Download Exoplanet Detection with Radial Velocities

Astronomical Data Analysis 2011: Lecture 6
Exoplanet Detection
with Radial Velocities
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanet Detection Rate
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Outline
1.  Introduction
2.  Radial Velocity Detection
3.  Exoplanet Properties from Radial Velocities
4.  Transits
5.  Exoplanet Atmospheres
6.  The Future
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
3
What is a Planet?
www.solarviews.org/cap/misc/solarsystem.htm
•  <13 Jupiter masses (no nuclear fusion), orbits star
•  Same for exoplanets and solar system planets
•  Earth-like: Earth-mass, -density, -temperature
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Planet vs. Star
Jupiter
Solar
Diameters
Jupiter
Masses
Convection
Fusion
Christoph U. Keller, [email protected]
Brown Dwarf
Sun
0.1
0.2
1
1
55
1000
partial
full
outer 30%
none
deuterium
hydrogen
Astronomical Data Analysis 2011: Exoplanet Detection
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Detection vs. Characterization
Detection:
•  Detect presence of exoplanet around a star
•  Determine mass to distinguish from brown dwarfs
•  Determine orbit around star
Characterization:
•  Determine radius
•  Determine surface properties
•  Determine atmosphere properties
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
6
Main Exoplanet Detection Methods
•  Radial velocity (stellar wobble): •  period
•  semi-major axis
•  eccentricity
•  lower limit to mass
•  Transits (stellar occultation): •  period
•  semi-major axis
•  inclination
•  radius
•  planet temperature
•  planet atmosphere
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Spectral Lines in the Solar Spectrum
www.noao.edu/image_gallery/images/d5/suny.jpg
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
9
Radial Velocity
•  Star, planet move around common center of mass
•  Doppler effect moves spectral lines
•  Look for periodic variations in stellar velocity
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Radial Velocity Signal
Semi-amplitude of radial velocity given by
!
" 13
2πG
MP sin i
1
K=
2 √
Porb
(M∗ + MP ) 3 1 − e2
•  Porb: orbital period
•  M*: mass of star
•  MP: mass of planet
•  i: inclination, angle between normal to orbital
plane and line of sight
•  e: excentricity Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Radial Velocity Signal
For circular orbits with MP << M* in meter/second:
MP sin i
vobs = 28.4 1/3 2/3
Porb M∗
•  MP in Jupiter masses
•  Porb in years
•  M* in solar masses
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Radial Velocity Signal Amplitude
K=
!
2πG
Porb
" 13
MP sin i
1
2 √
2
3
1
−
e
(M∗ + MP )
•  Observe: period, velocity amplitude and shape
•  Stellar mass unknown, but good estimate from
stellar spectrum
•  Jupiter: 12.4 m/s maximum velocity
•  Saturn: 2.8 m/s
•  Earth: 9 cm/s
•  Heavier stars reduce the signal
•  Lighter stars increase the signal
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Spectrometer
en.wikipedia.org/wiki/Extrasolar_planet
•  Wavelength cannot be measured directly
•  Spectrograph transforms wavelength into position
information
•  Must measure spatial location of spectral lines
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Positional Stability Requirement
•  Visible spectrum: ~ 500 nm
•  Typical high-resolution spectrograph: 0.005 nm
per camera pixel
•  One pixel in velocity: 3*108m/s/105 = 3000 m/s
•  Typical CCD camera pixel size: 15 μm
•  1 m/s is 15000/3000 nm = 5 nm
•  Need to keep this stability over years
•  1-meter aluminum bar expands 24 μm per deg C
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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HARPS
obswww.unige.ch/Instruments/Harps/
gallery/Integration_LSO/
•  Velocity noise 0.7 - 2 m/s over many years
•  Thorium-Argon calibration source simultaneous
with stellar spectrum
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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HARPS Polarimeter
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Astronomical Data Analysis 2011: Exoplanet Detection
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Example 1: 51 Pegasi b
Mayor and Queloz 1995 Figure 4
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Astronomical Data Analysis 2011: Exoplanet Detection
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Example 2: e=0
Butler et al. 2006, ApJ, 646, 505
