Chapter 12 The Origin of the Solar System Architecture of the solar system Planets Asteroids Comets Meteors and meteor

1. ASTA01 @ UTSC – Lecture 12 • Chapter 12The Origin of the Solar System • Architecture of the solar system • Planets • Asteroids • Comets • Meteors and meteoroids

2. A Survey of the Planets • To explore consequences of the solar nebula theory, astronomers search the present solar system for evidence of its past. • You should begin with the most general view of the solar system. • It is almost entirely empty space.

3. A Survey of the Planets • Imagine that you reduce the solar system until Earth is the size of a grain of table salt – about 0.3 mm in diameter. • The Sun is the size of a small plum 4 m from Earth. • Jupiter is an apple seed 20 m from the Sun. • Neptune, at the edge of the solar system, is a large grain of sand located 120 m from the central plum.

4. A Survey of the Planets • You can see from the rescaled models of the solar system that planets are tiny specks of matter scattered around the Sun, the last significant remains of the solar nebula.

5. Revolution and Rotation • The planets revolve around the Sun in orbits that lie close to a common plane. • The orbit around the Sun of Mercury, the planet closest to the Sun, is tipped 7.0°to Earth’s orbit. • The rest of the planets’ orbital planes are inclined by no more than 3.4°. • The solar system is basically ‘flat’ and disk-shaped.

6. Revolution and Rotation • The rotation of the Sun and planets on their axes also seems related to the same overall direction of motion. • The Sun rotates with its equator inclined 7.2° to ecliptic • Most planets’ equators are tipped less than 30°.

7. Revolution and Rotation • However, the rotations of Venus and Uranus are peculiar. • Compared with the other planets, Venus rotates backward. • Uranus rotates on its sides, with the equator almost perpendicular to its orbit.

8. Revolution and Rotation • The preferred direction of motion in the solar system (counterclockwise as seen from the north) is related to the rotation of a disk of material that became the planets. • All the planets revolve around the Sun in that direction.

9. Revolution and Rotation • Furthermore, nearly all the moons in the solar system, including Earth’s moon, orbit around their planets counterclockwise. • With only a few exceptions, most of which are understood, revolution and rotation in the solar system follow a common theme. Triton Neptune & Triton

10. Two Kinds of Planets 1. Four small Earth-like worlds, called terrestrial planets

11. Two Kinds of Planets 2. Four giant Jupiter-like worlds.

12. Two Kinds of Planets • The difference is so dramatic that you are led to say, • “Aha, this must mean something!”

13. Two Kinds of Planets • There are three important points to note about these categories. Terrestrial Jovian

14. Two Kinds of Planets • One, they are distinguished by their location. • The four inner planets are quite different from the outer four.

15. Two Kinds of Planets • Two, almost every solid surface in the solar system is covered with craters: • Terrestrial planets and moons have solid surfaces, while giant planets don’t have a visible surface (in fact, they don’t have surfaces)

16. Two Kinds of Planets • Three, the planets are distinguished by properties such as composition, rings, and moons. Rocky Gaseous

17. Two Kinds of Planets • The division of the planets into two families is a clue to how our solar system formed. • The present properties of individual planets, however, don’t reveal everything you need to know about their origins. • The planets have all evolved since they formed. • For further clues, you can look at smaller objects that have remained largely unchanged since the birth of the solar system.

18. Asteroids • The first asteroid (gr.: asteroidos = star-like) was Ceres, ~500 km diameter body discovered in 1801 • It is covered by ice and clay, and may have liquid water between the rocky core and icy crust Ceres filled an empty slot for m=3 in the so-called Titius-Bode law of planetary distances, but now is not considered a natural law any more. • This rule can be expressed as • a = (4 + 3 ⋅ 2m) AU /10 for m = -∞, 0, 1, 2, 3, 4, 5, 6..

19. Titius-Bode rule: 1766 • Astronomy professor Johann Daniel Dietz (Titius) from Wittenberg (Germany) inserted his observation on planetary distances into a German translation of Charles Bonnet's book Contemplation de la Nature • Johann E. Bode reformulated and popularized Titius rule in his textbook on astronomy in 1772

20. Titius-Bode rule and its role in planet discoveries • T-B rule was considered interesting but not important until Uranus was accidentaly discovered in 1781, and it fit the well! • Predictions of new planets were made Soon Ceres was found. G. Piazzi (1801) - W. Herschel (1781) -

21. Titius-Bode rule and its role in planet discoveries • T-B rule was considered interesting but not important until Uranus was discovered in 1781, and it fit the law rather precisely! • Predictions of new planets were made G. Piazzi (1801) - W. Herschel (1781) -

22. Asteroids • Why Titius-Bode law eventually fell out of favor and is now called a “Titius-Bode rule”: Neptune and Pluto fail to obey it. • However, the distribution of • Satellite systems and extrasolar planets resembles the power-law form of Titius rule to some extent : a ~ cn, where c=const different for different systems

23. T-B rule in exoplanetary (i.e., extrasolar) systems? • However, the distribution of many satellite systems and extrasolar planets resembles somewhat a power-law form of Titius rule to some extent : a ~ cn, where c=const different for different systems. • Why? We don’t fully know. • There are many exceptions.

