode law of planetary distances, but now is not considered a natural law any more.

• This rule can be expressed as • a = (4 + 3 2⋅ m) AU /10

for m = -∞, 0, 1, 2, 3, 4, 5, 6..

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Titius-Bode rule: 1766

• Astronomy professor Johann Daniel Dietz (Titius) from Wittenberg (Germany) inserted his observation on planetary distances into a German translation of Charles Bonnet's book Contemplation de la Nature

• Johann E. Bode reformulated and popularized Titius rule in his textbook on astronomy in 1772

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Titius-Bode rule and its role in planet discoveries

• T-B rule was considered interesting but not important until Uranus was accidentaly discovered in 1781, and it fit the well!

• Predictions of new

planets were made

Soon Ceres was found.

W. Herschel (1781) -

G. Piazzi (1801) -

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Titius-Bode rule and its role in planet discoveries

• T-B rule was considered interesting but not important until Uranus was discovered in 1781, and it fit the law rather precisely!

• Predictions of new

planets were made

W. Herschel (1781) -

G. Piazzi (1801) -

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Asteroids

• Why Titius-Bode law eventually fell out of favor and is now called a “Titius-Bode rule”: Neptune and Pluto fail to obey it.

• However, the distribution of • Satellite systems and extrasolar planets resembles

the power-law form of Titius rule to some extent : a ~ cn, where c=const different for different systems

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T-B rule in exoplanetary (i.e., extrasolar) systems?• However, the distribution of many satellite systems and

extrasolar planets resembles somewhat a power-law form of Titius rule to some extent : a ~ cn, where c=const different for different systems.

• Why? We don’t fully know. • There are many exceptions.

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Space Debris: Planet Building Blocks

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Asteroids - minor planets

Small rocky worlds.• Most of them orbit the Sun in a belt between the orbits of

Mars and Jupiter. • Roughly 20 000 asteroids asteroid belt (white)

have been catalogued• Most in asteroid belt• They are smaller than

the Moon

Vesta 525 km diameter

Ceres 950 km

Moon 3470 km

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Asteroids do have moons: Ida and its moon Dactyl

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Asteroids belong to orbital families: Ida is in the Koronis family, a group of 300 minor bodies

all of which have a = 2.86…2.89 AU , e=0.01…0.09, i=1o…3o

Asteroids in a family come from a disruption of a

common ancestor asteroids

This happened when two asteroids collided in the

asteroid belt between Mars and Jupiter >2 Gyr ago

Asteroids are as old as planets, actually planets

formed from asteroid-like primitive bodies called

planetesimals

Lacrimosa (41 km)

Koronis

Ida

Urda (40 km)

Elvira

Nassovia

Florentina

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Vesta • Spacecraft Dawn has orbited in Jul 2011- Sep 2012

asteroid Vesta; a=2.36 AU, e=0.088, P=3.63 yr, diameter 525 km, surface gravity acceleration = (1/40) g

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Asteroids

• About 2000 follow orbits that bring them into the inner solar system, where they can occasionally collide with a planet.• Earth has been struck many times in its

history.

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Asteroids

• Other asteroids share Jupiter’s orbit: the Greeks and Trojans

• Some other have

been found beyond

the orbit of Saturn.

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Asteroids

• About 200 asteroids are more than 100 km in diameter.• Tens of thousands are estimated to be more

than 10 km in diameter.• There are probably a million or more that are larger

than 1 km and billions that are smaller.

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Asteroids

• As even the largest are only a few hundred kilometres in diameter, Earth-based telescopes can detect no details on their surfaces.• The Hubble Space Telescope can image only

the largest features.

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Asteroid Eros

• Photos returned by robotic spacecraft such as NEAR Shoemaker in 2000-2001 (which landed as a 1st on asteroid surface) and space telescopes show that asteroids are generally irregular in shape and battered by impact cratering.

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Asteroids

• Some asteroids appear to be rubble piles of broken fragments.• A few are known to be double objects or to

have small moons in orbit around them. • These are understood to be evidence of multiple

collisions among the asteroids.

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Asteroids

• A few larger asteroids show signs of volcanic activity on their surfaces that may have happened when the asteroidwas young.

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Asteroids

• Astronomers recognize the asteroids as debris left over by a planet that failed to form at a distance of about 2.5-3 AU from the Sun. • A good theory should explain why a planet

failed to form there, leaving behind a belt of construction material.

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Kirkwood gaps in asteroid belt

• Daniel Kirkwood (1857)

Comets

• In contrast to the rocky asteroids, the brightest comets are impressively beautiful objects. Most comets are faint and are difficult to locate

even at their brightest.

• But in 2013 a very

bright comet is

expected

(mV= -14m !)

• A comet may take months to sweep through the inner solar system.• During this time, it appears as a glowing head with an

extended tail of gas and dust.

Comets

Comet Hartley 2, visited in Nov. 2010 by EPOXI • The beautiful tail of a comet can be longer than 1 AU.• However, it is produced by an icy nucleus only a few

tens of kilometres in diameter. Nucleus is covered with a dark crust, which breaks at places allowing gas (H2O +CO+…) + dust + sand + stones, to escape in jets.

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Comets: Halley’s comet

The nucleus remains frozen and inactive while it is far from the Sun.• As the nucleus moves along its elliptical orbit into

the inner solar system, the Sun’s heat begins to vapourize the ices, releasing gas and dust.

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Comets

• The pressure of sunlight and the solar wind push the gas and dust away, forming a long tail. Peter Apian’s drawing from 1532 shows that the tail always points away from the sun. (He drew comet Halley.)

