Leftovers

Gas is eventually captured or pushed out by wind from the star, but dust and planetesimals are left around

Formation of the Solar System

CLOUD COLLAPSE

ROTATING DISK

GAS CAPTURE?

CONDENSATION

STEPS:

ACCRETION

EVIDENCE:

•planets orbit in same direction and same plane

•Sun and planets rotate in same direction

•disks seen around other stars

•young stars seen in collapsing gas clouds

•terrestrial planets and asteroids found near Sun

•jovian planets, icy moons, comets found farther away

•Jupiter, Saturn are mostly hydrogen and helium

•gaps in disks around other stars

•many meteorites are made of smaller bits

•heavy cratering on oldest planet surfaces

•asteroids, comets are “leftovers”

Illustration of

Kepler planet candidates blocking their stars

SUN +JUPITER + EARTH

Transits - A Kind of Eclipse

• planet crosses in front of a star, making star appear fainter

1193 planets detected so far!

Planet Transits

planet

star

• amount of starlight blocked tells us about size of planet:

VIEW FROM EARTH:

• orbit period and estimate of star mass tells us about semi-major axis of planet:

Most planets that have been discovered around other stars are thought to be like Jupiter. The Sun is about 11 times the size of Jupiter. What fraction of the Sun’s light would get blocked if we were watching Jupiter transit the Sun from far away?

A. 99%

B. 90%

C. 10%

D. 1%

E. 0.1%

F. 0.01%

Thought Question

planet

star

Questions!!!Why are some jovian planets found near their stars?

(“Hot Jupiters”)

• Do jovian planets form differently than we think?

• Did jovian planets “migrate” in toward their stars?

Why doesn’t everything have the “special direction”?

• Why are Venus and Uranus rotating differently?

• Why do some planets orbit in the opposite way that their stars rotate?

Why are some jovian planets found far from their stars?

What are “super Earths” and “mini-Neptunes”?

Will terrestrial planet discoveries reveal something completely new?

“Hot Jupiters”• orbit in as little as

0.8 Earth days!• cloud-top temperatures

up to 1300 K• some are “puffy” – up to

30% larger than Jupiter

HOW DID THEY GET THERE?

“Super Earths”/”Mini Neptunes”• a variety of densities

• about 2-10 Earth’s mass, and up to 4x Earth’s radius

How common are they?

How do they form?

What could they be like?

still looking for these!!

Comparative Planetology

Questions:• How are planets similar and different?

surface

atmosphere• Why are they different?

Uncovering Planet History

CRATERING:All planets heavily bombarded in past…

If a planet has craters today:

• surface wasn’t completely protected (by atmosphere or oceans)

• surface wasn’t “cleaned” recently (erosion, lava flows, or tectonics)

Look at:

how heavily cratered surface is

where craters are

Thought Question:

What is the order from oldest to youngest?

A (volcano)

B (crater)

C (crater)D (lava flow from another volcano)

Mercury:heavily cratered, but

how long ago?

Impact Craters

Radiometric Dating of Rocks

When rock solidifies, radioactive atoms and their products are “frozen” into it … no escape!

A small fraction of atoms are radioactive - breaking up and forming atoms of a different chemical element

Potassium-40:

Uranium-238:

Radiometric Dating•Nuclei of parent species (like potassium-40) decay to become daughter products (like argon-40)

half-life: time for half of radioactive atoms to decay

potassium-40: 1.25 billion yr uranium-238: 4.468 billion yr

TOTAL K+Ar

Thought Question:Each atom of element Y will eventually decay to form one

atom of element X (with a half-life of 800 million years). When the planets solidified, there was no element X. If you find a rock where element Y is currently has 1/8th the total element X plus element Y,

…how old is the rock?

(Enter your answer in millions of years.)

…what fraction of the original Y is left today?

FRACTIONS:

PROPORTIONS: 1:1 1:3 1:7 1:15

Oldest Known Crystal4.374±0.006 billion yr old

Ages of Surfaces•Radiometric dating of Moon rocks allow us to measure when the rock was last melted…

…when were most craters made?

Thought Question:This is a picture of Saturn’s

moon Enceladus. Which of the following statements about its surface is probably true? (Enter the letters for all correct answers.)

A. The lower left part of the surface probably froze more recently than the upper right.

B. The oldest parts are about as old as the oldest parts of the Moon.

C. None of the above is true.

History of Cratering

• most cratering in first billion years of solar system

• heavily cratered surfaces have changed little in last 3 billion years

1/3 Earth

1/4 Earth

1/2 Earth

95% Earth

A Scatter Plot

Can this help us predict terrestrial

planet properties?

