4. Formation of Solar Systems

Transcription

1 Astronomy 110: SURVEY OF ASTRONOMY 4. Formation of Solar Systems 1. A Survey of the Solar System 2. The Solar System s Early History 3. Other Planetary Systems

2 The solar system s rich and varied structure points to its formation from a gas cloud collapsing due to selfgravity some 4.5 billion years ago. In this proto-solar system, small grains of solid matter clumped together, eventually producing the planets we see today. Other stars with planets have now been observed; a wide variety of planetary systems can arise when stars form.

3 1. A SURVEY OF THE SOLAR SYSTEM a. Overview of the solar system b. A brief tour of the premises c. Clues to its formation

4 Overview: Structure 100 AU S 5 AU 20,000 AU Inner system: terrestrial planets, asteroids. Outer system: giant planets and moons, KBOs. Oort Cloud: comets.

5 Overview: Motion All planets move in the same direction in nearly circular orbits; most rotate in the same direction as they orbit.

6 Overview: Planetary Types rocky & metallic terrestrial planets (inner system) hydrogen-rich jovian planets (outer system)

7 A Brief Tour: The Sun Over 99.9% of solar system s mass Largely H and He

8 A Brief Tour: Mercury Large iron core and desolate rock crust

9 A Brief Tour: Venus Earth s twisted sister Extreme greenhouse effect

10 A Brief Tour: Earth Liquid surface water! Complex and dynamic atmosphere Remarkably large moon

11 A Brief Tour: Mars Cold & dry, but water flowed long ago Complex and interesting topography

12 A Brief Tour: Jupiter Largest gas-giant planet no solid surface! Many different moons, some as big as planets

13 A Brief Tour: Saturn Spectacular ring system Titan, a moon with atmosphere

14 A Brief Tour: Uranus Water and other hydrogen compounds Extreme axis tilt

15 A Brief Tour: Neptune Most distant major planet Large moon with backwards orbit

16 A Brief Tour: Pluto and Other Icy Dwarfs Tiny compared to other planets Ices of H2O & other H compounds Many still to be discovered!

17 XXX (g/cm 3 )

18 Clues to Formation: Motion All planets and most moons orbit in the same direction and stay close to the same plane. The sun and most planets also rotate in that direction.

19 Conservation Laws: Angular Momentum An object with mass m moving in a circle of radius r with velocity v has angular momentum mvr. If there is no torque (roughly speaking, twisting force) on the object, its angular momentum is conserved. Wikipedia: Kepler s Laws

20 Clues to Formation: Two Types of Planets Terrestrial Planets Jovian Planets small size & mass large size & mass high density low density rock & metal H, He, H2O, CH4, NH3,... solid surface no solid surface few moons, no rings many moons & rings close to sun, warm far from sun, cold

21 Clues to Formation: Small Objects Asteroids: small (< 1000 km) rock & metal (?) inner system Dwarf planets: small (~ 1000 km) ice & rock Kuiper belt Comets: small (~ 10 km) ice & rock Kuiper belt & Oort cloud

22 Clues to Formation: Exceptions to the Rules Rotation of Venus: Reverse direction Extremely slow Rotation of Uranus: Axis tipped 98 Earth s Moon: Very large for our planet Iron deficiency Neptune s moon Triton: Reverse orbit direction

23 A SURVEY OF THE SOLAR SYSTEM: REVIEW 1. Ordered motion of Solar System Orbits & spins are fossils of motion in early Solar System. 2. Two types of planets Terrestrial planets near Sun, jovian planets further away. 3. Numerous small objects Asteroids in the inner system, icy dwarfs & comets further out. 4. Exceptions to the rules Earth s big Moon, and unusual spins and orbits of some objects.

24 2. THE SOLAR SYSTEM S EARLY HISTORY a. Collapse of the solar nebula b. Planet formation: the frost line c. The age of the solar system

25 The Nebular Theory

26 Collapse: Galactic Recycling Our solar system formed from gas which had already been cycled through many generations of stars. Each cycle increased the amount of heavy elements.

27 Collapse: Birthplace of the Solar System Hubble s sharpest image of the Orion Nebula

28 Hubble s sharpest image of the Orion Nebula

29 Collapse: From Cloud to Disk 1. A gas cloud starts to collapse due to its own gravity. 2. It spins faster and heats up as it collapses. 3. Vertical motions die out, leaving a spinning disk. 4. The solar system still spins in the same direction.

30 Collapse: Angular Momentum and Energy 1. Angular momentum conservation causes the cloud to spin faster as it contracts: (rotation speed) 1 (cloud diameter) Collapse stops when the cloud spins at orbital speed. 2. Energy conservation causes the cloud to heat up: potential energy kinetic energy thermal energy gravitational collapse gas shocks

31 1. What would have happened if the gas cloud had been rotating a little faster to begin with? A. The cloud would collapse even more before reaching orbital speed. B. The cloud would collapse a little less before reaching orbital speed. C. The cloud would fly apart instead of collapsing. D. The cloud would fall straight inward and never form a disk.

32 2. If the cloud collapsed further before forming a disk, would it be hotter or colder? A. Hotter, because more gravitational energy would be released. B. Colder, because less gravitational energy would be released. C. Neither hotter nor colder.

33 Collapse: Disks Around Other Stars We can see disks around other stars, as expected if these stars formed from collapsing gas clouds.

34 Planet Formation At the end of the collapse phase, the solar nebula was a uniform mixture of different materials. Some of these materials began condensing out of the gas.

