Transcript Search for Life in the Universe

Search for Life in the Universe
Chapter 10
Search for Habitable Worlds
(Part 1)
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Outline
• Are Habitable Planets Common?
• Distant Suns
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Properties of Stars
Stellar Types
Which Stars Make Good Suns?
Multiple Stars
• Extrasolar Planets
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Detecting Extrasolar Planets
Astrometry
Doppler Technique
Transit
Direct Detection
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Are Habitable Planets Common?
• Disks of young stars:
– Theory: disks inevitable  planets
– Observations: 2550% of stars have disks with a few percent of
the stellar mass (cf., <0.2% for our planets)
• Extrasolar planets (around main-sequence stars):
– 146 planetary systems (11/24/2005)
• 170 planets
• 18 multiple planet systems
– Encyclopedia: http://www.obspm.fr/encycl/encycl.html
• Characteristics of discovered extrasolar planets:
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Massive (mainly a selection effect): Jovian planets
Unlikely to be habitable
Orbit close to the parent stars
Highly elliptical orbits common
Affect habitability of Earth-like planets
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Properties of Stars
• Evolution
– Protostar  main sequence  giant or supergiant  white
dwarf, neutron star, or black hole
– Habitability: in main sequence phase, because it lasts longest
• Types
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Universal composition: >98% H + He
Universal burning: H  He  C + O
Mass: only free parameter
Types: OBAFGKM (politically incorrect mnemonic: “Oh, Be A
Fine Girl, Kiss Me”)
• Lifetimes
– Luminosity: steep function of mass  M(3.54)
– Lifetime: steep inverse function of mass  M(2.53)
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Stellar Types
Type
O
Percent
age
0.001%
Temp.
[C]
50,000
Lumin.
[solar]
106
Mass
[solar]
60
Lifetime
[years]
5x105
B
0.1%
15,000
1,000
6
5x106
A
1%
8,000
20
2
109
F
2%
6,500
7
1.5
2x109
G
7%
5,500
1
1
1010
K
15%
4,000
0.3
0.7
2x1010
M
75%
3,000
0.003
0.2
6x1011
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Which Stars Make Good Suns?
• O: much too short
• B: little life past accretion phase
• A and F:
– Life short but manageable
– UV light a potential problem
– Ways around that: e.g., more ozone
• G: OK (cf., the Sun)
• K and M:
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Long life
Very common
Less luminous  smaller habitable zone
Synchronous orbit: problem, unless the atmosphere rotates
Flares: main danger is UV, but it also produces more ozone
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Multiple Stars
• Orbit
– Planet pulled by two or more stars can have a
complicated orbit, moving in and out of the habitability
zones of the stars
• Stable cases
– Stars close, planet orbiting both at a safe distance
– Stars far, planet orbiting close to one of them
• Unstable cases
– Any other combination
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Detecting Extrasolar Planets
• Direct detection of the planet
– Difficulty: observe a dim planet near a bright star 
space missions > 10 yrs from now
– Absorption spectrum in a planetary atmosphere
during transit?
• Indirect detection of effect on parent star
– Extrasolar planets around main sequence stars
discovered in 1995
– Planets around neutron stars discovered previously
– Spectroscopy (>100 cases): detect Doppler shift of
stellar motion around center of mass
– Astrometry (1 case): detect angular motion
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Astrometry
• Binary or multiple stars
– Very practical, if they can be resolved
• Planets:
– Resolution: not an issue, observe only the star
– Amplitude: problem, e.g., at a distance of 10 ly, the amplitude of
the solar motion due to Jupiter is 0.003
– Angular motion  1/distance  harder for distant stars
– Angular motion  orbital radius  easier for outlying planets, but
orbital period longer and region less habitable
• Space Interferometer Mission (SIM, launch 2011?)
– Angular precision 10-6
– Earth-like planets: detect stellar wobble ~12 nearby stars
– Jupiter-size planets: detect wobble 3,000 ly away
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Doppler Technique
• Doppler effect
– / = vlos/c (nonrelativistic)
• Detection of planets
– Velocities ~30 m/s = 10-7c
– Accurate spectroscopy possible with iodine cells
instead of an arc
– Velocity independent of distance
– Velocity  Mplanet: favors massive planets
– Velocity  1/a: favors small orbits
– Orbital period  1/a3: shorter period favors small
orbit (observations take less time)
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Transit
• Transit: fancy name for a (very) partial eclipse
• HD209548:
– 1.7% decline in brightness during transit
– Due to Jupiter-size planet
– Measurable by inexpensive photometers
• Earth-size planet:
– 0.01% decline in brightness
– Very hard to measure from ground-based telescopes
– Kepler mission (2008?): monitor 105 stars from space
to detect even smaller than Earth-size transits
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Direct Detection
• Angular resolution
– Need to see dim planet near a bright star
– Angular resolution limited by diffraction and atmosphere
– Overcome atmosphere by going to space
• Infrared observations
– Improve the luminosity ratio between star and planet by
observing in the infrared
– Diffraction blurring , stronger in the infrared
• Interferometers
– Nulling: directly measure a difference instead of subtracting full
observations after the fact
– Interferometers: utilize the physical interference of light waves
– Terrestrial Planet Finder (TPF, schedule??) and/or Darwin
(schedule ??): Search for Earth-size planets around ~150 nearby
stars
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