Exoplanet Detection Techniques II
GUASA 12/10/2013
Prof. Sara Seager MIT
Exoplanet Detection Techniques II
• Planet Detection Techniques in More Detail
– Direct Imaging
– Microlensing
– Astrometry
Direct Imaging Lecture Contents
• Direct Imaging
– Planet and Star Spatial Separation
– Adaptive Optics
• Direct Imaged Candidates
• What is Being Measured?
• Planet-Star Flux Ratios
 National Geographic
used with permission
Direct Imaging
• Number 1 requirement is to spatially separate
planet and star
Direct Imaging
• Number 2 requirement is to literally block out
the glare of the star
Diffraction
• Light from a point source
passes through a small
circular aperture, it does
not produce a bright dot
as an image, but rather a
diffuse circular disc known
as Airy's disc
• The disk is surrounded by
much fainter concentric
circular rings.
Diffraction
• Light from a point source
passes through a small
circular aperture, it does
not produce a bright dot
as an image, but rather a
diffuse circular disc known
as Airy's disc
• The disk is surrounded by
much fainter concentric
circular rings.
Spatial Resolution
• Rayleigh criterion: the
minimum resolvable
angular separation of the
two objects
• Single slit
• Circular aperture
•  is the wavelength of
light, D is the aperture
diameter
Ground-Based Limitations
• Turbulence in the atmosphere blurs mixes up
photon paths through the atmosphere and
blurs images
Ground-Based Limitations
• Turbulence in the atmosphere blurs mixes up
photon paths through the atmosphere and
blurs images
• Adaptive optics can correct for this!
• http://planetquest.jpl.nasa.gov/Planet_Questmovies/AO_quickTime.html
Direct Imaging Lecture Contents
• Direct Imaging
– Planet and Star Spatial Separation
– Adaptive Optics
•
•
•
•
Direct Imaged Planet Candidates
What is Being Measured?
Planet-Star Flux Ratios
Direct Imaging Techniques for Earths
Direct Imaged Planet Candidates
Note this plot is somewhat out of
date
Based on data compiled by J. Schneider
TMR-1
NASA/Terebey
This is a discovery image of planet HD 106906
b in thermal infrared light from MagAO/Clio2,
processed to remove the bright light from its
host star, HD 106906 A. The planet is more
than 20 times farther away from its star than
Neptune is from our Sun. AU stands for
Astronomical Unit, the average distance of the
Earth and the Sun. (Image: Vanessa Bailey)
HR 8799
See also: http://www.space.com/20231-giant-exoplanets-hr-8799atmosphere-infographic.html
2M1207
Gl 229
a
NASA/Kulkarni, Golimowsk)
55 Cnc
Oppenheimer
GQ Lup
AB Pic
SCR 1845-6357
Biller et al.
2006
SCR 1845-6357
9 - 65 MJup
(likely T-dwarf)
Very close to Earth:
3.85 pc
~4.5 AU from primary
Biller et al.
2006
CT Cha
Schmidt et al. 2008
CT Cha
17±6 MJup
2.2±0.8 RJup
165±30 pc
Background star
~440 AU
T=2600±250 K
Star: classical T Tauri (0.9-3 Myr)
Schmidt et al. 2008
1RXS J160929.1-210524
Lafreniere et al. 2008
1RXS J160929.1-210524
330 AU
Young solar mass
star (5 Myr)
150 pc
T=1800±200 K
M=8 (+4 -1) MJup
Lafreniere et al. 2008
Direct Imaged Planet Candidates
Name
Mass
Estimate(MJ)
Radius
Estimate (RJ)
Semi-major
Axis (AU)
Distance
From Earth
(pc)
2M1207 b
4 +6-1
1.5
46 +/- 5
52.4 (+/-1.1)
GQ Lup b
21.5 +/- 20.5
1.8
103 +/- 37
140 (+/-50)
AB Pic b
13.5 +/- 0.5
275
45.6 (+/-1.2)
SCR 1845 b
> 8.5
> 4.5
3.85 +/-0.02
UScoCTIO
108b
14 +2-8
670 AU
145 +/- 2
CT Cha b
17 +/- 6
440 AU
165 +/- 30
This table is incomplete. Let’s look at a table online …
Direct Imaging Lecture Contents
• Direct Imaging
– Planet and Star Spatial Separation
– Adaptive Optics
• Direct Imaged Candidates
• What is Being Measured?
• Planet-Star Flux Ratios
What is Being Measured?
