How Planets Form

1. How Planets Form Gerard Williger U Louisville Public Astronomy Lecture Series 11 Feb 2016 With essential support from thousands of scientists who did the work over the centuries!

2. Formation of the Solar System How did Earth and Planets in the Solar System form? • Oldest known Rocks on the moon older than 4.4 Gyr  Solar System formed ~ 4.6 Gyr ago

3. Collapse of a Cloud • 4.6Gyr ago gas+dust cloud collapsed by own gravity • Triggering mechanism could be: - shock wave from a Supernova explosion - little gravity fluctuation in cloud of gas Orion Nebula

4. Cloud Collapse & Angular Momentum • Cloud spins faster as it collapses from conservation of angular momentum • angular momentum depends on: 1) rpm and 2) moment of inertia (distance of mass from spin axis) Angular momentum in action

5. Formation of Disk If cloud spins fast enough, rotational inertia (misnamed “centrifugal force”)counteracts gravitational contraction No spin perpendicular to rotation plane to stop contraction  formation of a disk When central region dense, hot enough to start nuclear reactions  formation of the Sun

6. DiskClumps Planetesimals  Protoplanets • Gas, dust start to clump • Smaller clumps form larger clumps and planetesimals (solid bodies, d 1km - ~100km, held together by stickiness, not gravity) • Gravity pulled together planetesimals protoplanets orbiting a protosun (not yet doing nuclear fusion) Computer simulation – view from above disk

7. Baby Solar System Effect of strong UV light from neighboring hot star on a young star and planet-forming disk. UV light heats up gas to 10000K, which evaporates the disk and forms a comet-like tail downstream.

8. “Pillars of Creation” Hubble Visible Hubble Infrared GAS DUST Eagle Nebula – Star Formation Region

9. Formation of Sun + Planets • Protoplanets contracted and cooled planets • Protosun contracted Sun • Remaining planetesimals may still be orbiting Sun (asteroids, meteoroids and comets) • Most of unused gas blown away by strong solar wind • Whole process: few Myr

10. From Cloud to Star + Planets

11. New Tools to See All This • Hubble Space Telescope • Infrared (IR) satellites • Atacama Large Millimeter Array

12. Satellite Telescopes Like “looking through a dusty old stained glass window” Compare with Fig. 2-2 in text <-Far IR-> Near/mid IR See most wavelengths near-infrared (1-2 μm) from ground, though sky glows See some wavelengths mid-infrared (2-10 μm) from ground, but sky glows A LOT – satellite better than from ground (1000s of times) We can only see a few wavelengths of far-infrared from ground (10-1000 μm) and sky glows incredibly brightly – satellite MUCH better than from ground (millions of times) Fig. 3-27, p. 52

13. Hubble Space Telescope Observes UV, optical, a little IR light Mirror polished wrong – had spherical aberration but was fixed Perfect seeing – no atmosphere! Airglow in near-IR is much lower, too! Fig. 3-22, p. 50

14. Spitzer Space IR Telescope (3-170 μm, NASA, 0.8m diameter, launched 2003) INFRARED Herschel Space IR Telescope (55-672 μm, ESA, 3.5m diameter), operated 2009-13 Fig. 3-32a, p. 54

15. MICROWAVE • Atacama Large Millimeter Array (ALMA) • Partners: US, Europe, Canada, Japan, Taiwan, Chile • Location: Chajnantor plateau, N Chile (elevation 5000m=16700’) • Cost: $1.5 billion (still under construction) • 66 dishes, area 7240 m2, maximum separation 16km, observes 0.32-3.6 mm

16. I – Background Star/Planet Formation Caselli & Ceccarelli, Astronomy & Astrophysics Review, Oct 2012

17. Proto-planetary/Debris Disks • Form around T Tau (low mass) or Herbig Ae/Be star (high mass) • First debris disk observed 1984 around Vega (IRAS)

18. Proto-planetary Disks: up to 25 Myr old • Dust emission strong in infrared/submm.microwaves • High resolution required (Space Telescope, ALMA) • Sizes up to 1000 AU From young stars in nearby star formation regions, Mamajek et al 2009, AIP, 1158, 3 HL Tau, ALMA, ESO public image 1436a

