Solar System Formation

1. Solar System Formation

2. Age of the Solar System The oldest rocks found on Earth are about 4.55 billion years old, not native but meteorites which fall from space. The oldest native Earth rocks are 3.85 billion years old.

3. Radioactive dating Unstable parent isotopes decay at a constant rate to stable daughter isotopes. By measuring how much of the parent isotope is still present, and how much of the daughter isotope there is, we can calculate the age of the rock.

4. parent daughter ½ life (millions of years)

5. Discussion The age of a rock given by radiometric dating is the time since the rock last solidified. What happens to the lighter daughter elements when the rock is in a molten or gaseous phase?

6. Formation of the Solar System The nebular hypothesis The Sun and planets formed from the gravitational collapse (possibly triggered by nearby supernova) of a single, spherical, slowly rotating cloud of cold interstellar gas and dust.

7. Discussion Why does the gas cloud need to be cold?

8. Consequence Planet formation is a natural outcome of star formation. Planetary systems should be common.

9. Dynamics of the Planets • The planets revolve counterclockwise around the Sun as viewed from above the Sun’s north pole, the same direction that the Sun rotates on its axis.

11. Discussion Why can’t all the planets just orbit in any arbitrary direction? Why should they all go around the Sun in the same way?

12. 2. The major planets have orbital planes that are only slightly inclined with the plane of the equator of the Sun’s rotation, i.e. the orbits are coplanar.

13. Orbital inclination

14. Orbital Inclination

15. Discussion What does this mean for the paths of the planets through the sky?

16. Discussion If the solar nebula started as a spherical cloud why do all the planets lie in a plane above the Sun’s equator. Shouldn’t they be spherically distributed about the Sun?

17. Gravity can collapse a rotating cloud only along the axis of rotation.

18. Discussion What two changes take place as the solar nebula collapses due to gravity?

23. 3. The planets move in elliptical orbits that are very nearly circular.

24. Eccentricities

25. Discussion Why are all the orbits nearly circular? What happens to planets that formed with highly eccentric (very elliptical) orbits?

27. 4. The planets rotate counterclockwise as viewed from above the north pole, the same direction as they revolve, except for Venus and Uranus.

28. Rotation of the Planets Period (days) Axis tilt

29. Discussion Why do you think the planets rotate in the same direction and why is this direction in the same sense as the planets orbit the Sun?

30. Discussion Unlike all the other planets Venus rotates backward. How would the diurnal and yearly motion of the Sun differ on Venus compared to that of the Earth?

31. Discussion Is the solar day longer or shorter than the sidereal day on Venus?

32. Discussion Which planet will have the most extreme seasons?

33. 5. The Planets’ orbital distance from the Sun follows a regular spacing. Titius-Bode rule Write down 0, 3, 6, 12, … each number, after the first, being double the previous value. Add 4 to each and divide by 10.

34. Titius-Bode Rule and Distance Distance AU T-B distance AU

35. A packed Solar System? The solar system may be as densely packed as possible. There do not appear to be any orbits stable over the lifetime of the solar system between the current planets.

36. 6. Most satellites revolve in the same direction as their parent planet’s rotation and lie close to their parent planet’s equatorial plane An exception is Neptune’s Triton

37. Discussion How would you explain this observation with our formation theory of the Solar System?

38. 7. The Sun contains 99.8% of the solar systems mass but only 0.5% of the angular momentum

39. Discussion If the Sun formed from a single spherical rotating cloud, wouldn’t you expect that all the pieces would have the same angular momentum as the original cloud? How must the solar system have changed since the time of its formation that this is no longer the case?

40. Discussion Either the Sun’s rotation rate has slowed over time, or the planet’s have been spun up in their orbits. How could we decide between these two possibilities?

41. The Sun rotates once every 33 days, but should rotate once in about 2 hours if angular momentum were distributed evenly. This two hour rotation rate is common among other young solar mass stars elsewhere in the galaxy as well as higher mass stars.

42. Slowing the Sun’s rotation Magnetic breaking – The Sun’s magnetic field might interact with the early solar nebula to slow the Sun’s rotation. Strong solar winds early in the history of the Sun might have carried the extra angular momentum away.

43. 8. Long period comets come from all directions and orbital inclinations in contrast to the coplanar orbits of the planets.

44. Discussion If the long period comets can have any inclination, what does this tell you about their distribution around the Sun?

45. 2 types of planets Terrestrial planets – iron-nickel cores and silicate mantles Jovian planets – silicate/hydrogen compound (methane, ammonia, water) cores and mostly H and He mantles

46. Terrestrial & Jovian planets

47. Density Mass of the planet divided by the volume of the planet. Higher density implies a larger percentage of high density materials, such as iron and nickel, lower density implies more silicates.

48. Mean density of planets Neptune 1.6 g/cm3 Sun 1.4 g/cm3 Jupiter 1.3 g/cm3 Uranus 1.3 g/cm3 Saturn 0.7 g/cm3 Earth 5.5 g/cm3 Mercury 5.4 g/cm3 Venus 5.2 g/cm3 Mars 3.9 g/cm3 Moon 3.4 g/cm3

49. 9. Most solid planetary surfaces are heavily cratered