What is Being Measured?
• Do we know the mass and radius of the
planet?
• Mass and radius are inferred from planet
evolution models
What is Being Measured?
• Astronomers are measuring the planet flux at the
detector
• Flux = energy/(m2 s Hz)
Flux from a Planet
• Stars become fainter with
increasing distance
• Inverse square law
– F ~ 1/D2
• Energy radiates outward
• Think of concentric spheres
centered on the star
• The surface of each sphere has the
same amount of energy per s
passing through it
• Energy = flux * surface area
The History of Pluto’s Mass
http://hoku.as.utexas.edu/~gebhardt/a309f06/plutomass.gif
Planets
• A flux measurement at visible wavelengths gives
albedo*area
• A flux measurement at thermal infrared wavelengths
gives temperature*area
• Same brightness from
– A big, reflective and hence cold planet
– A small, dark, and therefore hot planet
• A combination gives of the two measurements gives:
– Albedo, temperature, and area!
Direct Imaging Lecture Contents
• Direct Imaging
– Planet and Star Spatial Separation
– Adaptive Optics
• Direct Imaged Candidates
• What is Being Measured?
• Planet-Star Flux Ratios
• In the interests of time I will skip the planetstar flux ratio derivation and leave it for you if
you are interested
Flux from a Planet
•
•
Stars become fainter with increasing
distance
Inverse square law
– F ~ 1/D2
•
•
•
•
•
Energy radiates outward
Think of concentric spheres centered
on the star
The surface of each sphere has the
same amount of energy per s passing
through it
Energy = flux * surface area
Flux at Earth
Thermal Flux at Earth
• Fp() is the flux at the
planet surface
• Fp () is the planet
flux at Earth
Visible-Wavelength Flux at Earth
• Fp() is the flux at the
planet surface
• Fp () is the planet
flux at Earth
Planets at 10 pc
Sun
hot Jupiters
J
V
E
M
Solar System at 10 pc
(Seager 2003)
Planet-Star Flux Ratio at Earth
• Fp() is the flux at the
planet surface
• Fp () is the planet
flux at Earth
Thermal Emission Flux Ratio
• Planet-to-star flux ratio
• Black body flux
• Take the ratio
• Approximation for long
wavelengths
• Final flux ratio
• Thermal emission is
typically at infrared
wavelengths
Scattered-Light Flux Ratio
• Planet-to-star flux
ratio
• Black body flux
• Scattered stellar flux
• Take the planet-to-star
flux ratio
• Scattered flux is
usually at visiblewavelengths for
planets
Direct Imaging Lecture Summary
• Direct Imaging
– Diffraction limits detection
• Spatial resolution
• Diffracted light is brighter than planets
• Direct Imaged Candidates
– Four direct imaged planet candidates
– Mass and radiusi are inferred from models
– No way to confirm mass
• What is Being Measured?
– Flux at detector.
– Other parameters are inferred
• Planet-Star Flux Ratios
– Approximations are useful for estimates
Exoplanet Detection Techniques II
• Planet Detection Techniques in More Detail
– Direct Imaging
– Microlensing
– Astrometry
Microlensing Lecture Contents
• Gravitational Microlensing Overview
• Planet-Finding Microlensing Concept
• Tour of Planet Microlensing Light Curves
Gravitational Lensing
• Light from a very distant, bright source is
"bent" around a massive object between
the source object and the observer
• A product of general relativity
Gravitational Lensing
• According to general relativity, mass "warps"
space-time to create gravitational fields
• When light travels through these fields it bends
as a result
• This theory was confirmed in 1919 during a solar
eclipse when Arthur Eddington observed the
light from stars passing close to the sun was
slightly bent, so that stars appeared slightly out
of position
Strong Gravitational Lensing
Image is distorted into a ring if the lens and source are perfecty aligned (and the
lens is a “point” or spherical compact mass)
Strong
Gravitational
Lensing
Multiple distorted
images appear if the
lens and source are not
aligned (and the lens is
not spherical)
Can you pick out the
lensed objects?
Gravitational Microlensing
• The shape of the distortion in the background
object is not seen because the images cannot
be spatially resolved
• Instead, time is exploited: the amount of light
received from the background object changes in
time due to the relative motion of the source and
the lens and the distorted shape
• For exoplanets, the background source and the
lens are both stars in the Milky Way Galaxy
Microlensing
Sackett 1998
Microlensing
Sackett 1998
Bending angle from general relativity
Characteristic angular scale
Note degeneracy among D and M
E = Angular size of the ring image on the sky in the case of perfect lens-source alignment
Microlensing
Sackett 1998
Microlensing
Sackett 1998
Huge magnification is possible if source and lens are aligned
Alignment is rare!
