PLATO - 7 The outer solar system Tethis eclipsed by Titan; Cassini (NASA) 1
Titan (Saturn s largest moon) Cold temperature weather Thick Nitrogen atmosphere Similar atmospheric pressure to Earth Liquid methane rivers and lakes? Huygens probe landed in 2005 Icy rocks on surface Signs of erosion 2
Titan (Saturn s largest moon) Huygens panorama (NASA, 2005) 3
Enceladus 4
Enceladus An ice world Young, smooth surface Much like Europa? 5
Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean 6
Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean Possible responsible for E-ring Cassini image (NASA) 7
Enceladus An ice world Young, smooth surface Much like Europa? Water vapor from surface! Possibly liquid ocean Possible responsible for E-ring Synchronous orbit Tidal heating still possible Combined with radioactive heating? 8
Neptune s moon Triton Bigger than Pluto Retrograde elliptical orbit Captured? Atmosphere: Massive Cold (far out in solar system) Eruptions? 9
Pluto 10
Pluto Discovery 1930 (Clyde Tombaugh) Based on erroneous claims of perturbations to Uranus & Neptune orbits Properties: 0.002 Earth masses High eccentricity (elliptical orbit), crosses Neptune orbit In stable 3:2 resonance with Neptune (hiding from Neptune) Density: 2 kg/liter 11
The Pluto Controversy 2006, International Astronomical Union, General Assembly: A "planet" is defined as a celestial body that A) Is in orbit around the Sun B) Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape C) Has cleared the neighborhood around its orbit. All Terrestrial and Jovian planets satisfy these criteria Objects that satisfy (A) and (B) called dwarf planets 12
Trans-Neptunian Objects Name Diameter Mass Semi-major axis Orbital Eccentricity Eris 2600 km 1.7 10 22 kg 67.8 AU 0.44 Pluto 2310 km 1.3 10 22 kg 39.4 AU 0.25 2005 FY9 ~1600 km? 45.8 AU 0.16 Sedna ~1500 km? 526. AU 0.85 2003 EL61 ~1900 1000 km 4.2 10 21 kg 43.3 AU 0.19 Charon 1210 km 1.5 10 21 kg 39.4 AU satellite of Pluto Quaoar ~1000 km? 43.6 AU 0.038 Orcus ~950 km 7.5 10 20 kg 39.4 AU 0.22 13
Trans-Neptunian Objects TNOs: Unkown, but large number of Kuyper Belt objects Crossing orbits (not cleared ) Small, icy objects Some are spherical (e.g., Eris) TNOs that are dwarf planets are called Plutoids Some TNOs have similar orbits to Pluto TNOs in 2:3 resonance with Neptune are called Plutinos 14
Comets Comet Hale-Bopp; (Rob Jones) 15
Comets Comet West 16
Comets Orbital periods From ~ 100 years to millions of years Objects from outer Solar System Kuyper Belt (TNOs) Short period comets Roughly in plane of ecliptic, short periods Oort Cloud (out to 100,000 AU) Long period comets Random orientations, long periods 17
Comet Origins Oort cloud or Kuyper belt objects Kicked onto elliptical orbits by gravitational encounters 18
Comet Structure Core: ~ 1-10 km snow ball Coma: ~ 10 5 km gas & dust cloud Halley s Comet Comet C/2001 Q4 Envelope: ~ 10 6 km Hydrogen cloud Tail: ~10 8 km gas a Comet Hale-Bopp 19
Comet Structure 20
Comet Probes Deep impact: Crashed probe into comet Tempel 1 Stardust: Collected samples from comet Wild 2 Returned them to Earth Contains crystalline material Must have formed at high temperature Early transport of matter out from inner Solar system to Kuyper belt? 21
Comet Tails Gas and dust supply: Comets spend most of their time far from Sun When they approach, they heat up Ice sublimates (goes from solid to gas) Core releases gas and dust cloud But why the tail? Recall: Tail always points away from the Sun Radiation pressure Solar wind 22
Comet Tails Radiation pressure: Photons have energy and momentum When a photon is absorbed, its momentum lives on... Both gas & dust particle pushed away from Sun Solar wind: Energetic particles and magnetic fields Streaming away from Sun Pushes only gas particles 23
Tails Pin the tail on the comet: Where is the gaseous tail going to point at the location of the comet along its path? 24
A Tale of Two Tails Why are there two tails? Gas particles Are locked to the Solar Wind magnetic field They move straight out from the Sun Dust grains Get pushed out only by radiation pressure This is like orbiting a star of lower mass (weaker gravity) Dust particles go into trailing Keplerian orbits (slower, further out) 25
Comet Tails 26
Meteor Showers Comets leave behind pebbles/dust grains In orbit around Sun When dust orbits cross Earth s obit Comet bits rain down Meteor shower Most prominent: Perseids (August 13) 27
Question Which direction would you see the asteroid coming from, given the Earth s motion and the asteroid s motion? A) B) Earth C) 28
Meteors Meteoroid: Burns up in atmosphere Meteorite: Reaches the ground 29
