4.1ORIGIN OF THE SOLAR

SYSTEM

HISTORY OF THE ATOM IN THE UNIVERSE

We are linked through a chain of origins extending back through time, to the instant when the universe began estimated 13.7 billion years ago.

Accepted theory of the Big Bang explains the beginning of the universe. After only a few minutes into this, the

subatomic particles (protons, neutrons, electrons) making up your body came into existence very old matter.

Most of the matter in the universe at this time was hydrogen, and about 25% helium.

HISTORY OF THE ATOM IN THE UNIVERSE Within a few hundred million years,

matter collected, forming galaxies containing billions of stars. Massive stars produce iron (Fe) in their

cores, but much of that is destroyed when the core collapses and the star explodes (known as a supernova).

Our galaxy, the Milky Way, contains at least 100 billion stars, of which the Sun is one. Believed to have formed about 5 BYA

from a cloud of gas and dust.

Geologist, Preston Cloud, once remarked: “Stars have died so that we might live.”

EARLY HYPOTHESES 1st physical theory proposed by French philosopher,

Rene Descartes (1596-1650). Prior to Newton, therefore didn’t recognize gravity.

Proposed the Sun and planets formed when a large vortex contracted and condensed.

A century later, Georges Louis de Buffon (1707-1788) proposed an alternative hypothesis planets were formed when a passing comet collided with, or passed close to, the Sun. Astronomers later revised his hypothesis saying a star,

rather than a comet, interacted with the Sun.

Became known as the Passing Star Hypothesis. Stars collide very infrequently Gas pulled from stars would be too hot to make planets Planets would not have stable orbits around the Sun.

Descartes

de Buffon

EARLY HYPOTHESES Two categories of

hypotheses:

Evolutionary Involves common, gradual

processes to produce the Sun and planets.Descartes

Catastrophic Involves unlikely, sudden events to

produce the solar system, implying planetary systems are very rare.Buffon (Passing Star Hypothesis)

NEBULAR HYPOTHESIS The modern theory for the origin of the

solar system falls under the evolutionary category and had its true beginning with Pierre-Simon de Laplace (1749-1827), a French astronomer and mathematician.

In 1796, he combined Descartes vortex idea with Newton’s law of gravity and motion to produce a model of a rotating cloud of matter that contracted under its own gravitation and flattened into a disk. This idea became is known as the Nebular

Hypothesis.

He reasoned that as it spun faster, the disk would shed its outer edge, leaving behind a ring of matter eventual planet formation.

Laplace

SOLAR NEBULA THEORY

The modern explanation of the origin of the planets is the Solar Nebula Theory, which proposes the planets, including Earth, formed in a rotating disk of gas and dust that surrounded the Sun as it formed.

Astronomers continue to refine the details of this theory every day with new discoveries being made.

If planet formation is a natural part of star formation, most stars should have planets.

RELATIVE SIZES OF PLANETS

Assume we reduce all bodies in the solar system so the Earth has a diameter of 0.3 mm. (size of a grain of salt):

Sun ~ plum

Mercury, Venus, Earth, Mars ~ salt grains

Jupiter ~ apple seed

Saturn ~ sunflower seed

Uranus, Neptune ~ peppercorns

Pluto ~ powdered sugar grain

REVOLUTION AND ROTATION The planets revolve around the Sun in orbits

close to a common plane: The orbit of Mercury is tipped 7º to Earth’s orbit.

The rest of the planets’ orbital planes are inclined by no more than 3.4º.

Preferred direction of motion in the solar system counterclockwise as seen from the north. Curl of the fingers of your right hand if you point

your thumb toward your eyes.

All the planets revolve counterclockwise around the Sun, and with the exception of Venus and Uranus, they also rotate counterclockwise on their axes.

PLANETARY ORBITS

Plut

o

Nep

tune

Ura

nus

Saturn

Jupi

ter

Mar

s

Earth

VenusMercury

All planets in almost circular

(elliptical) orbits around the sun, in approx. the same plane (ecliptic).

Sense of Revolution:

counter-clockwiseSense of Rotation: counter-clockwise (w/ exception of Venus, Uranus, and Pluto)

Orbits generally inclined by no more than 3.4o

Exceptions:

Mercury (7o)

Pluto (17.2o)

(Distances and times reproduced to scale)

TWO KINDS OF PLANETS The two kinds of planets are

distinguished by their location and characteristics.