Infinite magnification is theoretically possible for the “point caustic”
Microlensing
Sackett 1998
Infinite magnification is potentially possible on the caustic
Microlensing
Sackett 1998
Microlensing Animation
http://www.youtube.com/watch?v=J_w1OJlXTzg
http://www.eso.org/public/videos/eso0847b/
Microlensing Lecture Contents
• Gravitational Microlensing Overview
• Planet-Finding Microlensing Concept
• Tour of Planet Microlensing Light Curves
http://www.hinduonnet.com/fline/fl2303/images/20060224003010304.jpg
http://bulge.princeton.edu/~ogle/ogle3/blg235-53.html
OGLE235-MOA53 (1)
Bond et al. 2004
OGLE235-MOA53 (2)
Zoom in of (1)
Bond et al. 2004
OGLE235-MOA53 (2)
Bond et al. 2004
OGLE-2005-BLG-169
Gould et al. 2006
OGLE 2005-BLG-390Lb (1)
Beaulieu et al. 2006
OGLE 2005-BLG-390Lb (2)
Beaulieu et al. 2006
OGLE 2005-BLG-071
0.08M sun < M* < 0.5M sun
1.5kpc < DL < 5kpc
Mp
M*
@ 7.1± 0.3 ´10-3
0.05M J < M p < 4 M J
Udalski et al. 2005
MOA-2007-BLG-192
Bennett et al. 2008
OGLE2006BLG109Lb,c
Gaudi et al. 2008
Bennett et al. 2008
OGLE2006BLG109Lb,c
Gaudi et al. 2008
Microlensing Lecture Summary
• Microlensing Exoplanet Discovery Technique
– Sensitive to low-mass planets down to Earth-mass
(for high magnification events)
– Actual mass of star and planet, and planet semimajor axis are discernable with high magnification
events
– Planet cannot be followed up after event
Exoplanet Detection Techniques II
• Planet Detection Techniques in More Detail
– Direct Imaging
– Microlensing
– Astrometry
Astrometry Lecture Contents
• Astrometry Overview
• Tour of Planet Astrometry Light Curves
Astrometry
• Astrometry is the branch of astronomy that
relates to precise measurements and
explanations of the positions and
movements of stars and other celestial
bodies.
M p ap = M*a*
Astrometry
• Recall that radial velocity measured the 1D
line of site motion of the star (about the star
and planet common center of mass)
• Astrometry measures the 2D motion of the
star on the sky (about the star and planet
common center of mass)
M p ap = M*a*
http://csep10.phys.utk.edu/astr162/lect/binaries/astrometric.html
M p ap = M*a*
http://csep10.phys.utk.edu/astr162/lect/binaries/astrometric.html
For animation see:
http://en.wikipedia.org/wiki/Astrometric_binary
http://csep10.phys.utk.edu/astr162/lect/binaries/astrometric.html
Astrometry Estimates
• What is the maximum angular motion on the sky of
a sun-like star due to a Jupiter-mass companion at 5
AU separation? Due to an Earth-mass companion at
1 AU separation?
– 1 degree?
– 1 arc sec?
• Make an estimate in degrees, arc min (60 arc min in
1 degree), or arc sec (60 arc sec in 1 arc min)
• Star is 10 pc from Earth
• 1 arcsec = 1 AU/10 pc
p p
* *
M a =Ma
Astrometry Lecture Contents
• Astrometry Overview
• Tour of Planet Astrometry Light Curves
Barnard’s Star (1)
Van de Kamp 1963
Barnard’s Star (2)
Van de Kamp 1982
GJ 876
Benedict et al. 2002
Epsilon
Eridani
(1)
Benedict et al. 2006
Epsilon Eridani (2)
Benedict et al. 2006
Lecture Summary
• Astrometry Exoplanet Discovery and
Characterization Technique
– No discoveries to date because high precision over
long time scales
– Used currently as a characterization technique
– GAIA mission is about to launch
Lecture I Summary
Exoplanets come in all masses, sizes,
orbit parameters
Many different exoplanet discovery
techniques are known
Radial velocity and transit finding are
the most successful to date
Direct Imaging is next with GPI
coming online
Based on data compiled by J. Schneider