Asteroids Not all particles hitting Earth are small cometary dust grains... Wolf Creek Crater, Australia 30
Asteroids ~150,000 catalogued so far Sizes: Densities: Many times more uncounted for yet Few km to 1000 km (Ceres) about 3 kg/liter Loose rock 31
Asteroids Most occupy asteroid belt Between Mars & Jupiter Trojan Asteroids Jupiter Trojan Asteroids Found based on Bode s rule Planets spacing follows pattern Mars orbit Probable coincidence Jupiter s orbit Planet missing between Jupiter and Mars Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Ceres was first called a planet 32
Asteroids Jupiter has cleared out resonance gaps Small fraction of asteroids are Earth-crossing 33
Impacts 34
Impacts 0.7 miles Barringer impact crater in Arizona 35
Impacts 15 miles Ries, Bavaria (Germany) 36
Impacts 40 miles Manicougan Crater (Canada) 37
Impacts Energy: A 1 km object has the impact energy of 70 million kilotons of TNT (100 x the destructive power of all nuclear weapons during height of cold war combined). A 10 km object has 1000 x more energy yet! Barringer crater, AZ 38
Impacts Tunguska event, 1908, Siberia 5-30 megatons of TNT 39
Impacts A 10 km asteroid can lead to mass extinctions 65 Million years ago: Cretaceous (dinosaur) extinction Yukatan Peninsula 180 km Fossil records: Mass extinction coincident with deposition of Iridium all over globe Not usually found in Earth s crust, But common in asteroids 40
Extinction rate KT-event today 41
Cratering Rate Decreased with time Maria formed after cratering rate declined Late heavy bombardment? Read off from the chart how old a surface is Current rates on Earth: Size Rate: once every Energy (kilotons) < 1 cm 15 minutes 10-6k 5 m 1 yr 10 50 m 1000 yrs 10,000 1 km ~500 thousand yrs 7.5x10 7 5 km ~10 million yrs 10 10 k 10 km ~200 million yrs 7.5x10 10 k 42
Crater Structure Size ~ 10-20 x size of impacting object 43
Cratering Paintball simulation vs. Tycho (Moon) Rays 44
Comet impacts Comet Shoemaker Levy 9 45
Exoplanets Direct image of a Jovian planet around a brown dwarf sub-star 46
The Exoplanet Quest Solar nebula theory: Other stars should also form planetary disks They should be broadly similar the the Solar System Why is it hard to find planets around other stars? They are very dim They are very close to a much brighter object They have much lower masses than stars They are very small compared to stars We don t know where to look for them a priori 47
Successful Methods We have found and/or observed planets using these methods: Radial velocity detection Planetary transit observations (eclipses) Direct imaging Pulsar timing Gravitational lensing Binary timing 48
Kepler Orbits Sun moves due to planet s gravity Observe long and accurately enough: Infer masses and periods of planets Without ever having to actually see a planet! Other stars with planets must do the same thing! Look for wobbling stars 49
Radial Velocity 50
Radial velocity 51
Radial Velocity Most successful method Pioneered by Marcy & Butler Finding lots of exoplanets 494 planets to date Goeff Marcy In 414 planetary systems It can measure: Lower limit on planet mass Orbital Periods Semi-major axis Paul Butler 52
orbital axis Orbital Inclination i The angle between the orbital axis and the observer i=0 : i=90 : i=60 : No Maximum Some Doppler Doppler Doppler shift shift shift i=90 i=60 i=0 orbital axis orbital axis 53
We measure: Radial Velocity Radial velocity vrad of the star (Doppler shift) Orbital period P We know: Mass of star Mstar (from stellar structure theory) Kepler s 3rd law: a 3 planet = GM starp 2 4π 2 54
Radial Velocity Example: 51 Pegasi 51 Pegasi P = 4 days, v rad =57ms 1 M star =2.1 10 30 kg Planet mass: M 51Peg =0.47M Jupiter / sin (i) A lower limit in mass (inclination) 55
Planet Mass (Jupiter masses) Planet Demographics What kinds of planets have been found? 100.0 10.00 539 planet systems total Mostly massive planets Mostly close to star 1.000 0.100 High eccentricities Jupiter-size planets at close distance to star: Called Hot Jupiters 0.010 0.001 Jupiter s orbit 0.10 1.00 10.0 100 average orbital distance (AU) 56
Planet Demographics Orbital types: Many have high eccentricity! Much higher than Solar System Many exoplanets: On elliptical orbits 57
Bias: The way we design observations influences the results We must be careful to account for this Example: We are much more likely to find objects that produce a large radial velocity signal, i.e., planets that... have a large mass (more gravity) are close to the star (more gravity) Basically: Radial velocity searches can only find hot Jupiters. That does not mean that all exoplanets are hot Jupiters. 58