35 Planet Formation: The Frost Line The disk was hot at the center, and cool further out. Inside the frost line, only rocks & metals can condense. Outside, hydrogen compounds can also condense. The frost line was between the present orbits of Mars and Jupiter roughly 4 AU from the Sun.

36 Planet Formation: Terrestrial Planets 1. Within the frost line, bits of rock and metal clumped together to make planetesimals. 2. As the planetesimals grew, they became large enough to attract each other. 3. Finally, only a few planets were left.

37 Planet Formation: Jovian Planets 1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth. 2. These planets attracted nearby gas, building up giant planets composed mostly of H and He. 3. The disks around these planets produced moons.

38 Planet Formation: Asteroids and Comets Leftovers from early stages of planet formation Asteroids form inside frost line, comets outside Scattered by jovian planets into present orbits

39 Planet Formation: Explaining the Exceptions 1. Giant impacts in early solar system: explain rotation of Venus, Uranus form Moon from collision debris 2. Satellite capture after near-miss: moons of Mars captured from asteroid belt Triton captured from Kuiper belt

40 3. How would the solar system be different if the nebula had been cooler? A. Jovian planets would form closer to the Sun. B. There would be no asteroids. C. There would be no comets. D. Terrestrial planets would be larger.

41 4. Which of these facts is not explained by the nebular theory? A. There are two kinds of planets: terrestrial and jovian. B. All planets orbit in the same direction. C. Asteroids and comets are common. D. There are four terrestrial and four jovian planets.

42 The Age of the Solar System Radioactive elements decay into stable ones; e.g., 40 K 40 Ar + e + (Potassium-40) (Argon-40) (positron) The rate of decay is fixed by the element s half-life, the time for 50% to decay; for 40 K, this time is 1.25 Gyr (1 Gyr = 1 billion years). Rocks contain no 40 Ar when they form; by measuring the ratio of 40 Ar to 40 K, the rock s age can be found.

43 5. If a mineral has Ar atoms for every 40 K atom, how old is it? A. about 3 Gyr B. about 4 Gyr C. about 5 Gyr D. about 6 Gyr E. about 7 Gyr

44 The Age of the Solar System: Dating Rocks The oldest Earth minerals are 4.4 Gyr old. The oldest Moon rocks are also 4.4 Gyr old. The Cartoon History of the Universe The oldest meteorites are 4.55 Gyr old; this is how long ago minerals started condensing in the disk.

45 THE SOLAR SYSTEM S EARLY HISTORY: SUMMARY a. Collapse of the solar nebula b. Planet formation: the frost line c. The age of the solar system

46 3. OTHER PLANETARY SYSTEMS a. How to find em b. What we find c. What it means

47 How To Find Other Planetary Systems 1. The Doppler Technique measure effect of planet on motion of star can detect systems with multiple planets 2. Transits and Eclipses measure dimming of star s light by planet can be done using small telescopes 3. Direct Detection seeing is believing, but hard to do need for accurate analysis of planetary surfaces

48 The Doppler Method: Gravitational Tug-Of-War As a planet orbits, the star must move slightly in response (Newton s 3 rd law). The combined effect of eight planets (mostly J. & S. ) makes the Sun dance around. These motions are small how can we detect them?

49 The Doppler Shift Doppler Effect Doppler Effect A stationary source sends out waves of the same wavelength in all directions. If the source is moving, the waves bunch up ahead of its motion, and spread out behind.

50 The Doppler Shift: Light We get a similar effect with light. The change in wavelength λ depends on the source s velocity v toward or away from us: red-shift blue-shift λshift - λrest λrest = v c where λshift is the observed (shifted) wavelength, λrest is the wavelength with the source at rest, and c is the speed of light.

51 The Doppler Shift: Astronomy Applications 1. All lines in a spectrum shift by same amount: Hydrogen lines in lab (no shift). Shift to red star receding. Shift to blue star approaching. 2. No shift from sideways motion.

52 The Doppler Method A planet orbiting a star induces alternating red and blue shifts in the star s spectrum (unless orbit is face-on). 51 Pegasi These tiny shifts can be used to find the planet s mass and the properties of its orbit.

53 Transits and Eclipses A star with a planet in an edge-on orbit dims slightly every time the planet crosses (transits) its face. HD Pegasi Half an orbit later, there s a slight drop in the combined brightness as the star hides (eclipses) the planet.

54 Direct Detection Hubble Directly Observes Planet Orbiting Fomalhult

55 What We Find: Orbits Many planets found so far orbit very close to their central stars. Some orbit in only a few days! Many have very elliptical orbits. These systems are very different from our solar system!

56 What We Find: Masses and Orbital Periods Most planets found Hot Jupiters so far are even more massive than Jupiter. Transit and doppler methods tend to find planets near stars. M (MJ) Systems like ours are hard to detect direct detection doppler method transit method P (yr) Wikipedia: Extrasolar planet

57 What We Find: Hot Jupiters Many known because they are easy to find they may not be especially common.

58 What it Means 1. Are Earth-like planets rare or common? hot (or warm) jupiters would disrupt Earth s orbit many stars do not have hot (or warm) jupiters not yet possible to detect Earth-like planets 2. Do Hot Jupiters imply the Nebular theory is wrong? jovian planets cannot form inside frost line planetary migration may explain this puzzle

59 Planet Migration A planet embedded in a disk around a star can excite spiral waves this process robs the planet of angular momentum, causing it to spiral inward.

60 Planet Migration 1. Can explain hot jupiters and eccentric orbits migration can move planets very close to star encounters between planets disturb orbits 2. Why didn t this happen in our solar system? disk cleared by Sun s wind or external effects some migration may be needed to form Oort cloud