Exoplanet Detection Techniques IIGUASA 12/10/2013Prof. 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 Geographicused 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_Quest-

movies/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

Based on data compiled by J. Schneider

Note this plot is somewhat out of date

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-8799-atmosphere-infographic.html

2M1207

a

Gl 229

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

Schmidt et al. 2008

Background star

Star: classical T Tauri (0.9-3 Myr)

17±6 MJup

2.2±0.8 RJup

165±30 pc

~440 AU

T=2600±250 K

1RXS J160929.1-210524

Lafreniere et al. 2008

1RXS J160929.1-210524

Lafreniere et al. 2008

330 AU

150 pc

T=1800±200 K

M=8 (+4 -1) MJup

Young solar mass star (5 Myr)

Direct Imaged Planet CandidatesName Mass

Estimate(MJ)Radius Estimate (RJ)

Semi-majorAxis (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 planet-star 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

Sun

J

M

VE

Solar System at 10 pc (Seager 2003)

hot Jupiters

Planets at 10 pc

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 visible-wavelengths 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

Huge magnification is possible if source and lens are alignedAlignment is rare!Infinite magnification is theoretically possible for the “point caustic”

Sackett 1998

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

Udalski et al. 2005

MOA-2007-BLG-192

Bennett et al. 2008

OGLE-2006-BLG-109Lb,c

Bennett et al. 2008

Gaudi et al. 2008

OGLE-2006-BLG-109Lb,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 semi-major 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.

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)

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

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

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

Based on data compiled by J. Schneider

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