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Astronomical Data Analysis 2011: Exoplanet Detection
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Example 3: e=0.5
Butler et al. 2006, ApJ, 646, 505
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Multiple Planets: One-Planet Fit
exoplanets.org/esp/55cnc/55cnc.shtml
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Multiple Planets: Two-Planet Fit
exoplanets.org/esp/55cnc/55cnc.shtml
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Multiple Planets: Three-Planet Fit
exoplanets.org/esp/55cnc/55cnc.shtml
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Radial Velocity Observables
•  Period
•  Lower limit to mass: (MP sin i)
•  Eccentricity of orbit
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Problems with Radial velocity
•  Correction of earth orbital motion (up to 30 km/s)
and earth rotation (0.5 km/s)
•  Period analysis (see Lecture on Periodicity)
•  Only good for cool stars such as the Sun
•  Hot stars (O,B,A) do not have enough narrow,
spectral lines
•  Stellar rotation, starspots, oscillations, convection
impact Doppler signal
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Radial Velocity Limits
vobs = 28.4
MP sin i
1/3
2/3
Porb M∗
•  Kepler’s 3rd law: Porb1/3 ~ a1/2/M*1/6
•  Limit given by MP sin i ~ a1/2 •  Need to observe at least one full orbit
•  Detection limit proportional to a1/2
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Problems with Statistics
•  Selection effects:
•  some aspects of observed distributions are
inconsistent with real population of exoplanets
•  depend on exoplanet detection approach
•  Mass is mostly a lower limit to real mass
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
27
Exoplanet Orbital Distances
Compared to Earth and Mercury
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanet Masses & Lower Limits
Compared to Jupiter and Earth
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Mass Distribution Interpretation
•  Low end of mass distribution:
•  Heavily affected by selection
•  low-mass planets induce small velocity
variations, difficult to detect, underrepresented
•  High end of mass distribution:
•  Massive planets easier to detect
•  Apparent decrease for M > 3MJ real
•  Apparent decrease for M >12MJ real,
‘‘brown dwarf desert’’
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Exoplanet Masses and Orbits
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanet Eccentricities
Mercury
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Astronomical Data Analysis 2011: Exoplanet Detection
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Eccentricity
•  Exoplanets within 0.1 AU on nearly circular orbits
•  Beyond 0.3 AU, distribution of eccentricities is
essentially uniform between 0 and 0.8
•  Radial velocity survey sensitivity not a strong
function of eccentricity for 0 < e < 0.7 and a < 3
AU
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Eccentricity
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
34
Distance vs. Eccentricity
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Orbital Eccentricity Interpretation
•  Overestimation of eccentricity because of e > 0
•  Probably 30-40% have e < 0.05
•  For a<0.1 mostly circular orbits due to tidal
circularization
•  Large eccentricities for more distant exoplanets may
be due to
•  Perturbations by other planets
•  Resonances
•  Interactions with protoplanetary disk
•  No clear correlation between eccentricity and mass
•  But high-mass exoplanets (M sin i > 5MJ) have higher
median eccentricity than lower mass exoplanets
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
36
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Transits
•  Can use small
telescopes (<1 m
diameter)
•  Space missions
avoid problems
with Earth’s
atmosphere
•  Transit timing
variations may
reveal hidden
exoplanets
Christoph U. Keller, [email protected]
sci.esa.int/science-e/www/object/index.cfm?fobjectid=35225
Astronomical Data Analysis 2011: Exoplanet Detection
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Mercury Transit 8 November 2006
sohowww.nascom.nasa.gov/hotshots/2006_11_06/
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Transit of Kepler 10b
0.017%
Christoph U. Keller, [email protected]
kepler.nasa.gov/Mission/discoveries/kepler10b/
40
Astronomical Data Analysis 2011: Exoplanet Detection
Transit Signals
•  Intensity signal
!
