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

Definition • The Solar System comprises the Sun and all objects gravitationally bound to it Our Our Place Place in in the the Cosmos Cosmos • It includes the planets, their moons, asteroids, comets, gas and dust Lecture 9 The Solar System Formation Formation • HST images of newly formed stars show • Clues: accretion disks • Orbits of the planets lie close to a single plane • The Solar system probably formed from the • All planets orbit the Sun in the same direction Sun’s accretion disk • Meteorites are formed from from small fragments • Thus study of the formation of the Solar • These clues suggest that the Solar System System is closely linked with the formation of began as a rotating disk of gas and dust, with the Sun, and with star formation in general larger bodies forming from aggregation of smaller bodies 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 object 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 Spherical Collapse • Suppose that an approximately spherical cloud of • Suppose that a cloud about 1 light-year (10 16 interstellar gas is collapsing due to self-gravity m) across takes one million years to complete • Cloud would maintain its spherical shape except for one rotation its angular momentum • By the time such a cloud has collapsed to the • Interstellar clouds are several light-years in size size of the Sun (10 9 m across, or one ten- • Tidal forces from other objects “stir” them up and millionth of the size of the original cloud), it give them some rotation will be rotating 50 trillion times faster, • Due to large extent of cloud, even very slow rotation rotating once every 0.6 seconds corresponds to huge angular momentum • This is 3 million times faster than Sun’s actual • As cloud shrinks, in order for angular momentum to be rotation speed conserved, rotation rate must speed up [cf a spinning ice skater pulling in their outstretched arms] • 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 Material rains down from collapsing, • As material falls towards forming star it rotating cloud travels on elliptical orbits as predicted by Newton’s law of gravity Vertical motion of • However when material reaches the centre of material from above the cloud, it runs into material falling in from the opposite direction and below cancels… • The material piles up in the centre …leaving net • Vertical motions cancel but rotational motion rotational motion remains, resulting in a rotating accretion disk • Angular momentum of infalling material is transferred to the disk Forming Structures Small particles are blown into larger ones by • Small dust particles within a protoplanetary gas motions disk will be blown around by gas motion forming larger • Occasionally they will be blown into larger and larger particles and stick - a process known as aggregations 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 Disk Heating Planetessimals • Clumps that reach a size of 1 km are known • As material falls into accretion disk it will be as planetessimals (“tiny planets”) heated up • These are massive enough that their gravity • This heating results from the conversion of starts to attract other nearby bodies and the systematic motion of the infalling their growth is no longer limited by chance particles to random motions as they collide encounters with particles falling in from opposite side • Larger planetessimals quickly sweep up most • It is the random motion of the particles that remaining bodies in the vicinities of their make up a body that make it hot orbits • Equivalently, one can think of the • The final survivors of this process are the gravitational potential energy of the outlying planets material being converted into kinetic energy

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 Marbles released together Similarly, a cold cloud of objects; it is lost as objects fall towards each fall with little relative infalling gas is heated when other and is converted into kinetic energy motion but have large it collides with the • Total energy is conserved, but may be freely relative motions after accretion disk converted from one form to another bouncing off a rough surface Disk Temperature Planet composition • The inner part of the disk (that part closer to the • The composition of planets at different radii is protostar) will be hotter than the outer disk since: expected to reflect these differences • Material has had further to fall and so has lost • The inner planets will be made up mostly of more GPE and gained more kinetic energy rocks and metals • Inner disk is radiated by hot protostar • Outer planets can also contain refractory • Here only solids that can withstand high temperatures materials, but also contain large quantities before melting or being vapourized (refractory of ices and organic materials materials, eg. rocks and metals) can exist • Volatile materials (eg. water and organic molecules) • This trend in composition with distance from can only exist in solid form in the outer parts of the the Sun is found in our Solar System disk Atmospheres and Moons Brief Solar System History • A solid planet can capture gas from the accretion Around 5 billion years ago the Sun was a protostar • disk but must act quickly as young stars are sources surrounded by a protoplanetary disk of gas and dust of “winds” and intense radiation that can disperse • Over a few hundred thousand years dust collected gaseous remains of accretion disk into planetessimals comprising rocks and metals close • Giant planets such as Jupiter have an advantage in to the Sun with the addition of ice and organic attracting and keeping a primary atmosphere - a compounds further away mini-accretion disk will form around them About six planetessimals within a few AU of the Sun • • Moon’s can then form from this mini-disk grew to become dominant masses • In the case of small planets such as Earth, the Their growing gravitational fields either captured • primary atmosphere is lost, but a secondary the remaining planetessimals or ejected them from atmosphere forms from the later release of carbon the inner part of the disk dioxide and other gases from volcanic activity

Terrestrial Planets Outer Planets These dominant • Beyond about 5 AU from the Sun, planetessimals • planetessimals coalesced to form a number of bodies with masses 10- became the 15 times that of the Earth terrestrial planets: • Being in a cooler region of the disk, they contained Mercury, Venus, ices and organic compounds as well as rock and metal Earth and Mars • Four of these became the cores of the giant planets: Remaining debris • Jupiter, Saturn, Uranus and Neptune continued to rain • All possessed mini-accretion disks funneling in down on these hydrogen and helium gas and from which moons young planets, as eventually formed evidenced by the • Jupiter’s solid core captured around 300 M � of gas, large impact Saturn around 100 M � craters seen on Mercury Asteroids and Comets Other Solar Systems? • These are planetessimals that survive to this • Models of star formation generically predict the existence of proto-planetary disks around protostars day and so we expect other planetary systems like the • Gravitational field of Jupiter prevented Solar System to be quite common planets from forming in the region between it • Planets around other stars (extra-solar planets) are and Mars - the asteroid belt extremely hard to see due to glare from the host star • Icy planetessimals from outer Solar System • However, since stars and massive planets are in orbit remain as comets about each other we can detect a “wobble” in the • Some are on very eccentric orbits and so position of stars with nearby massive planets occasionally travel very close by the Sun and • The existence of many extra-solar planets is now Earth inferred from such observations