Comets

• The gas and dust respond differently to the forces acting on them.• So, they often separate into two separate sub-tails.

Comet Hale-Bopp in 1995

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Comets

• Comet nuclei contain ices of water and other volatile compounds such as carbon dioxide, methane, and ammonia.• These ices are the kinds of compounds that

should have condensed from the outer solar nebula. The comets never fully melted. • That makes astronomers think that comets are

ancient samples of the gases and dust from which the outer planets formed.

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Comets

• Five spacecraft flew past the nucleus of Comet Halley when it visited the inner solar system in 1985 and 1986. • Since then, spacecraft have visited the nuclei

of several other comets. • Images show that comet nuclei are irregular in

shape and very dark, with jets of gas and dust spewing from active regions on the nuclei.

Comets

Comets

• In general, crusts of these nuclei are darker than a lump of coal.• This suggests that they have composition

similar to certain dark, water- and carbon-rich meteorites.

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Comets

• Since 1992, astronomers have discovered roughly a thousand small, dark, icy bodies orbiting in the outer fringes of the solar system beyond Neptune.

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Comets

• This collection of objects is called the Kuiper belt. • It is named after the Dutch-American

astronomer Gerard Kuiper, who predicted their existence in the 1950s.

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Comets

• There are probably 100 million bodies larger than 1 km in the Kuiper belt.• Any successful theory should explain how

they came to be where they are. • We will return to them later in this course

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Comets

• Astronomers believe that some comets,those with the shortest orbital periodsand orbits in the plane of thesolar system, come from the Kuiper belt.

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Comets

• The longer-period comets do not always orbit in the plane of the solar system, but can approach the Sun from random directions. • They are believed to originate from the Oort

cloud, a roughly spherical cloud of comets that lies much farther away at a distance of almost one light-year from the Sun. • The solar nebula theory can explain these two

different sources of comets.

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Kuiper belt and Oort cloud

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Comets

• American astronomer Carolyn Shoemaker holds the record for being the most successful comet hunter alive, although, remarkably, she only took up astronomy at age 51. • Canadian amateur astronomer David Levy is

also one of the most successful comet discoverers in history. • He is the first person to have discovered comets

using visual, photographic, and electronic methods.

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Comets

• Carolyn and Eugene Shoemaker, David Levy, and Philippe Bendjoya were co-discoverers of Comet Shoemaker–Levy 9, which flew by Jupiter, disintegrated under tidal force in 1992, and in 1994 collided with Jupiter

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Meteoroids, Meteors, and Meteorites

• Unlike the stately comets, meteors flash across the sky in momentary streaks of light.• They are commonly called “shooting stars.”

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• They are not stars but small bits of rock and metal falling fast into Earth’s atmosphere.• They burst into incandescent vapour about 80

km above the ground because of friction with the air.

• This hot vapour condenses to form dust, which settles slowly to the ground, adding ~~ ~40 000 tons per year to the planet’s mass.

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• Technically, the word meteor refers to the streak of light in the sky. In space, before its fiery plunge, the object is called a meteoroid.• Some meteoroids hitting Earth are large (60 m

body hit over Siberia in 1908 and disintegrated in the air – no meteor found on the ground - causing

• Tunguska catastrophe

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• Most meteoroids are specks of dust, grains of sand, or tiny pebbles. • Almost all the meteors you see in the sky are produced

by meteoroids that weigh less than 1 g. • Only rarely is one massive enough and strong enough to

survive its plunge, and reach Earth’s surface.• Such a rock is called a meteorite (this one was found in

Canada)

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• Meteorites can be divided into three broad categories. • Iron,• Stony, and• Stony-iron.


• Iron meteorites are solid chunks of iron and nickel. • Stony meteorites are silicate masses that resemble

Earth rocks. • Stony-iron meteorites are iron-stone mixtures.


• One type of stony meteorite called carbonaceous chondrites has a chemical composition that resembles a cooled lump of the Sun with the hydrogen and helium removed.


• These meteorites generally contain abundant volatile compounds including significant amounts of carbon and water.• They may have similar composition to comet

nuclei.

• Allende meteorite

found in Mexico

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• Heating would have modified and driven off these fragile compounds.• So, carbonaceous chondrites must not have been

heated since they formed. • Astronomers conclude that carbonaceous chondrites, unlike

the planets, have not evolved and thus give direct information about the early solar system.

• Ca+Al inclusion in a chondrite• Sign of original heating:• Lightnings in the protoplanetary

disk?


• You can find evidence of the origin of meteors through one of the most pleasant observations in astronomy.• You can watch a meteor shower, a display of

meteors that are clearly related by a common origin.

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• For example, the Perseid meteor shower occurs each year in August.• During the height of the shower, you might

see as many as 40 meteors per hour. • The shower is so named because all its meteors

appear to come from a point in the constellation Perseus.


• Meteor showers are seen when Earth passes near the orbit of a comet.• The Eta Aquariids meteor shower, best

viewed from the southern hemisphere, is caused by the Earth passing through dust released by Halley’s Comet.


• The meteors in meteor showers must be produced by dust and debris released from the icy head of the comet. • In contrast, the orbits of some meteorites

have been calculated to lead back into the asteroid belt.

1 chapter 12 the origin of the solar system -architecture of the solar system - planets -asteroids...

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solar system planets

kinds of planets

terrestrial planets

inner planets

planets equators

giant planets dont

present solar system

solar system architecture