"2
∆I
RP
=
I
R∗
•  R* stellar radius
R*
RP
•  Rp planet radius
•  About 1% for Jupiter and Sun
•  Transit duration proportional to Porb1/3R*/M*1/3
•  Transit duration: also estimate of stellar radius
•  Intensity change then provides planetary radius
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Transit Observables
•  Period
•  Orbit inclination (i≈90°)
•  Planet radius
•  Planet temperature from secondary eclipse
•  Stellar limb darkening
•  Good for large planets close to the star
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Problems with Transits
•  Low probability, but simple observation
•  Many false positives:
•  Grazing eclipse from main-sequence star
•  Eclipsing binary (giant and main-sequence)
•  Eclipsing binary close by (foreground or background)
kepler.nasa.gov/files/mws/
aas2010-5nbFalsePositives.jpg
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Kepler Object of Interest 106
•  Eclipsing
binary in
background
•  Odd and
even
transits are
different
Christoph U. Keller, [email protected]
Batalha et al. 2010
Astronomical Data Analysis 2011: Exoplanet Detection
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Kepler Exoplanet Candidates
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Problems with Transits
PLANETS
TARGET EB
NUMBER
BAKCGROUND EB
0.001
0.01
0.1
DEPTH OF SIGNAL
www.oca.eu/cassiopee/COROTWeek10/Communications/F_Pont.pdf
•  Only few percent of candidates are real exoplanets
•  Need radial velocity confirmation with large
telescopes (≥4m)
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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First Rocky Exoplanet
CoRoT (Leger et al. 2009)
HARPS (Queloz et al. 2009)
•  4.8 Earth masses
•  density 5.6 g/cm3
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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CoRoT-7b
www.eso.org/public/outreach/press-rel/pr-2009/pr-33-09.html
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Second Rocky Exoplanet: Kepler-10b
Batalha et al. (2011)
•  4.6±1.2 Earth masses
•  Density 8.8±3 g/cm3
•  0.84 days orbital period
•  0.017 AU orbital radius
49
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Kepler 10b: Vulcan
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanet Densities
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanets as Black Bodies
Seager & Deming 2010
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Habitable Zone
•  Radiative equilibrium temperature: Incoming stellar
radiation = planet radiation into space
•  Corot 7b: 1300 °C
•  Kepler 10b: 1600 °C
•  Earth: -20 °C
•  Venus: -43 °C
•  Habitable zone: surface temperature between 0 and
100 Celsius: liquid water possible
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
ExoPlanet Atmosphere Observations
•  Transits: planet size
•  Thermal emission: emitting atmosphere,
temperature and its gradient, thermal phase curve
•  Transmission Spectra: upper atmosphere,
exosphere
•  Reflection: albedo, reflected light phase curve,
(polarization), scattering atmosphere
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Exoplanet Atmospheres
Seager & Deming 2010
•  Light curve provides information on star + planet
•  Secondary eclipse: no planet light -> albedo
•  Difference at different wavelengths -> exoplanet
spectrum -> atmosphere composition
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Secondary Eclipse
photojournal.jpl.nasa.gov/catalog/PIA11390
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Secondary Eclipse from Corot
Snellen et al. 2009
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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λ-Dependence of Secondary Eclipse
Charbonneau et al. 2008
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Exoplanet Atmospheres (Swain et al. 2009)
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Measured Exoplanet Atmospheres
Seager & Deming 2010
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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The Future: Direct Imaging
•  Central star much brighter than disks and planets
•  Disks: at least 104 times fainter than central star •  Jupiter: 109 times fainter than Sun
•  Telescope optics, Earth atmosphere make halo
•  halo: 102 to 106 times fainter than central star
•  Space telescopes show significant halos
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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1000 Million times Larger
Christoph U. Keller, [email protected]
en.wikipedia.org/wiki/File:Everest_kalapatthar_crop.jpg
Astronomical Data Analysis
2011: Exoplanet Detection
Fomalhaut b (Kalas et al. 2008)
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Very Young, Very Big Exoplanets
Marois et al. 2008
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Astronomical Data Analysis 2011: Exoplanet Detection
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Beta Pic
www.eso.org/public/images/eso1024c/
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Scattering Polarization
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
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Scattering Polarization
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Astronomical Data Analysis 2011: Exoplanet Detection
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ExPo at William Herschel Telescope
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Utrecht Extreme Polarimeter (ExPo)
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
SU Aurigae, a T-Tauri Star with Jet
ExPo
Christoph U. Keller, [email protected]
VS
HST
Astronomical Data Analysis 2011: Exoplanet Detection
Saturn with ExPo
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
Exoplanet in the Lab in Utrecht
Intensity
Christoph U. Keller, [email protected]
Polarization
Reconstructed Polarization
Astronomical Data Analysis 2011: Exoplanet Detection
ZIMPOL for SPHERE at VLT
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
EPICS for 42-m E-ELT
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection
EPICS for the E-ELT
Top view
Christoph U. Keller, [email protected]
Nasmyth implementation
Astronomical Data Analysis 2011: Exoplanet Detection
Rocky Planet in HZ (Gliese 581 d)
Christoph U. Keller, [email protected]
Astronomical Data Analysis 2011: Exoplanet Detection