What Made Them This Way?

VOLCANIC ACTIVITY

HOW MUCH OUTGASSING?

STRENGTH OF GRAVITY

PLANET MASS/RADIUS

DISTANCE FROM SUN

TEMPERATURE

CONDENSATION?HOLDS ONTO

GASES?

VOLCANOES:

If a planet has volcanoes:

• interior was hot enough for rock to move

• crust was thin enough to allow lava to reach surface

Clues:

number of volcanoes

their pattern

are they active?

Uncovering Planet History

Marsvery few volcanoes, but they are VERY large…

volcanoes probably not

active

largest number of volcanoes among terrestrial planets

evidence of active volcanoes

Venus

Thought Question:Suppose a terrestrial planet the same age as Earth is

discovered orbiting another star, and it is your job to predict what it is like. If the planet is known to have a mass and size smaller than Venus but larger than Mars, what would be the best prediction?

A. It should have no volcanoes.

B. It should have volcanoes, but they may or may not be active.

C. It should have a moderate number of active volcanoes spread evenly over the surface.

D. It should have a large number of active volcanoes found only in small areas of the surface.

Moon Oceans?

Ganymede(Jupiter moon)

Enceladus(Saturn moon)

Temperature of interior involves competition between:

Terrestrial Planet Interiors

HEAT NOW IN PLANET

SOURCE OF HEAT

HEAT LOST FROM

SURFACE

LEAKY BUCKET ANALOGY:• thermal energy added by radioactivity:radioactive atoms release heat when they decay

• thermal energy lost to space:

heat can only escape from planet’s surface

Terrestrial Planet Interiors• thermal energy

added by radioactivity:

every bit of mass contains a small fraction of radioactive atoms:

• thermal energy lost to space:

heat can only escape from planet’s surface

energy added per

second

average density

energy lost per second

planet area

is related to

planet mass

is related to

Loss of Internal HeatTerrestrial planets are losing heat (cooling) because radioactive heating is smaller, but exactly how small affects how quickly the inside cools off:

Thought Question:Based on the rates of energy being added and lost,

which terrestrial planet should cool off most slowly? (Hint: compare rates of adding and losing thermal

energy by thinking about the ratio.)

A. Mercury

B. Venus

C. Earth

D. Moon

E. Mars

Thought Question:Mars is about half the diameter of Earth and

1/10th the mass of Earth. Based on the energy added and lost per second, Mars probably cooled off…

A. about 10 times as fast as Earth.

B. about 2 times as fast as Earth.

C. at about the same rate as Earth.

D. about 1/2 as fast as Earth.

E. about 1/10th as fast as Earth.

The Heat Inside:

Earth, Venus:

still active today

Mars:

active volcanoes once, but not many

Mercury, Moon:

dead for a long time

• large planets take longer to cool off and solidify inside:

• As planets formed, collisions and radioactivity melted them into sphere shape:

Venus from the Ground

Atmosphere Conditions

Average Temperature:

850 F 60 F -60 F

737 K 288 K 210 K

Atmospheric Pressure:

90x Earth’s 0.007x Earth’s

Chemicals:

96% CO2 78% N2 95% CO2

Addition of Gases• Outgassing by volcanoes• BombardmentLoss of Gases:• Gas escape into space• Condensation• Chemical reactions

Terrestrial Planet Atmospheres

Where Did It Come From?

Comet impacts bring “ices”:water vapor (H2O)carbon monoxide (CO)ammonia (NH3)methane (CH4)

Volcanoes release gas from molten rock:

carbon dioxide (CO2)nitrogen (N2)water vapor (H2O)

Escape Speeds

• Larger planet mass gas molecules have to move faster to escape

escape speed:

• Higher temperature faster gas molecules move easier to escape

Temperature• Temperature relates to average speed of motions of atoms• absolute zero (0 K) is when thermal motion stops

box increases the average particle speed

Gas animation

Speeds of Gas Molecules

Average particle kinetic energy in a gas only depends on temperature:

(Boltzmann’s constant)

Gas of a particular chemical has a chance to escape over the history of the solar system if:

Average particle speed in a gas:

Thought Question:What combination of factors makes it more likely

to lose a gas from a planet’s atmosphere? (Enter a three letter answer.)

A. High mass planetB. Low mass planet

C. High temperature planetD. Low temperature planet

E. High-mass gas moleculesF. Low-mass gas molecules

Thought Question:How does the average speed of a hydrogen molecule

(H2) compare to the average speed of an oxygen molecule (O2) at the same temperature?

(H2 has twice the mass of hydrogen atom, and O2 has 32 times the mass of a hydrogen atom.)