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 discAiry'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 discAiry'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! 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Direct Imaged Planet Candidates Based on data compiled by J. Schneider Note this plot is somewhat out of date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 b in thermal infrared light from MagAO/Clio2, processed to remove the bright light from its host star, HD 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: atmosphere-infographic.html

2M1207

a Gl 229 NASA/Kulkarni, Golimowsk)

55 Cnc Oppenheimer

GQ Lup

AB Pic

SCR Biller et al. 2006

SCR 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 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 J Lafreniere et al. 2008

1RXS J Lafreniere et al AU 150 pc T=1800±200 K M=8 (+4 -1) MJup Young solar mass star (5 Myr)

Direct Imaged Planet Candidates NameMass Estimate(M J ) Radius Estimate (R J ) Semi-major Axis (AU) Distance From Earth (pc) 2M1207 b / (+/-1.1) GQ Lup b21.5 +/ / (+/-50) AB Pic b13.5 +/ (+/-1.2) SCR 1845 b> 8.5> /-0.02 UScoCTIO 108b AU145 +/- 2 CT Cha b17 +/ AU165 +/- 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?

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/(m 2 s Hz)

Flux from a Planet Stars become fainter with increasing distance Inverse square law – F ~ 1/D 2 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

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/D 2 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 F p ( ) is the flux at the planet surface F p  ( ) is the planet flux at Earth

Visible-Wavelength Flux at Earth F p ( ) is the flux at the planet surface F p  ( ) is the planet flux at Earth

Sun J M V E Solar System at 10 pc (Seager 2003) hot Jupiters Planets at 10 pc

Planet-Star Flux Ratio at Earth F p ( ) is the flux at the planet surface F p  ( ) 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 aligned Alignment 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

Microlensing Lecture Contents Gravitational Microlensing Overview Planet-Finding Microlensing Concept Tour of Planet Microlensing Light Curves

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 BLG- 109Lb,c Bennett et al Gaudi et al. 2008

OGLE 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)

For animation see:

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