Our Place in the Cosmos

1. Our Place in the Cosmos Lecture 9 The Solar System

2. Definition • The Solar System comprises the Sun and all objects gravitationally boundto it • It includes the planets, their moons, asteroids, comets, gas and dust

3. 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

4. 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

6. 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

8. 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

9. 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]

10. Spherical Collapse • Suppose that a cloud about 1 light-year (1016 m) across takes one million years to complete one 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?

11. 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

13. 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

14. Material rains down from collapsing, rotating cloud Vertical motion of material from above and below cancels… …leaving net rotational motion

15. 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

16. Small particles are blown into larger ones by gas motions forming larger and larger aggregations

17. 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 orbits • The final survivors of this process are the planets

18. 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

19. 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

20. 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 objects; it is lost as objects fall towards each other and is converted into kinetic energy • Total energy is conserved, but may be freely converted from one form to another

21. 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

22. 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 of ices and organic materials • This trend in composition with distance from the Sun is found in our Solar System

23. Atmospheres and Moons • A solid planet can capture gas from the accretion disk but must act quickly as young stars are sources of “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

24. 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

25. 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

26. 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

27. 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 occasionally travel very close by the Sun and Earth

28. 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