How Giant Planets Shape the Characteristics of Terrestrial Planets

A colloquium talk I gave at UC Santa Cruz.

Transcript

1. Tom Barclay NASA Ames Research Center UC Santa Cruz April Fool’s Day 2016 How Giant Planets Shape the Characteristics of Terrestrial Planets
2. Collapse of molecular cloud core proto-star + disk (Shu, Adams, & Lizano 1987) Planets Form From Disks
3. Early stage dust grains planetesimals ~ μm ~1-10 km • non-gravitational sticking process remains poorly understood Classical Solar Nebular Theory
4. Early stage dust grains planetesimals ~ μm ~1-10 km Middle stage planetesimals planetary embryos ~103 km Kokubo & Ida 2002 Classical Solar Nebular Theory
5. Early stage dust grains planetesimals ~ μm ~1-10 km Middle stage planetesimals planetary embryos ~103 km Late stage embryos planets Classical Solar Nebular Theory
6. Classical Solar Nebular Theory Timescales/composition for Solar System support In Situ formation
7. N-body models are widely-used tools to study planet formation “Mercury” hybrid symplectic integrator package (Chambers 1999) Dynamical Models
8. Solar System Example Quintana & Lissauer 2014
9. N-body Models Challenges: 1. Models assume perfect accretion (fragmentation increases N) 2. N-body systems are chaotic, need lots of simulations This new study addresses these two issues Mercury modified to include state-of-the-art collisions model We performed hundreds of N-body simulations to infer results statistically Chambers (2013) Quintana et al. 2016 (arxiv 1511.03663)
10. New Collision Model Leinhardt & Stewart 2012 Stewart & Leinhardt 2012 Collision model maps outcomes of a two-body collisions based on masses and impact geometry in gravity regime
11. rimp ℓ ℓ B B rimp rtar rtar θ θ ℓ = rtar + rimp - B ℓ = 2 rimp vimp vimp (a) (b) New Collision Model Based on model by Stewart & Leinhardt (2012) Mercury N-body integration package modified to include collision model that maps outcomes of a two-body collisions based on masses and impact geometry Outcomes include: -collision with central star, giant planet -perfect accretion -fragmentation -hit-and-run collisions (Asphaug 2006) Chambers (2013) Quintana et al. 2016 (arxiv 1511.03663)
12. Hundreds of Simulations Sun + Jupiter + Saturn (at present orbits) Bimodal protoplanetary disk: 26 embryos (0.1 MEarth ) 260 planetesimals (0.01 MEarth ) Smallest fragments = 0.5 lunar mass Small change in initial conditions in each simulation 2 Gyr simulations, where all bodies fully interact gravitationally and collisionally
13. Quintana et al. 2016, in press Effects of Fragmentation Collisions matter!
14. Collisions • Shape of the final architecture of planetary systems • Alter bulk compositions (e.g. Mercury, Earth-moon) • Affect potential habitability - alter spin and rotation rates (weather) - strip off oceans and atmospheres - wipe out life Timing of final giant collision (eg. Moon forming impacts) is important because you need enough time and enough residual material for Earth-like planets to accrete and retain water and other volatiles
15. Quantifying Impacts Stewart, Lock and Mukhopadhyay (2015) Qs ~ 107 J/kg strip 100% atmosphere Qs ~ 108 J/kg strip 100% ocean
16. Stewart, Lock and Mukhopadhyay (2015) Defining a “Giant Impact” Qs >= 2 x 106 J/kg Comparable to MFI and stripping ~50% atmosphere
17. Earth analogs Impacts onto Earth-analogs Difficult to strip atmospheres/oceans!
18. Properties of giant impacts On average proto-earths receive 3 giant impacts Most proto-earths experience their last giant impacts <100 Myr into simulation Last giant impact
19. The Small Mars Problem N-body models are widely used tools to study the late stages of terrestrial planet formation (i.e., SWIFT, Mercury) Simulations typically form a Mars analog that is far too massive
20. Mars Formed Rapidly Hf/W isotopic evidence from Martian meteorites suggests Mars accreted most of its mass within about 5 Myr. Earth continued to grow for an additional 30-100 Myr (Dauphas and Pourmand 2011)
21. The Grand Tack?
22. Distribution of final planets We are able to from planets that resemble the solar system Mars analogs defined as a planet with 0.05-0.2 Mearth, within 1.25-2 AU, and accreted no more than 10% of mass after 2 Myr Could the Grand Tack be a small number statistics issue? - Mars’ are uncommon but not rare
23. Generalized Solar System Look at terrestrial planet formation in a probabilistic manner. 1. Take best-guess initial conditions for disk that have been successful in broadly reproducing the inner Solar System 2. Run a large number of simulations 3. Infer distribution of physical properties (mass, number, water content, etc.) 4. Consider Solar System as one draw from these distributions Not necessary to always form the Solar System, occasionally forming the Solar System is ok to validate model
24. Tracking the Composition
25. Jupiter analogs are likely scarce! Occurrence Rates of Jupiter (RV + Transits) ~ 6% (Wittenmyer et al. 2016)
26. Jupiter+Saturn 26 No giant planets
27. Effect of Giant Planets With no giant planets, more planets are formed but inner systems looks similar
29. The Impact of Giant Planets Without giant planets there are more total impacts but fewer giant impacts
30. Last Giant Impact Times Final giant impacts happen earlier if there are giant planets
31. 31 WFIRST is going to be searching for free-floating planets. How many will it find?
32. 32 WFIRST is going to be searching for free-floating planets. How many will it find?
33. 33 Properties of Ejected Material
34. 34 With giant planets ejections happen early Two epochs of ejection: first primordial material followed by processed material Giant planets only Times of Ejections
35. 35 Outer disk is ejected first Efficient radial mixing with no giant planets Fragments from inner system ejected later Dependence on Initial Semimajor Axis
36. 36 WFIRST will find plenty of Mars’ but few earths If Giant planets are rare, WFIRST finds no FFP WFIRST Detections
37. 37 Sun-Only
38. 38 Micro-Oort Clouds?
39. Conclusions New N-body model with fragmentation and hit-and-run events Allows detailed modeling of planet accretion, erosion, water delivery, bulk compositions, giant impacts that can strip atmospheres/oceans With giant planets: collisions less frequent but have higher energies ~1/3 disk mass ejected Without giant planets: Collisions continue for Gyrs, but lower energy Very little mass ejected