Our Our Place Place in in the the Cosmos Cosmos It includes the - - PDF document

Our Our Place Place in in the the Cosmos Cosmos

Lecture 9 The Solar System

Definition

• The Solar System comprises the Sun and

all objects gravitationally bound to it

• It includes the planets, their moons,

asteroids, comets, gas and dust

Formation

• Clues:
• Orbits of the planets lie close to a single plane
• All planets orbit the Sun in the same direction
• Meteorites are formed from from small fragments
• These clues suggest that the Solar System

began as a rotating disk of gas and dust, with larger bodies forming from aggregation of smaller bodies

Formation

• HST images of newly formed stars show

accretion disks

• The Solar system probably formed from the

Sun’s accretion disk

• Thus study of the formation of the Solar

System is closely linked with the formation of the Sun, and with star formation in general

Sun’s Formation

• Around 5 billion years ago, newly-formed Sun

was still a protostar - a large ball of gas whose collapse converts gravitational energy to thermal energy

• Surrounding the protostar was a flat, orbiting

disk of gas and dust - a protoplanetary accretion disk, containing about 1% of the mass of the protostar

Why Disks?

• A rotating object possesses a quantity called

angular momentum

• Angular momentum depends on:
• rotation speed
• mass
• mass distribution
• Just as the linear momentum p = mv of an
• bject moving in a straight lines is conserved

(ie will remain unchanged until an external force acts upon it, Newton’s 1st law of motion), angular momentum is also conserved

Spherical Collapse

• Suppose that an approximately spherical cloud of

interstellar gas is collapsing due to self-gravity

• Cloud would maintain its spherical shape except for

its angular momentum

• Interstellar clouds are several light-years in size
• Tidal forces from other objects “stir” them up and

give them some rotation

• Due to large extent of cloud, even very slow rotation

corresponds to huge angular momentum

• As cloud shrinks, in order for angular momentum to be

conserved, rotation rate must speed up [cf a spinning ice skater pulling in their outstretched arms]

Spherical Collapse

• Suppose that a cloud about 1 light-year (1016

m) across takes one million years to complete

• ne rotation
• By the time such a cloud has collapsed to the

size of the Sun (109 m across, or one ten- millionth of the size of the original cloud), it will be rotating 50 trillion times faster, rotating once every 0.6 seconds

• This is 3 million times faster than Sun’s actual

rotation speed

• Why isn’t the Sun rotating faster?

Non-Spherical Collapse

• Cloud’s angular momentum opposes collapse

towards rotation axis but allows collapse parallel to axis

• An initially spherical, rotating cloud will thus

collapse not into a ball, but into a flattened “pancake” structure which becomes the accretion disk

• Infalling (accreting) material arrives first at

the accretion disk before becoming part of the central star

Accretion Disk Formation

• As material falls towards forming star it

travels on elliptical orbits as predicted by Newton’s law of gravity

• However when material reaches the centre of

the cloud, it runs into material falling in from the opposite direction

• The material piles up in the centre
• Vertical motions cancel but rotational motion

remains, resulting in a rotating accretion disk

• Angular momentum of infalling material is

transferred to the disk Material rains down from collapsing, rotating cloud Vertical motion of material from above and below cancels… …leaving net rotational motion

Forming Structures

• Small dust particles within a protoplanetary

disk will be blown around by gas motion

• Occasionally they will be blown into larger

particles and stick - a process known as aggregation

• Eventually some aggregates will grow into

structures around 100 m across

• Two such boulder-sized structures may stick

together if they bump into each other very gently - at around 10 cm/s or less Small particles are blown into larger ones by gas motions forming larger and larger aggregations

Planetessimals

• Clumps that reach a size of 1 km are known

as planetessimals (“tiny planets”)

• These are massive enough that their gravity

starts to attract other nearby bodies and their growth is no longer limited by chance encounters

• Larger planetessimals quickly sweep up most

remaining bodies in the vicinities of their

• rbits
• The final survivors of this process are the

planets

Disk Heating

• As material falls into accretion disk it will be

heated up

• This heating results from the conversion of

the systematic motion of the infalling particles to random motions as they collide with particles falling in from opposite side

