PHYS 162 1

Planet Formation

• contracting gas/dust cloud forms stars swirling disk of material (H, He, C, O,

heavier elements, molecules, “dust”) form planets

• New born star heats up material, blows out of solar nebula planets (or at least protoplanets) need to form before material dissipates

PHYS 162 2

PHYS 162 3

see disks around new stars in Orion nebula

PHYS 162 4

Planet Formation II• material condensates if cool enough (gasliquid/solid)• heavier elements (metals, silicates) condense first, at

higher temperatures, then molecules like water H and He remain gases

• density and temperature falls with distance from star. Planet formation occurs when not too far and not too close

• “snow line” separates type of planets being formed

PHYS 162 5

Temperature in early Solar Nebula

PHYS 162 6

“snow line” in early Solar Nebula

Very, very simplistic

PHYS 162 7

Planet Formation III• condensation starts, protoplanets grow in size

-objects collide; stick together

• over millions of years sweep out most smaller objects as collide with larger objects existing planets

• only ~circular orbits won’t collide any further (asteroid belt between Mars and Jupiter)

• Possible motion of planets to/from star may be critical

PHYS 162 8

planetisimals (little dust grains) protoplanets by accretion: collisions, gravity smaller objects to stick together

PHYS 162 9

Planet Formation IV• close to star: planets = heavy elements (iron,silicon)

-water may be trapped at beginning in dust grains or come later from comets hitting surface???

• further from star: Gas Giants ices (water H2O, methane CH4) froze out early larger protoplanets more material to accrete

• comets, meteors, asteroids give clues to composition of early solar system

PHYS 162 10

PHYS 162 11

PHYS 162 12

Planets in other Star Systems• test out how planets are formed with more examples

• first extrasolar planet observed in 1995. In Jan 2000, 28 observed and now >4500 candidates about 1000 confirmed (11/2013). Many systems with 2 or more observed planets

• difficult to observe directly

• mostly look for impact on Star: wobbles due to gravity of planets or reduction of light due to “eclipse”

• Planet orbits obey Kepler’s laws. If multiple planets, will have to add effects of planets (our solar system: Jupiter with 12 year orbit, Earth with 1 year, etc)

PHYS 162 13

Observe Directly

block out star in telescope optics

Do if found exoplanet by other means

PHYS 162 14

Observe by Star’s Wobble: Doppler Shift or Proper Motion

the larger the planet the larger the gravitational pull

the smaller the orbit the larger the gravitational pull

the smaller the orbit the more rapid is the wobble

easiest to see large planets which are close to their stars

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47 Ursae Majoris (one of first discovered)

Doppler shift 2 large planets

PHYS 162 16

55 Cancri (one of first discovered)

Doppler shift very complicated. One close large planet plus 3-4 more?

PHYS 162 17

Summary: some of the exoplanets found by Doppler shift

-- easiest to find big planets close to a star

PHYS 162 18

Observe by planet eclipsing star

Jupiter would reduce Sun’s light by 1%; Earth reduces by .01%

“easy” (done by 7th grader at NIU Science Fair)

once spotted can also analyze Doppler shift and try and observe atmosphere

WASP-4 Wide Angle Search for Planets

PHYS 162 19

Kepler telescope• launched in 2009, designed to detect Earth-sized (or smaller) planets by

observing them eclipsing their stars• In orbit around the Sun….away from the Earth, points away from the Sun• Looked 150,000 main sequence stars (every 20 minutes) measures luminosity

20 parts per million (0.000002)• Has discovered over 3000 possible planets, ~600 confirmed (Jan 2014), rest

available for analysis. Candidates often binary star systems• Database at www.planethunters.org • 2013: two motors fail, can no longer “point”.

PHYS 162 20

Kepler telescope• orbit field of view (Cygnus + Lyra)

PHYS 162 21

Kepler Results: many Earth-like planetssome in habitable zones (more later)

Kepler 35 – binary star

Planet orbiting 4-star (2 close binaries)

PHYS 162 22

Kepler telescope• collected data from

2009-2013. pointing failed. Not sensitive to long periods like Jupiter’s 12 years

• Sees a lot of planets between Earth and Neptune size. 603 in plot (1/14)

PHYS 162 23

Kepler telescope• Kepler data can estimate the number of Earth-like planets orbiting

Sun-like stars. Didn’t take data long enough to do 1 year periods

PHYS 162 24

Exoplanet Highlights Jan 2014• Smallest: Kepler-37b. About the size of our Moon

• Earthiest: Kepler-78b has earthlike mass and diameter but 8.5 hour orbit and temp>2000 degrees C• Intriguing 1: Kepler-62e and 62f are 2 Earthlike planets in habitable zone. Both ~twice Earth size at .4

and .7 AU from star ~0.2 Sun’s luminosity

• Intriguing 2: star Gliese667C has three planets in habitable zone. Bit 3 star system with “C” being a M1 class with 1% of Sun’s luminosity poor for “intelligent” life

PHYS 162 25

Planetary Atmospheres• composition of a planet’s atmosphere depends on

Surface Gravity

Temperature• light atoms/molecules move faster than heavy molecules • if velocity = escape velocity gas leaves planet• Mercury, Moon: all escape• Earth: lightest (H,He) escape• Jupiter: none escape

PHYS 162 26

Familiar Molecules molecule mass

H2 hydrogen 2

He helium 4

CH4 methane 16

NH3 ammonia 17

H20 water 18

N2 nitrogen 28

O2 oxygen 32

CO carbon monoxide 28

CO2 carbon dioxide 44

PHYS 162 27

Atmosphere of Venus vs Earth

96.5% CO2, 3% N2

runaway greenhouse effect

78% N2, 21% O2, 0.04% CO2, ~1% H20. most CO2 absorbed by oceans

PHYS 162 28

Atmosphere of Jupiter and Saturn

ammonia, sulfuric acid, water

interiors are helium and hydrogen, core of ice/rock

Titan, moon of Saturn, has ~90% nitrogen rest mostly methane and argon; pressure similar to Earth