Formation of the Solar System

1. Formation of the Solar System

2. How did the Solar System Form? We weren't there. We need a good theory. Check it against other forming solar systems. What must it explain? - Solar system is very flat. - Almost all moons and planets orbit and spin in the same direction. Orbits nearly circular. - Planets are isolated in space. - Terrestrial - Jovian distinction (esp. mass, density, composition). - Leftover junk and its basic properties (comets, asteroids, TNOs). Not the details and oddities – such as Venus’ and Uranus’ retrograde spin.

3. General theory – the Nebular Model • Idea goes back to Rene Descartes (1556-1650). • Interstellar cloud of dust and gas • Slow rotation, original spherical shape • Gravitational collapse, dissipation into a plane due to conservation of angular momentum • Differing temperature environments

4. A cloud of interstellar gas a few pc, or about 1000 times bigger than Solar System The associated dust blocks starlight. Composition mostly H, He. Molecular. Cold. But collisions still cause rotational transitions in molecules – observe at mm wavelengths. Doppler shifts of lines indicate clouds rotate at a few km/s. Some clumps within clouds collapse under their own weight to form stars or clusters of stars. Clumps spin at about 1 km/s.

5. So how can you get a flat, rapidly rotating Solar System? Conservation of Angular Momentum For a spinning object: where L is angular momentum, I is moment of inertia, and Ω is angular rotation rate (of spinning or orbiting object). For uniform density sphere, In general So if R decreases, Ω increases. (For orbiting object, is conserved.)

6. Cloud starts collapsing under its own gravity. It’s pressure cannot support it. The cloud spins more rapidly as it collapses (conservation of angular momentum). We observe these now in star forming regions, using the Hubble.

7. How does the nebular model explain planets and “debris”? • Solar Nebula composed of 71% H (by mass), 27% He, traces of heavier elements in gas, and dust grains (only about 2% of mass). • After collapse, now so dense that solid material can grow by collisions and accretion. In warm inner nebula, growth of dust grains. • Further from Sun, ice chunks, and ice mantles on dust grains, formed readily (lots of gas to make ice from => much more solid material). But most matter still gas.

8. Condensation temperature

9. The "frost line" • Rock and metals forms where T<1300 K • Carbon grains and ices form where T<300 K • Inner Solar System is too hot for ices and carbon grains • In the Outer Solar System carbon grains and ices form beyond the "frost line"

10. Temperature distribution in Solar Nebula at time of formation of the planets

11. Grains => planetesimals => protoplanets The planets formed by the collision and sticking of solid particles, leading to km-scale planetesimals (few 106 yrs). Collisions of planetesimals enhanced by gravity, growth of proto-planets. Larger ones grew faster – end result is a few large ones.

12. Hubble observation of disk around young star with ring structure. Unseen planet sweeping out gap?

13. Terrestrial planets • Only rocky planetesimals inside the frost line • Energy of collisions heats growing protoplanets. Along with heat from radioactivity, they become molten • Hotter close to the Sun => protoplanets cannot capture H, He gas and retain thick atmosphere. • Solar wind also dispersing nebula from the inside, removing H & He => Rocky terrestrial planets with few ices

14. Jovian planets • Addition of ices increases masses of grains - large proto-planets of rock and H compounds result (few to 15 MEarth) • Larger masses & colder temps: can accrete H & He gas from the solar nebula • Planets with biggest cores grow fast, thereby increasing gas accretion • Form large Jovian planets with massive cores of rock and ice and heavy H, He atmospheres • Alternative: formed directly and rapidly by gravitational collapse in disk. Denser material sunk to center. Should take 100’s to 1000’s of years only!

15. Moons & Asteroids Some gas attracted to proto-Jovians formed disks: • Mini solar nebula around Jovians • Rocky/icy moons form in these disks (later, more detail on four Galilean moons of Jupiter as mini “solar” system) • Later moons added by asteroid/comet capture Asteroid Belt: perhaps a planet was going to form there. But Jupiter's strong gravity disrupted planetesimals' orbits, ejecting them from this zone. The Belt is the few left behind.

16. Icy bodies and comets • Leftover bodies from planet building in Jovian planet zone. Hence more icy than asteroids. • Gravitational encounters with giant planets flung them outwards beyond Neptune. • Result is TNOs, some of which encounter Neptune and sent into inner Solar System, where they appear as comets.

17. What evidence do we have for the Nebular model? • Theory is reasonable, but needs to make predictions to compare with observations. Saw some evidence from other forming systems. • But what about other formed planetary systems?

