Transcript Time From the Perspective of a Particle Physicist

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
1
PHYS 162
2
see disks around new stars in Orion nebula
PHYS 162
3
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
PHYS 162
4
Temperature in early Solar Nebula
PHYS 162
5
“snow line” in early Solar Nebula
Very, very simplistic
PHYS 162
6
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
7
planetisimals (little dust grains)  protoplanets by
accretion: collisions, gravity  smaller objects to stick
together
PHYS 162
8
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
9
PHYS 162
10
PHYS 162
11
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
12
Observe Directly
block out star
in telescope
optics
Do if found
exoplanet by
other means
PHYS 162
13
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
PHYS 162
14
47 Ursae Majoris (one of first discovered)
Doppler shift 2 large planets
PHYS 162
15
55 Cancri (one of first discovered)
Doppler shift very
complicated. One close
large planet plus 3-4 more?
PHYS 162
16
Summary: some of the exoplanets found by Doppler shift - easiest to find big planets close to a star
PHYS 162
17
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
PHYS 162
18
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
19
Kepler telescope
• orbit
field of view (Cygnus + Lyra)
PHYS 162
20
Kepler Results: many Earth-like planets
some in habitable zones (more later)
Kepler 35 – binary star
Planet orbiting 4-star (2
close binaries)
PHYS 162
21
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
22
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
23
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
24
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
25
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
26
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
27
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
PHYS 162
28