Download Exoplanets

Exoplanets
Astrobiology Workshop
June 29, 2006
Exoplanets:
Around Solar-Type Stars
Discovery (since 1995) by
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Doppler shifts in spectral lines of stars
Transits of stars by planets
Microlensing
Maybe imaging
Web Sites
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exoplanet.org
exoplanet.eu
Solar System Planets
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Earth
Neptune
Terrestrial
Gas Giant
Ice Giant
Saturn
Jupiter
Exoplanets:
Around Solar-Type Stars
Characteristics
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All (or almost all?) are gas or ice giants
• Masses from 7ME up to > 13MJ (MJ = 320 ME)
Orbits are mostly unlike the Solar System
• “Hot Neptunes” & “Hot Jupiters” (a < 0.4 AU) are
common
• Orbits are often very eccentric
Earths cannot be detected yet
Numbers (>180)
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Probably at least 10-15% of nearby Sun-like Stars
18 Planetary Systems (stars with 2 or more planets)
Doppler Shift
due to Stellar Wobble
Doppler Shift
due to Stellar Wobble
Doppler Shift for a Star
Orbited by a Planet
So How Hard Is It?
Difficulty of Doppler Searches
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Jupiters
• C.O.M. of Jupiter-Sun system (5.2 AU orbit radius) is
near the Sun’s surface (M = 1,000 MJ)
• Jupiter orbits the C.O.M. at 13 km/s
• The Sun’s speed is smaller by the ratio of Jupiter’s
mass to the mass of the Sun (10-3)
• The Sun’s wobble due to Jupiter is only 13 m/s
• The speed of light is 3x108 m/s
• For the Doppler effect: / = v/c
• So, we have to detect changes in wavelength  of
spectral lines of less than one part in 107 to measure
this!
• Massive, close-in gas giants are much easier to
detect
So How Hard Is It?
Difficulty of Doppler Searches
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Earth
• The Sun’s wobble due to the Earth is only about 10
cm/s !
Requirements for Any Planet
• Very stable reference spectrum
• Use of all the spectral lines in the spectrum
• Problem: Velocity “noise” from motions in the star’s
atmosphere is typically 1 to10 m/s !
Exoplanets from Doppler Shifts:
General Picture
M V E
M
J
Latest Version
Extrasolar Planet Discovery Space
brown dwarfs
gas giant planets
Right of the blue line,
the orbit period is more
than the time these
systems have been
observed.
Below the dashed
line, the stellar wobbles
are less than 10 m/s.
First Detection of an Exoplanet:
51 Pegasi
First Exo-Planetary System:
Upsilon Andromedae
F8V
4.2 MJ
0.7MJ
1.9MJ
Eccentric Orbit Example:
16 Cygni b
1.7 MJ
G5V
S.S. Analog:
47 Ursa Majoris
47 Ursa Majoris
0.76MJ
2.5MJ
55 Cancri:
A Four Planet System
QuickTi me™ and a Sor enson Video 3 decompressor are needed to see thi s pi cture.
Planet
Msini = 4.05 MJ
a = 5.9 AU (5,360 days)
Planet
Msini = 0.21 MJ
a = 0.24 AU (44.3 days)
Planet
Msini = 0.84 MJ
a = 0.12 AU (14.7 days)
Planet
Msini = 0.045 MJ (14 ME)
a = 0.038 AU (2.81 days)
Star Mass = 0.95 M G8V
Gliese 876 System:
Gas Giants in 2:1 Resonance
Gliese 876 System:
6 to 8 Earth Mass Planet
Gliese 876 System:
Three Known Planets
Planet
Msini = 1.89 MJ
a = 0.21 AU (61.0 days)
Planet
Msini = 0.56 MJ
a = 0.13 AU (30.1 days)
Planet
Msini = 5.9 ME
a = 0.021 AU (1.94 days)
Star Mass = 0.32 M M4V
Gliese 876 System:
The Movie
QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture.
