Origin of the Solar System

1. Origin of theSolar System Chapter 19

2. Objectives • What is the theory for the origin of the solar system? • What are the observed properties of the solar system that the theory of its origin can explain? • How do planets form? • What do astronomers know about other planetary systems?

3. EarlyHypotheses • Catastrophic • Sudden events, like passing stars • Solar systems would be rare • Evolutionary • Common, gradual events • Most stars would have solar systems

4. Solar Nebula Hypothesis • Planets form at the same time, from the same dust cloud as the star • Planets are by-products of star formation

5. Dust Disks in Orion Nebula

6. Characteristic Properties • The solar system is almost entirely empty space. • If it were scaled down, you might see the following: • Sun – size of a plum • Mercury, our Moon, & Pluto – speck of pepper • Venus, Earth, & Mars– grain of salt • Jupiter – apple seed • Saturn – smaller apple seed • Uranus & Neptune – oversized salt grain

7. Rotation & revolution follow a disk theme with only few exceptions, which can be explained.

8. Planetary Orbits 0 All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane (ecliptic) Orbits generally inclined by no more than 3.4o Mercury Venus Sense of revolution: counter-clockwise Mars Exceptions: Mercury (7o) Jupiter Earth Sense of rotation: counter-clockwise (with exception of Venus, Uranus, and Pluto) Uranus Saturn Neptune (Distances and times reproduced to scale)

9. Two types of planets Terrestrial & Jovian • The 4 inner planets are very different from the 4 outer planets. • Almost all solid surfaces are covered in craters. • Planets have characteristic rings, clouds, and moons.

10. Space Debris… planets are not the only objects orbiting the Sun • Asteroids • Comets • Meteoroids

11. Asteroids • Sometimes called minor planets • Irregular, rocky bodies ranging in size up to 60 miles in diameter • Left over material from a planet that failed to form between Mars & Jupiter (the asteroid belt – 2.8 AU) • Also found in the Kuiper (KI-per) Belt beyond Neptune & Pluto

12. Asteroid Pics

13. Comets • Icy nucleus, which evaporates and gets blown into space by solar wind pressure • Mostly objects in highly elliptical orbits, occasionally coming close to the sun

14. Comet Pics • Hale-Bopp • Halley • Last seen 1986 • Will return about every 74 yrs

15. Meteors, Meteoroids, & Meteorites • Meteoroid – small dust grains (usually less than 1 gram) in the solar system, theycan tell us the age of the solar system • Meteor – commonly called a “shooting star” • Meteorite – any part that survives the trip through our atmosphere and lands on Earth

16. Meteorite Pics

17. The Age of the Solar System 0 Sun and planets should have about the same age Ages of rocks can be measured through radioactive dating: Measure abundance of a radioactively decaying element to find the time since formation of the rock Dating of rocks on Earth, on the Moon, and meteorites all give ages of ~ 4.6 billion years

18. Planet Building 0 Planets formed from the same protostellar material as the sun, still found in the Sun’s atmosphere Rocky planet material formed from clumping together of dust grains in the protostellar cloud Mass of more than ~ 15 Earth masses: Mass of less than ~ 15 Earth masses: Planets can grow by gravitationally attracting material from the protostellar cloud. Planets can not grow by gravitational collapse. Terrestrial planets (Earthlike) Jovian planets (gas giants)

19. Condensation of Solids • The density of the planets tends to follow a pattern. The planets closer to the Sun tend to be more dense. • The temperature near the protostar (Sun) would be higher so compounds with high melting points would be able to condense there. • As the distance from the protostar decreases, the temperature also decreases. Materials with a lower melting point would be able to condense there. • The temperature of the nebula would have decreased over time. This would explain how some planets could have a rocky interior and a low-density gas exterior.

20. Planetesimals • Planet formation starts with clumping together of grains of solid matter by condensation & accretion. • The larger clumps are called planetesimalsand range in size from a few cm to a km in size. • Planetesimals stick together and will eventually form a protoplanet.

21. The Growth of Protoplanets 0 Simplest form of planet growth: Unchanged composition of accreted matter over time As rocks melted, heavier elements sank to the center differentiation This also produces a secondary atmosphere outgassing Improvement of this scenario: Gradual change of grain composition due to cooling of nebula and storing of heat from potential energy

22. The Jovian Problem 0 Two problems for the theory of planet formation: 1) Observations of extrasolar planets indicate that Jovian planets are common. 2) Protoplanetary disks tend to be evaporated quickly (typically within ~ 100,000 years) by the radiation of nearby massive stars Too short for Jovian planets to grow! Solution: Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky planetesimals.

23. Extrasolar Planets 0 Modern theory of planet formation is evolutionary Many stars should have planets! planets orbiting around other stars = “Extrasolar planets” Extrasolar planets can not be imaged directly. Detection using same methods as in binary star systems: Look for “wobbling” motion of the star around the common center of mass

24. Direct Detection of Extrasolar Planets 0 Only in exceptional cases can extrasolar planets be observed directly. Preferentially in the infrared: Planets may still be warm and emit infrared light; stars tend to be less bright in the infrared than in the optical

25. Project • Each group will research the assigned planet. • Pluto has been included even though it has been down-graded to a “dwarf planet”. • The group who teaches us about Pluto should address why it was down-graded. • There are several other Dwarf Planets in our solar system. • Eris (2005 – Kuiper Belt) • Haumea (2004 – Kuiper Belt) • Makemake (2005 – Kuiper Belt) • Ceres (1801 – Asteroid Belt)