The four inner Terrestrial planets include:

Mercury

Venus

Earth

Mars

Small, dense rocky worlds with little or no atmosphere.

TWO KINDS OF PLANETS The four outer Jovian

planets include:

Jupiter

Saturn

Uranus

Neptune

Large, low-density worlds with thick atmospheres and liquid or ice interiors.

CRATERS Craters, like on

our Moon’s surface, are common throughout the solar system.

Do not exist on Jovian planets (no solid surface).

SPACE DEBRIS The Sun and planets are not

the only products of the solar nebula.

Our solar system is littered with 3 kinds of space debris: Asteroids

Comets

Meteoroids

These objects represent a tiny fraction of the mass of the solar system, but are a rich source of information about the origin of the planets.

ASTEROIDS Asteroids, sometimes called minor planets, are

small rocky worlds, most of which orbit the Sun in a belt between the orbit of Mars and Jupiter. More than 100,000 asteroids have orbits that are

charted.

About 200 asteroids are more than 100 km (60 mi) in diameter.

HST cannot detect details on their surface, just larger features due to their relatively small size.

Spectroscopic observations indicate rocky and metallic surfaces.

Spacecraft photographs show irregular shape and cratered surfaces for most asteroids.

Eros (Asteroid)

Series of photographs from NEAR spacecraft.

KUIPER BELT Since 1992, astronomers have discovered more

than a thousand small, dark, icy bodies orbiting the outer edges of our solar system beyond Neptune (Trans-Neptunian Objects, TNO’s). This collection of objects is called the Kuiper Belt.

100 million bodies larger than 1 km.

COMETSSmall, icy bodies known as comets, also orbit in this region, and produce tails of gas and dust when they near the Sun.

The motion of the nucleus along its orbit, the effects of sunlight and the outward flow of solar wind create long tails pointed away from the Sun.

METEOROIDS, METEORS, METEORITES

Smaller particles, possibly coming from asteroids or comets, orbiting the Sun are known as meteoroids.

Most have diameter < 1 mm.

When a meteoroid enters Earth’s atmosphere, friction heats the surface and causes it to burn up.

As it burns it produces a bright streak of light known as a meteor, which is commonly referred to as a “shooting star”.

Any part of a meteoroid making contact with Earth’s surface is known as a meteorite.

AGE OF THE SOLAR SYSTEM According to the solar nebula theory, the

planets should be about the same age as the Sun. Most accurate way to find the age of a rocky body is

to bring a sample into a laboratory and analyze the radioactive elements it contains.

Certain isotopes of elements gradually decay into other isotopes. Example: Potassium-40 (parent isotope) decays into

Calcium-40 and Argon-40 (daughter isotopes).

The half-life of a radioactive substance is the time it takes for half of the parent isotope atoms to decay into daughter isotope atoms.

RADIOMETRIC DATING The process of using

radioactive decay to determine the age of a substance is called radiometric dating.

Measure the abundance of a radioactively decaying element to find the time since the formation of the rock.

Dating on the rocks of Earth, the moon, and meteorites all give ages of approximately 4.6 billion years.

RADIOMETRIC DATING METHODS

Method Parent Isotope

Daughter Isotope

Half-Life Effective Range

Radiocarbon Dating

14C 14N 5730 years < 70,000 years

39Argon – 40Argon

39Ar 40Ar 1.25 billion years

10,000 to 4.6 billion years

40Potassium – 40Argon

40K 40Ar 1.25 billion years

50,000 to 4.6 billion years

87Rubidium – 87Strontium

87Rb 87Sr 48.1 billion years

10 million to 4.6 billion

years235Uranium –

207Lead

235U 207Pb 704 million years

10 million to 4.6 billion

years238Uranium –

206Lead

238U 206Pb 4.5 billion years

10 million to 4.6 billion

years232Thorium –

208Lead

232Th 208Pb 14 billion years

> 200 million years

COMPOSITION OF THE SOLAR NEBULA Analysis of the solar spectrum

shows the Sun is mainly Hydrogen, with a quarter of its mass being Helium, and only 2% being heavier elements. This must have been the

composition of the solar nebula. Also seen in the chemical

composition of the planets:

Terrestrial: rock and metal

Jovian: low-density gases (H, He) If you allowed those gases to

escape, the relative proportions of the heavier elements remaining would resemble terrestrial compositions.

