Chapter 8 Formation of Planetary Systems Our Solar System and Beyond

1. Chapter 8Formation of Planetary SystemsOur Solar System and Beyond

2. Where did the solar system come from? • According to the nebular theory, our solar system formed from a giant cloud of interstellar gas. • (nebula = cloud)

3. Evidence from Other Gas Clouds • We can see stars forming in other interstellar gas clouds, lending support to the nebular theory. The Orion Nebula with Proplyds

4. What caused the orderly patterns of motion in our solar system?

5. Conservation of Angular Momentum • The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted.

6. Flattening • Collisions between particles in the cloud caused it to flatten into a disk.

7. The spinning cloud flattens as it shrinks. Formation of the Protoplanetary Disk

8. Disks Around Other Stars • Observations of disks around other stars support the nebular hypothesis.

9. Why are there two major types of planets?

10. Conservation of Energy As gravity causes the cloud to contract, it heats up.

11. Inner parts of the disk are hotter than outer parts. Rock can be solid at much higher temperatures than ice. Temperature Distribution of the Disk and the Frost Line

12. Fig 9.5 Inside the frost line: Too hot for hydrogen compounds to form ices Outside the frost line: Cold enough for ices to form

13. Formation of Terrestrial Planets • Small particles of rock and metal were present inside the frost line. • Planetesimals of rock and metal built up as these particles collided. • Gravity eventually assembled these planetesimals into terrestrial planets.

14. Tiny solid particles stick to form planetesimals. Summary of the Condensates in the Protoplanetary Disk

15. Gravity draws planetesimals together to form planets. This process of assembly is called accretion. Summary of the Condensates in the Protoplanetary Disk

16. Accretion of Planetesimals • Many smaller objects collected into just a few large ones.

17. Formation of Jovian Planets • Ice could also form small particles outside the frost line. • Larger planetesimals and planets were able to form from ices as well as metals • The gravity of these larger planets was able to draw in surrounding H and He gases.

18. Moons of jovian planets form in miniature disks.

19. Radiation and outflowing matter from the Sun — the solar wind — blew away the leftover gases.

20. Where did asteroids and comets come from?

21. Asteroids and Comets • Leftovers from the accretion process • Rocky asteroids inside frost line • Icy comets outside frost line

22. How do we explain the existence of our Moon and other exceptions to the rules?

23. Heavy Bombardment • Leftover planetesimals bombarded other objects in the late stages of solar system formation.

24. Giant Impact Giant impact stripped matter from Earth’s crust Stripped matter began to orbit Then accreted into Moon

25. Origin of Earth’s Water • Water may have come to Earth by way of icy planetesimals from the outer solar system.

26. Odd Rotation • Giant impacts might also explain the different rotation axes of some planets.

27. Captured Moons • The unusual moons of some planets may be captured planetesimals.

28. Review of nebular theory

29. Thought Question How would the solar system be different if the solar nebula had only been half as hot? • Jovian planets would have formed closer to the Sun. • There would be no asteroids. • There would be no comets. • Terrestrial planets would be larger.

30. When did the planets form? • We cannot find the age of a planet, but we can find the ages of the rocks that make it up. • We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

31. Radioactive Decay • Some isotopes decay into other nuclei. • A half-life is the time for half the nuclei in a substance to decay.

32. Thought Question Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form? • 1.25 billion years ago • 2.5 billion years ago • 3.75 billion years ago • 5 billion years ago

33. Dating the Solar System Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

34. Dating the Solar System • Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old. • The oldest meteorites are 4.55 billion years old. • Planets probably formed 4.5 billion years ago.

35. How do we detect planets around other stars?

36. Planet Detection • Direct: Pictures or spectra of the planets themselves • Indirect: Measurements of stellar properties revealing the effects of orbiting planets • For more check out this website: http://astro.unl.edu/naap/esp/detection.html

37. Direct Detection • Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of a very few extrasolar planets.

38. Indirect detection : Gravitational Tugs • The Sun and Jupiter orbit around their common center of mass. • The Sun therefore wobbles around that center of mass with the same period as Jupiter.

39. Gravitational Tugs • Sun’s motion around solar system’s center of mass depends on tugs from all the planets. • Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

40. Astrometric Technique • We can detect planets by measuring the change in a star’s position in the sky. • However, these tiny motions are very difficult to measure (~0.001 arcsecond).

41. Doppler Technique • Measuring a star’s Doppler shift can tell us its motion toward and away from us. • Current techniques can measure motions as small as 1 m/s (walking speed!).

42. First Extrasolar Planet Detected • Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period • Short period means small orbital distance • First extrasolar planet to be discovered (1995)

43. First Extrasolar Planet Detected • The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.

44. Thought Question Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? • It has a planet orbiting at less than 1 AU. • It has a planet orbiting at greater than 1 AU. • It has a planet orbiting at exactly 1 AU. • It has a planet, but we do not have enough information to know its orbital distance.

45. Transits and Eclipses • A transitis when a planet crosses in front of a star. • The resulting eclipse reduces the star’s apparent brightness and tells us the planet’s radius. • When there is no orbital tilt, an accurate measurement of planet mass can be obtained.

46. How do extrasolar planets compare with those in our solar system?

47. Measurable Properties • Orbital period, distance, and shape • Planet mass, size, and density • Composition

48. Orbits of Extrasolar Planets • Most of the detected planets have orbits smaller than Jupiter’s. • Planets at greater distances are harder to detect with the Doppler technique.

49. Orbits of Extrasolar Planets • Most of the detected planets have greater mass than Jupiter. • Planets with smaller masses are harder to detect with the Doppler technique.

50. Planets: Common or Rare? • Observations indicate ~2-5% of sun-like stars have Jupiter size planets • The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques. • Current estimations range between 20 and 50% of sun-like stars having planets.