PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
PLATO - 7• The outer solar system
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Tethis eclipsed by Titan; Cassini (NASA)
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Titan (Saturn’s largest moon)
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Huygens panorama (NASA, 2005)
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Enceladus
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Enceladus• An ice world
★ Young, smooth surface
★ Much like Europa?
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Enceladus• An ice world
★ Young, smooth surface
★ Much like Europa?
• Water vapor from surface!
★ Possibly liquid ocean
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Enceladus• An ice world
★ Young, smooth surface
★ Much like Europa?
• Water vapor from surface!
★ Possibly liquid ocean
★ Possible responsible for E-ring
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Cassini image (NASA)
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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?
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Neptune’s moon Triton
• Bigger than Pluto
• Retrograde elliptical orbit
★ Captured?
• Atmosphere:
★ Massive
★ Cold (far out in solar system)
★ Eruptions?
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Pluto
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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”
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✔
✘
✔
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Trans-Neptunian Objects
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Name Diameter Mass Semi-major axis Orbital Eccentricity
Eris 2600 km 1.7 × 1022 kg 67.8 AU 0.44
Pluto 2310 km 1.3 × 1022 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 × 1021 kg 43.3 AU 0.19
Charon 1210 km 1.5 × 1021 kg 39.4 AU satellite of Pluto
Quaoar ~1000 km ? 43.6 AU 0.038
Orcus ~950 km 7.5 × 1020 kg 39.4 AU 0.22
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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”
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comets
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Comet Hale-Bopp; (Rob Jones)
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comets
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Comet West
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comet Origins• Oort cloud or Kuyper belt objects
• Kicked onto elliptical orbits by gravitational encounters
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comet Structure• Core:
~ 1-10 km snow ball
• Coma:
~ 105 km gas & dust cloud
• Envelope:
~ 106km Hydrogen cloud
• Tail:
~108 km gas a
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Halley’s Comet
Comet C/2001 Q4
Comet Hale-Bopp
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comet Structure
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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?
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Tails
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• Pin the tail on the comet: Where is the gaseous tail going to point at the location of the comet along its path?
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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)
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comet Tails
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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)
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Question• Which direction would you see the asteroid coming
from, given the Earth’s motion and the asteroid’s motion?
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Earth
A)B)
C)
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
• Meteoroid: Burns up in atmosphere
• Meteorite: Reaches the ground
Meteors
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Asteroids• Not all particles hitting Earth are small cometary dust
grains...
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Wolf Creek Crater, Australia
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Asteroids
• ~150,000 catalogued so far
★ Many times more uncounted for yet
• Sizes:
★ Few km to 1000 km (Ceres)
• Densities:
★ about 3 kg/liter
⇒ Loose rock
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Asteroids• Most occupy asteroid belt
★ Between Mars & Jupiter
• Found based on Bode’s rule
★ Planets spacing follows pattern
★ Probable coincidence
★ Planet “missing” between Jupiter and Mars
★ Ceres was first called a planet
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Jupiter
Trojan Asteroids
Mars’ orbit
Jupiter’s orbit
Trojan Asteroids
Mer
cury
Venu
sEa
rth
Mar
s
Cer
es
Jupi
ter
Satu
rn
Ura
nus
Nep
tune
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Asteroids• Jupiter has cleared out resonance gaps
• Small fraction of asteroids are Earth-crossing
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Impacts
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Barringer impact crater in Arizona
Impacts
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0.7 miles
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Impacts
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Ries, Bavaria (Germany)
15 miles
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Impacts
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Manicougan Crater (Canada)
40 miles
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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!
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Barringer crater, AZ
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Impacts
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Tunguska event, 1908, Siberia5-30 megatons of TNT
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Impacts• A 10 km asteroid can lead to mass extinctions
• 65 Million years ago:
★ Cretaceous (dinosaur) extinction
★ Yukatan Peninsula
• Fossil records:
★ Mass extinction coincident with deposition of Iridium all over globe
★ Not usually found in Earth’s crust,
★ But common in asteroids
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180 km
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Extinction rate
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KT-event
today
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Cratering Rate• Decreased with time
★ Maria formed after cratering rate declined
★ Read off from the chart how old a surface is
• Current rates on Earth:
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Late heavy bombardment?
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 7.5x107
5 km ~10 million yrs 1010 k
10 km ~200 million yrs 7.5x1010 k
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Crater Structure• Size ~ 10 - 20 x size of impacting object
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
• Paintball “simulation” vs. Tycho (Moon)
Cratering
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“Rays”
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Comet impacts• Comet Shoemaker Levy 9
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Exoplanets
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Direct image of a Jovian planet around a brown dwarf sub-star
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Radial Velocity
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Radial velocity
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PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Radial Velocity• Most successful method
★ Pioneered by Marcy & Butler
★ Finding lots of exoplanets
✦ 494 planets to date
✦ In 414 planetary systems
★ It can measure:
✦ Lower limit on planet mass
✦ Orbital Periods
✦ Semi-major axis
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Goeff Marcy
Paul Butler
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
• The angle between the orbital axis and the observer
• i=0°: No Doppler shift
Orbital Inclination i• The angle between the orbital axis and the observer
• i=90°: Maximum Doppler shift
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orbi
tal a
xis
i=90°or
bita
l axi
si=60°
• The angle between the orbital axis and the observer
• i=60°: Some Doppler shift
orbital axis
i=0°
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Radial Velocity• We measure:
★ 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:
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a3planet =
GMstarP 2
4π2
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Radial Velocity• Example:
★ 51 Pegasi
★
★
★
• Planet mass:
★
★ A lower limit in mass (inclination)
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51 Pegasi
M51Peg = 0.47MJupiter/ sin (i)
P = 4 days, vrad = 57 m s−1, Mstar = 2.1× 1030 kg
P = 4 days, vrad = 57 m s−1, Mstar = 2.1× 1030 kg
P = 4 days, vrad = 57 m s−1, Mstar = 2.1× 1030 kg
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
Planet Demographics• What kinds of planets have
been found?
★ 539 planet systems total
★ Mostly massive planets
★ Mostly close to star
★ High eccentricities
• Jupiter-size planets at close distance to star:
★ Called “Hot Jupiters”
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average orbital distance (AU)0.10 1.00 10.0 100
Plan
et M
ass
(Jupi
ter
mas
ses)
0.001
0.010
0.100
1.000
10.00
100.0
Jupi
ter’s
orb
it
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets 57
• Orbital types:
★ Many have high eccentricity!
★ Much higher than Solar System
⇒ Many exoplanets:
★ On elliptical orbits
Planet Demographics
PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets
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.
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