Download Lecture 20: Formation of Planets, Exoplanets 3/30

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
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see disks around new stars in Orion nebula
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Planet Formation II
• material condensates if cool enough (gasliquid/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
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Temperature in early Solar Nebula
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“snow line” in early Solar Nebula
Very, very simplistic
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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
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planetisimals (dust grains)  protoplanets by accretion,
collisions, gravity  smaller objects stick together
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Craters on Moon (and other planets’ moons) are fossil
remnants of collisions in early solar system
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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
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Planets in other Star Systems
first extrasolar planet observed in 1995. In Jan 2000, 28
observed and now about 3700 confirmed. 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”
• If multiple planets, will have to add effects of planets
(our solar system: Jupiter with 12 year orbit, Earth with
1 year, etc)
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Planets in other Star Systems
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Observe Directly
block out star in
telescope optics
Do if found exoplanet
by other means. Can
study atmosphere
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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
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55 Cancri (one of first discovered)
Doppler shift very
complicated. One close
large planet plus 3-4 more?
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Summary: some of the exoplanets found by Doppler shift - easiest to find big planets close to a star
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Observe by planet eclipsing star
WASP-4 Wide Angle
Search for Planets
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
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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)
• Database at www.planethunters.org
• 2013: two motors fail, can no longer “point”. Now mostly
just points away from Sun.
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Kepler telescope
• orbit
original field of view (Cygnus + Lyra)
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Kepler telescope after motor failure
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Kepler Results: many Earth-like planets
some in habitable zones (more later)
Kepler 35 – binary star
Planet orbiting 4-star (2
close binaries)
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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) 
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2325 found by Kepler 5/10/16
3586 found by everyone 3/2017
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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
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Exoplanet Highlights 2014-2016
• Smallest: Kepler-37b. About the size of our Moon
• Closest: Alpha CentauriB has Earth-sized but close to star and hot
surface
• Earthiest: Kepler-78b earthlike mass/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. But
3 star system with “C” being a M1 class with 1% of Sun’s
luminosity  poor for “intelligent”
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Extra Slide
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