Systems Where Planets
Transit the Star
Transiting Planet
HD209458b
Planet Mass = 0.69  0.05 MJ
Planet Radius = 1.43  0.04 RJ
Orbit a = 0.045 AU
Orbit Period = 3.52 days
Star Mass = 1.05 M (F8V)
Transiting Planet
HD209458b
Transiting Planet HD209458b:
Absorption Line of Sodium
Transit Surveys
Transiting Planet HD149026b:
A Massive Heavy Core
Transiting Planet HD149026b:
A Massive Heavy Core
Planet Mass = 0.36 MJ
Planet Radius = 0.72  0.025 RJ
Orbit a = 0.042 AU
Orbit Period = 2.88 days
Star Mass = 1.31 M G0IV
Image of a Planet?
Doppler-Shift Exoplanets:
Masses, Eccentricities, & Orbits
Brown
Dwarf
Desert
Doppler-Shift Exoplanets:
Masses & Orbits
Highest
Mass
ALL
Average
Mass
30 m/s
10 m/s
NEPTUNES
JUPITERS
Doppler-Shift Exoplanets:
Eccentricities & Orbit Periods
Doppler-Shift Exoplanets:
Metallicity of the Host Star
Some statistics
[Fe/H] is the log10 of Fe/H in the
star divided by the Sun’s value.
Transiting Exoplanets:
Are They Like Jupiter and Saturn?
1.3 g/cc
J
S
0.3 g/cc
Issues and Concerns:
Planet Formation
Planet Formation
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Gas Giant Formation Theories
• Solid Core Accretion followed by gas capture
– Pro: Mechanism that can work
– Con: Slow, expect formation at > few AU, may not be able to
make super-Jupiters
• Disk Instability due to self-gravity of the protoplanetary
disk
– Pro: Fast formation
– Con: Real protoplanetary disks may not cool fast enough to
fragment, may be hard to explain large solid cores
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• Hybrid: Core Accretion sped up by Disk Instability?
Evidence
• Metallicity correlation may favor Core Accretion
Issues and Concerns:
Planet Formation
Hot Neptunes & Jupiters?
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Formation in Place
• Probably not possible
Planet “Migration”
• Planets can drift inward due to planet-disk interaction
Eccentricities?
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How Are They Attained?
• Multi-body interactions
• Perturbations by nearby stars
• Planet-disk interactions
• Migration into orbital resonances
Overall
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Incredible Diversity of Planetary Systems!
Formation of the Solar System:
The “Solar Nebula” Theory
Dense, Cold, Rotating Interstellar
Cloud
Collapses and Flattens
105 yrs
Sun Forms with “Solar Nebula”
(Protoplanetary Disk)
106-107 yrs
Solid Planetesimals and Gas
Giant Planets Form,
Then Gas Dissipates
107-3x107 yrs
Terrestrial Planets Form by
Accretion of Planetesimals
Gas Giant Planet Formation:
The Two Theories
Core Accretion
few x106 yrs
Disk Instability
102 - 103 yrs
Issues and Concerns:
Life
Why Are Hot Jupiters Bad?
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Origin
• Probably exist due to inward “migration” during planet formation
Effects
• Sweep terrestrial planet material into the star as they migrate
• Gas Giants near or inside the habitable zone make stable orbits
for terrestrial planets difficult or impossible
Why Are Eccentric Gas Giants Bad?
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Effects
• Tend to disrupt terrestrial planet formation
• Tend to destabilize terrestrial planet orbits and/or force the orbits
to be eccentric, producing extreme seasons
Issues and Concerns:
Life
Hope?
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There ARE Solar System Analogs!
• Gas giants at > few AU in nearly circular orbits
• Over the next decade, more are likely to be found
Incredible Diversity of Environments!
And…
Maybe Close-In Gas Giants Have
Earth-Like Moons