The outer solar system - UW-Madison Astronomyheinzs/Homepage/PLATO_2011_files/session_… · PLATO...

PLATO 2011: Planets - Foreign Worlds (7) Moons, TNOs, Exoplanets

PLATO - 7• The outer solar system

1

Tethis eclipsed by Titan; Cassini (NASA)


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)

3

Huygens panorama (NASA, 2005)


Enceladus

4


Enceladus• An ice world

★ Young, smooth surface

★ Much like Europa?

5





• Water vapor from surface!

★ Possibly liquid ocean

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★ Possible responsible for E-ring

7

Cassini image (NASA)







★ Possible responsible for E-ring

• Synchronous orbit

★ Tidal heating still possible

★ Combined with radioactive heating?

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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

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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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✔

✘

✔


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


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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Comets

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Comet Hale-Bopp; (Rob Jones)


Comets

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Comet West


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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Comet Origins• Oort cloud or Kuyper belt objects

• Kicked onto elliptical orbits by gravitational encounters

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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


Comet Structure

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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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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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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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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?


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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Comet Tails

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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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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)


• Meteoroid: Burns up in atmosphere

• Meteorite: Reaches the ground

Meteors

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Asteroids• Not all particles hitting Earth are small cometary dust

grains...

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Wolf Creek Crater, Australia


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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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


Asteroids• Jupiter has cleared out resonance gaps

• Small fraction of asteroids are Earth-crossing

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Impacts

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Barringer impact crater in Arizona

Impacts

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0.7 miles


Impacts

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Ries, Bavaria (Germany)

15 miles


Impacts

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Manicougan Crater (Canada)

40 miles


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


Impacts

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Tunguska event, 1908, Siberia5-30 megatons of TNT


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


Extinction rate

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KT-event

today


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


Crater Structure• Size ~ 10 - 20 x size of impacting object

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• Paintball “simulation” vs. Tycho (Moon)

Cratering

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“Rays”


Comet impacts• Comet Shoemaker Levy 9

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Exoplanets

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Direct image of a Jovian planet around a brown dwarf sub-star


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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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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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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Radial Velocity

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Radial velocity

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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


• 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°


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:

54

a3planet =

GMstarP 2

4π2


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




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


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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The outer solar system - UW-Madison Astronomyheinzs/Homepage/PLATO_2011_files/session_… · PLATO...

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