Planets formed from the same protostellar material as the Sun, still found in the Sun’s

atmosphere.Rocky planet material formed from clumping

together of dust grains in the protostellar cloud.

Mass of less than 15 Earth masses:

Planets can not grow by gravitational

collapse.

Mass of more than 15 Earth masses:

Planets can grow by gravitationally attracting

material from the protostellar cloud.Terrestrial planets

(Earth-like) Jovian planets (Gas Giants)

THE STORY OF PLANET BUILDING

CONDENSATION OF SOLIDS

The uncompressed densities of the planets are their calculated densities if gravity did not compress them (average density of their original construction materials). Dependent upon the temperature.

Inner regions ~ 1500 K The only materials able to form grains

at this temperature are metal oxides and pure metals.

Farther out, silicates (rocky materials) could also condense, along with metals.

Terrestrial planets are evidently composed of a mixture of metals, metal oxides, and silicates. More metals – closer to Sun

More silicates – farther from Sun

CONDENSATION OF SOLIDS Even farther from the Sun was a boundary

called the ice line, beyond which water vapor could freeze to form ice particles. Farther yet, compounds such as methane and

ammonia could condense to form other types of ice. Those ices are low-density materials Jovian planets.

CONDENSATION OF SOLIDS

The sequence in which the different materials condense from the gas as you move away from the Sun toward lower temperature is called the condensation sequence.

Suggests the planets, forming at different distances from the Sun, should have accumulated from different materials in a predictable manner.

PLANETESIMALS AND PROTOPLANETS

In the development of a planet, 3 processes operate to collect solid bits of matter (i.e. rock, metal, ice) into small bodies called planetesimals, which eventually build planets:

1. Condensation• A particle grows by condensations when it adds

matter one atom or molecule at a time from a surrounding gas.

2. Accretion• Defined as the sticking together of solid particles.

• The coalescing of planetesimals eventually formed protoplanets, the name for massive objects destined to become planets.

3. Gravitational Collapse• Once a protoplanet gets large enough, it could

begin to grow by gravitational collapse, the rapid accumulation of large amounts of in-falling gas from the nebula.

PLANETESIMALS AND PROTOPLANETS

PLANETESIMALS AND PROTOPLANETS Once the planet formed, heat would

begin to accumulate in its interior from the decay of short-lived radioactive elements.

The violent impacts of in-falling particles would have also released energy called heat of formation.

These two heating sources would eventually have melted the planet and allowed it to differentiate. Differentiation is the separation of material

according to density. Heavy metals settled to the core, such as Iron

(Fe) and Nickel (Ni).

Lighter silicates (combination of Si and O) floated to the surface, forming a low-density crust.

PLANETESIMALS AND PROTOPLANETS

If planets formed by accretion of plantesimals, later melted by radioactive decay and heat of formation, then Earth’s early atmosphere may have consisted of a combination of gases. Delivered by planetesimal impacts and release

from the planet’s interior during differentiation.

The creation of a planetary atmosphere from a planet’s interior is called outgassing. Not as much water back then; astronomers

believed it formed later in Earth’s formation.

Massive planets (like Jovians), may be able to form through direct collapse, the in-falling of gas without first forming a rocky, dense core.

CLEARING THE NEBULA Remains of the solar nebula were cleared away

by:

Radiation Pressure (Sun) Pushed specks of dust and individual atoms out of the

system.

Solar Wind Flow of ionized (charged) particles away from the Sun’s

upper atmosphere.

Steady breeze; 400 km/s (250 mi/s).

Sweeping-Up of Space Debris By Planets Old, solid surfaces of the solar system are heavily

cratered.

Believed to have formed approximately 4 BYA in what is called the heavy bombardment.

Ejection by Close Encounters with Planets Small objects can gain energy from larger ones

(gravitational field) and be ejected from the solar system.

CLEARING THE NEBULA

Surfaces of Earth’s moon and Mercury show evidence of heavy bombardment for asteroids.

EXTRASOLAR PLANETS Recall, the solar nebula

theory falls under the classification of an evolutionary one. Many stars should have

planets. A planet orbiting another star is

called an extrasolar planet.Faint, difficult to see (glare)Detection is same as in a

binary star system: Wobbling of a star around a

common center of mass (barycentre).

EXTRASOLAR PLANETS Only in exceptional cases can we directly

observe an extrasolar planet. Primarily in the infrared portion of the spectrum.

Planets may still be warm and emit infrared light.