19. Disk structure/variety High aspect ratio (flat); often associated with jets Variety seen in HST infrared images from NICMOS -- note dust lanes

20. Disk Composition: Function of Temperature Grains / refractories (high melting points: metals, silicates) Water / volatiles

21. Observing Proto-planetary Disks C Dullemond, ITA, Heidelberg

22. Disk Types/Evolution (Oldest)

23. Observing Formation: Coronography to observe Proto-planetary Disks Images of protoplanetary disks around stars Observe in IR because disks are COOL & DUSTY Coronagraphy – blocks light from star so can see faint disk KECK: Mid-IR (18 micron) Near-IR, adaptive optics image, Keck Near-IR image, Keck Hubble (optical, occulted)

24. Proto-planetary disks Systems are bright in IR due to blackbody emission from cool dust. HL Tau, ALMA (mm image) HL Tau, HST

25. Transitional Disks • Up to 10% develop holes, gaps • Maintain optically thin outer disks • often inner disks, too Espaillat et al. arXiv:1402.7301 0.88mm dust emission from pre-transitional disks

26. Forming Giant Planets – takes 1-10 Million years Model 1) Disk Instability –Gas first: clumps in proto-planetary disk video: start to collapse gravitationally form giant planets with gas+dust rocky core forms slowly from impacts of rocky bodies

27. Forming Giant Planets – takes 1-10Myr Model 2) Core Accretion – Rock first: Rocky core of ~10 Earth masses forms first: video gas comes from proto-planetary disk (nearby space)

28. Temperature drives planet formation - I • Temperature: dictates how fast gas molecules move, likelihood of escape from a planet’s gravity • Temperature: energy per sq. meter on a planet depends on amount of sunlight received (proportional to 1/d2) • Temperature: determines which molecules stay, which go (evaporate) from growing planet

29. Temperature drives planet formation - II • Free hydrogen (H), helium (He) lightest -> fastest to evaporate, closest to sun • Iron (Fe), nickel (Ni) last to evaporate • Outer planets – cooler -- naturally rich in light gases (H, He, methane, ammonia) • inner planets – hotter – mostly Fe-Ni, rocky, might take 100 Myr to form • Comets, asteroids, Kuiper Belt Objects left over • Asteroid belt – where Jupiter’s gravity prevented planetesimals from coming together by gravity • Comets -- may have brought water, organic molecules to inner planets after formation

30. TEMPERATURE VS. COMPOSITION http://astro.unl.edu/classaction/animations/solar/formationtemps.html

31. Planets slowed down by disk • Watch the planet spiral in

32. NEW IDEA: PLANETS MIGRATE! • “Bullies” like Jupiter throw around other planets in a giant cosmic dance of “Swing Your Partner”

33. IMPLICATION: ORIGINAL “EARTH” MIGHT HAVE BEEN DESTROYED! • “Grand Tack” model: Jupiter and Saturn migrated toward Sun, due to interactions with smaller bodies (thrown to outer solar system)

34. Nice Model Formation of terrestrial planets: excentricité vs. axe semi-majeur • Computer Simulations – Alessandro Morbidelli Planetesimals asteroids - - - - combination (e vs. a) of instabilityJupiter

35. Planets Can Change Orbits “Drag”from proto-planetary nebula gas? Gravitational interactions with each other? We think that Uranus & Neptune formed closer to Sun, were flung to their present orbits by interactions with Jupiter, maybe Saturn

36. Nice Model excentricity vs. semi-maj. axis top view • Simulations numériques – A. Morbidelli JupiterSaturnNeptuneUranus asteroide/planetesimals

37. After Planet Formation/Migration • LATE HEAVY BOMBARDMENT • Many small bodies changed orbit, impacted planets about 3.1 billion years ago • Made many craters: video

38. Solar System Impacts • Mercury: Caloris Basin • Venus: retrograde rotation? • Earth: formation of Moon • Mars: northern basin, loss of mag. field? • Vesta, Mimas: giant impacts which nearly destroyed them • Uranus: sideways rotation Mimas