18. Planets around other stars • Test solar system formation process • Possibility of life on other planets Techniques: • Direct detection (images) • Transit of star by planet • Detection of star’s wobble by spectroscopy • Detection of star’s wobble by imaging • Microlensing

19. Detecting a star’s wobble Idea: a planet and its star both orbit around their common center of mass, staying on opposite sides of this point.

20. The massive star is closer to center of mass, and moves more slowly than the planet, but it does move! Example: Sun and Jupiter orbit their common center of mass every 11.86 years. Jupiter’s orbit has semi-major axis of 7.78 x 108 km, while Sun’s semi-major axis is 742,000 km. (Compare to Sun’s radius of 696,000 km).

21. A wobbling star might be seen by careful observations of its position, called “astrometry”:

22. More successful method: Use the Doppler shift of star’s spectral lines due to its radial (= back and forth) motion:

23. Calculate orbital speed of Sun, assuming Jupiter is only planet: Moves in circular orbit of radius 742,000 km How much Doppler shift? Consider H-alpha absorption line, at rest wavelength 656 nm: This is tiny!

24. 51 Pegasi • Mayor & Queloz at Geneva Observatory saw observed wobble in star 51 Peg in 1995 • Sun-like star ~40 ly distance • Wobble was 53 m/s, period 4.15 days • Implied a planet with 0.5 Jupiter mass orbiting at 0.05 AU! • First planet found around sun-like star

25. Selection effects • Doppler wobble biased towards massive planets close to their star (leads to larger velocities and shorter periods). Now getting close to Earth-mass planets. • Limited by the orbital speed sensitivity (few m/s but always improving) and length of orbital period: for more than several year periods, hard to tell if motion is periodic • Inclination of binary orbit unknown (unless transits observed). More likely to be close to edge-on for detection. If not, wobble is larger than measured and so is planet.

26. Extrasolar planet count is 463 as of June, 2010! 370 by Doppler wobble method. See: exoplanet.eu

27. Characteristics of detections • Hot Jupiters on small orbits (migration or formation?) • Very elliptical orbits (in the Solar System this is a sign of perturbation => supports migration idea?) • Rare around low-mass stars (smaller disks thus less material) • Rare around metal-poor stars

28. Extrasolar planet 3 – 4 Earth masses (Gliese 581g) found in star’s habitable zone by Doppler method – Sept 2010 g f

29. Planetary transits If orbital plane of extrasolar planet is aligned with the line of sight, planet will transit face of parent star

30. Dims by about 1-2% during the transit • Requires precision photometry, better done from space • Can extract info on planet’s atmosphere, size, density! 87 of these as of June 2010

31. Kepler transit mission. Should find warm, Earth-mass planets – launched March 2009 and is working. also CoRoT mission in Europe

32. Gravitational microlensing • If two stars line up, one near and one far, the light from the background star will bend around the foreground star (due to gravity) • A planet around the foreground star will cause an intense amplification if passing close to the line of sight • Nine thus detected as of June 2010

34. Direct Imaging 13 found this way as of June 2010

37. For a little fun, see: http://tauceti.sfsu.edu/~chris/SIM/ http://vo.obspm.fr/exoplanetes/encyclo/catalog.php

38. Problem 5.31 • The bright star Sirius in the constellation of Canis Major has a radius of 1.67 Rsolar, and a luminosity of 25 Lsolar. • Use this information to calculate the energy flux at the surface of Sirius. • Use your answer in part a) to calculate the surface temperature of Sirius.

39. Atmosphere composition • 78% N2, 21% O2, 1% everything else • Initial atmosphere had more H, CH4, NH3 • Composition depends on • Original formation (what gases were trapped) • Chemical processes (including life) • Escape speed

40. Molecular weights Hydrogen 2 Helium 4 Methane 16 Ammonia 17 Water 18 Neon 20 Nitrogen 28 Oxygen 32 Argon 40 CO2 44 Air: 29 (Ar, CO2, Ne, He and other rare gases). Early atmosphere: CO2, steam, NH3 and CH4 (volcanoes). No oxygen. Steam condensed to water => seas Photosynthesis: CO2 and H2O to O2. NH3 and CH4 reacted with O2.

42. Problem 7.24 • Find the mass of a hypothetical spherical asteroid 2 km in diameter and composed of rock with average density 2500 kg m-3. • Find the speed required to escape from the surface of this asteroid. • A typical jogging speed is 3 m/s. What would happen to an astronaut who decided to go for a jog on this asteroid?