• It is the random motion of the particles that

make up a body that make it hot

• Equivalently, one can think of the

gravitational potential energy of the outlying material being converted into kinetic energy

Marbles released together fall with little relative motion but have large relative motions after bouncing off a rough surface Similarly, a cold cloud of infalling gas is heated when it collides with the accretion disk

Types of Energy

• Energy can take several forms
• Anything moving possesses kinetic energy
• Heat is a form of kinetic energy due to random

motions of the constituent particles

• Anything that feels a gravitational force possesses

gravitational potential energy (GPE)

• GPE is increased by separating two massive
• bjects; it is lost as objects fall towards each
• ther and is converted into kinetic energy
• Total energy is conserved, but may be freely

converted from one form to another

Disk Temperature

• The inner part of the disk (that part closer to the

protostar) will be hotter than the outer disk since:

• Material has had further to fall and so has lost

more GPE and gained more kinetic energy

• Inner disk is radiated by hot protostar
• Here only solids that can withstand high temperatures

before melting or being vapourized (refractory materials, eg. rocks and metals) can exist

• Volatile materials (eg. water and organic molecules)

can only exist in solid form in the outer parts of the disk

Planet composition

• The composition of planets at different radii is

expected to reflect these differences

• The inner planets will be made up mostly of

rocks and metals

• Outer planets can also contain refractory

materials, but also contain large quantities

• f ices and organic materials
• This trend in composition with distance from

the Sun is found in our Solar System

Atmospheres and Moons

• A solid planet can capture gas from the accretion

disk but must act quickly as young stars are sources

• f “winds” and intense radiation that can disperse

gaseous remains of accretion disk

• Giant planets such as Jupiter have an advantage in

attracting and keeping a primary atmosphere - a mini-accretion disk will form around them

• Moon’s can then form from this mini-disk
• In the case of small planets such as Earth, the

primary atmosphere is lost, but a secondary atmosphere forms from the later release of carbon dioxide and other gases from volcanic activity

Brief Solar System History

• Around 5 billion years ago the Sun was a protostar

surrounded by a protoplanetary disk of gas and dust

• Over a few hundred thousand years dust collected

into planetessimals comprising rocks and metals close to the Sun with the addition of ice and organic compounds further away

• About six planetessimals within a few AU of the Sun

grew to become dominant masses

• Their growing gravitational fields either captured

the remaining planetessimals or ejected them from the inner part of the disk

Terrestrial Planets

• These dominant

planetessimals became the terrestrial planets: Mercury, Venus, Earth and Mars

• Remaining debris

continued to rain down on these young planets, as evidenced by the large impact craters seen on Mercury

Outer Planets

• Beyond about 5 AU from the Sun, planetessimals

coalesced to form a number of bodies with masses 10- 15 times that of the Earth

• Being in a cooler region of the disk, they contained

ices and organic compounds as well as rock and metal

• Four of these became the cores of the giant planets:

Jupiter, Saturn, Uranus and Neptune

• All possessed mini-accretion disks funneling in

hydrogen and helium gas and from which moons eventually formed

• Jupiter’s solid core captured around 300 M of gas,

Saturn around 100 M

Asteroids and Comets

• These are planetessimals that survive to this

day

• Gravitational field of Jupiter prevented

planets from forming in the region between it and Mars - the asteroid belt

• Icy planetessimals from outer Solar System

remain as comets

• Some are on very eccentric orbits and so
• ccasionally travel very close by the Sun and

Earth

Other Solar Systems?

• Models of star formation generically predict the

existence of proto-planetary disks around protostars and so we expect other planetary systems like the Solar System to be quite common

• Planets around other stars (extra-solar planets) are

extremely hard to see due to glare from the host star

• However, since stars and massive planets are in orbit

about each other we can detect a “wobble” in the position of stars with nearby massive planets

• The existence of many extra-solar planets is now

inferred from such observations