24. Space Debris: Planet Building Blocks • The solar system is littered with three kindscontains four kinds of space debris (officially known as minor bodies): • Asteroids, • Comets, and • Meteoroids • Planetoids (dwarf planets) like Pluto, Eris, Sedna, Quaoar and other large, round bodies • Although these objects represent a tiny fraction of the mass of the system, they are a rich source of information about the origin of the planets.

25. Asteroids - minor planets Small rocky worlds. • Most of them orbit the Sun in a belt between the orbits of Mars and Jupiter. • Roughly 20 000 asteroids asteroid belt (white)have been catalogued • Most in asteroid belt • They are smaller than the Moon Vesta 525 km diameter Ceres 950 km Moon 3470 km

26. Asteroids do have moons: Ida and its moon Dactyl

27. Asteroids belong to orbital families: Ida is in the Koronis family, a group of 300 minor bodies all of which have a = 2.86…2.89 AU , e=0.01…0.09, i=1o…3o Asteroids in a family come from a disruption of a common ancestor asteroids This happened when two asteroids collided in the asteroid belt between Mars and Jupiter >2 Gyr ago Asteroids are as old as planets, actually planets formed from asteroid-like primitive bodies called planetesimals Florentina Lacrimosa (41 km) Elvira Ida Urda (40 km) Koronis Nassovia

28. Vesta • Spacecraft Dawn has orbited in Jul 2011- Sep 2012 asteroid Vesta; a=2.36 AU, e=0.088, P=3.63 yr, diameter 525 km, surface gravity acceleration = (1/40) g

29. Asteroids • About 2000 follow orbits that bring them into the inner solar system, where they can occasionally collide with a planet. • Earth has been struck many times in its history.

30. Asteroids • Other asteroids share Jupiter’s orbit: the Greeks and Trojans • Some other have been found beyond the orbit of Saturn.

31. Asteroids • About 200 asteroids are more than 100 km in diameter. • Tens of thousands are estimated to be more than 10 km in diameter. • There are probably a million or more that are larger than 1 km and billions that are smaller.

32. Asteroids • As even the largest are only a few hundred kilometres in diameter, Earth-based telescopes can detect no details on their surfaces. • The Hubble Space Telescope can image only the largest features.

33. Asteroid Eros • Photos returned by robotic spacecraft such as NEAR Shoemaker in 2000-2001 (which landed as a 1st on asteroid surface) and space telescopes show that asteroids are generally irregular in shape and battered by impact cratering.

34. Asteroids • Some asteroids appear to be rubble piles of broken fragments. • A few are known to be double objects or to have small moons in orbit around them. • These are understood to be evidence of multiple collisions among the asteroids.

35. Asteroids • A few larger asteroids show signs of volcanic activity on their surfaces that may have happened when the asteroidwas young.

36. Asteroids • Astronomers recognize the asteroids as debris left over by a planet that failed to form at a distance of about 2.5-3 AU from the Sun. • A good theory should explain why a planet failed to form there, leaving behind a belt of construction material.

37. Kirkwood gaps in asteroid belt • Daniel Kirkwood (1857)

38. Comets • In contrast to the rocky asteroids, the brightest comets are impressively beautiful objects. Most comets are faint and are difficult to locate even at their brightest. • But in 2013 a very bright comet is expected (mV= -14m !)

39. Comets • A comet may take months to sweep through the inner solar system. • During this time, it appears as a glowing head with an extended tail of gas and dust.

40. Comet Hartley 2, visited in Nov. 2010 by EPOXI • The beautiful tail of a comet can be longer than 1 AU. • However, it is produced by an icy nucleus only a few tens of kilometres in diameter. Nucleus is covered with a dark crust, which breaks at places allowing gas (H2O +CO+…) + dust + sand + stones, to escape in jets.

41. Comets: Halley’s comet The nucleus remains frozen and inactive while it is far from the Sun. • As the nucleus moves along its elliptical orbit into the inner solar system, the Sun’s heat begins to vapourize the ices, releasing gas and dust.

42. Comets • The pressure of sunlight and the solar wind push the gas and dust away, forming a long tail. Peter Apian’s drawing from 1532 shows that the tail always points away from the sun. (He drew comet Halley.)

43. Comets • The gas and dust respond differently to the forces acting on them. • So, they often separate into two separate sub-tails. Comet Hale-Bopp in 1995

44. Comets • Comet nuclei contain ices of water and other volatile compounds such as carbon dioxide, methane, and ammonia. • These ices are the kinds of compounds that should have condensed from the outer solar nebula. The comets never fully melted. • That makes astronomers think that comets are ancient samples of the gases and dust from which the outer planets formed.

45. Comets • Five spacecraft flew past the nucleus of Comet Halley when it visited the inner solar system in 1985 and 1986. • Since then, spacecraft have visited the nuclei of several other comets. • Images show that comet nuclei are irregular in shape and very dark, with jets of gas and dust spewing from active regions on the nuclei.

46. Comets

47. Comets • In general, crusts of these nuclei are darker than a lump of coal. • This suggests that they have composition similar to certain dark, water- and carbon-rich meteorites.

48. Comets • Since 1992, astronomers have discovered roughly a thousand small, dark, icy bodies orbiting in the outer fringes of the solar system beyond Neptune.

49. Comets • This collection of objects is called the Kuiper belt. • It is named after the Dutch-American astronomer Gerard Kuiper, who predicted their existence in the 1950s.

50. Comets • There are probably 100 million bodies larger than 1 km in the Kuiper belt. • Any successful theory should explain how they came to be where they are. • We will return to them later in this course