Lecture 16. How did it happen? How long did it take? Where did it occur? Was there more than 1 process?

Transcription

1 Planet formation in the Solar System Lecture 16 How did it happen? How long did it take? Where did it occur? Was there more than 1 process?

2 Planet formation How do planets form?? By what mechanism?

3 Planet formation How do planets form?? By what mechanism? How long does it take? Inner: tens of Myr Outer : tens of years hundreds of Myr

4 Planet formation How do planets form?? By what mechanism? How long does it take? Is it the same for all planets? for all planetary systems?

5 Planet formation How do planets form?? By what mechanism? How long does it take? Is it the same for all planets? for all planetary systems? Where do we get constraints from? properties of the planets (tough) current positions/sizes/chemistry primordial? NO. Evolved. But some...

6 Disks are now directly imaged around other stars DISK BUT, details of what is going on inside are still inaccesible to observations

7 Solar System has 3 'types' of planetary bodies 2 ICE Rocky inner 2 GAS Giant outer planets (terrestrial) planets Pluto (same as inner) (nothing is to scale in the picture above!)

8 Planet formation Is accepted to occur in an accretion disk of gas and dust around the star

9 Planet formation Two main models 1) Aggregation via planetesimal accretion seems only way for rocky planets/moons

10 Planet formation Is accepted to occur in an accretion disk of gas and dust around the star Two main models 1) Aggregation via planetesimal accretion 2) Direct collapse at the planetary scale via gravitational instability appealing for gas giants

11 1) Dust sedimentation 1) No direct planetesimal creation 2) µm grains settle to midplane 3) Grains stick together to build macroscopic (~cm and larger) objects

12 Dust is product of condensation 1) The composition of solids (that is, dust) which condensed in the solar nebula depended on the temperature and pressure at that distance from the Sun 2) Thus, silicate rich near Sun, ice rich further out

13 2) Planetesimal creation 1) Form ~1 km objects (decouple from gas) 2) These planetesimals then accrete together as they collide.

14 3) Form planetary embryos via local 'runaway' 1) Well understood analytically+numerically 2) Planetesimal swarm on very circular and low inclination orbits 3) The biggest objects get bigger faster (simple to understand) 4) In inner S.S., go from 'asteroids' to Moon >>10 in mass>> 9

15 3) Runaway accretion, cont'd 1) Increase in physical cross section 2 Big planets (large Vesc) grow faster (Escape speed done on blackboard) At any given distance, one object (embryo) sucks up most of the mass

16 Near 1 AU, reach lunar size 1) Finish with 'nested' set of embryos 2) Note embyros are on low eccentricity orbits 3) Ready for next stage

17 4) Put the lunar embryos together One gets planets at the end! Number and location is random, but similar to our Solar System

18 Time scale Isotopic evidence (eg: from the terrestrial mantle) indicates the Earth had formed its core at most 100 Myr (likely less) T=0 here is defined relative to chondrule and CAI formation

19 Question: Why are the inner planets denser than moons in the outer Solar System? A)The Sun's gravity pulled denser materials to the inner part of the nebula B)In the beginning, when the protoplanetary disk was spinning faster, centrifugal forces flung the lighter materials toward the outer parts of the solar nebula C)In the inner part of the nebula only metals and rocks were able to condense because of the high temperatures, whereas hydrogen compounds, although more abundant, were only able to condense in the cooler outer regions. D)It was too cold for metals to condense in the outer Solar System, so everything that could condense was ice.

20 Composition check AGREES with the ideas of the Lewis model for the rocky terrestrial planets (eg. dropping water content with distance from Sun) AGREES with fact that outer planet satellites are ice rich

21 But, the giant planets... This sequence of steps does NOT work for the giant planets Unlike terrestrial planets, giants have gas (majority for J/S, several Earth mass for U/N) Standard way to get this is core accretion

22 Core Accretion models X: H Y: He Z: the rest Build a roughly 10 Earth mass core via runaway accretion (solid) Add gas envelope slowly for millions of years while core cools, then quickly Jupiter/Saturn had full envelope collapse, while U/N had gas 'run out'?

23 PROBLEMS While runaway mass is bigger outside 5 AU, it's NOT 10 Earth masses

24 PROBLEMS While runaway mass is bigger outside 5 AU, it's NOT 10 Earth masses The embryos start interacting, and the system 'self destructs' Giant planets fling each other out of S.S.!

25 OTHER PROBLEMS It takes too long to build the Uranus and Neptune cores (gas disk leaves!) Why should gas inflow stop???

26 Instant solution? Why not direct collapse? Nebula unstable locally and collapses into planets; should have solar composition Uranus/Neptune didn't; why have 2 mechanisms? Requires very massive disk Such planets migrate

27 How can we get more information? Use small bodies Planetary satellites; regular and irregular Small bodies Meteorites Interplanetary dust Asteroids Comets

28 Constraints from small bodies Comets and Asteroids much more primitive Easier to sample Physical properties Orbital distribution BOTH tell us about what was going on during planet formation