Characterizing and Detecting Extrasolar Planets David Spergel February 2004

PREDICTION Some time in the next decade, SIM, Kepler, Eclipse, JPF, or some other telescope will be detect an Earth-like planet. This will revolutionize astronomy

Will We Find Life?  The necessary ingredients of life are widespread  Observation reveals uniformity of physical and chemical laws  Origin of the elements and their dispersal is well understood  Life on Earth can inhabit harsh environments  Micro- and environmental biology reveal life in extremes of temperature, chemistry, humidity  Life affects a planetary environment in a detectable way  Our own atmosphere reflects the presence of primitive through advanced life  Planets are a common outcome of star formation  Modern theory of star formation makes planet formation likely

m Optical Telescope as Life Finder  TPF will likely be a 4- 8 meter class optical telescope or a “small” mid-IR interferometer  It will be capable of detecting Earths out to pc  m telescope would be the next step: Characterize planets and detect a large sample

Direct Planet Imaging: Good News Much faster detections Immediate detection of entire system Enormous additional science Size and Albedo Spectroscopy Biomarkers

Ford/Seager/Turner Model See FST, Nature, 412, 885 (2001).See FST, Nature, 412, 885 (2001).  180x360 deg resolution map of surface  Pixel auto-classification by satellite imagery  BDRFs - in 4 bands for 6 pixel types  Single scattering, no elevation variations  Gray cummulus clouds only  Monte Carlo to 1% accuracy: B, G, R, NIR Water with waves (specular & isotropic components) Permanent ice (strong backscattering) Seasonal/sea ice (80% dirty ice, 20% dirt) Bare ground (90% sand, 10% clay) Grass/brush land (67% dirt, 33% clover) Forested land (75% leaves, 25% peat)

Scattered Light The scattered light comes from a small part of the planet surface

Viewing Geometry No Clouds: high contrast between land and ocean Ford, Seager,Turner, Nature, 2001

Clouds Clouds: bright, variable, correlated in space and time. Ford, Seager,Turner, Nature, 2001

Extrasolar Planets Ford, Seager,Turner, Nature, 2001 TIME (Days)

Plants in visible versus near infrared light

Optical Plant Signature as a Biomarker Chlorophyll causes strong absorption blueward of 0.7  m. The high reflectance red-ward of 0.7  m is from light scattering in the gaps between plant cells. This “red edge” is an evolutionary adaptation which helps plants stay cool enough to allow efficient photosynthesis.

Red Edge chlorophyl l absorption water absorptio n wavelength （ nm ） rose chinquapin Benjamin Reflection by plants hibiscus pothos baseline

APO 3.5m Earthshine Spectra (Feb 2002) water vapor water vapor water vapor oxygen (A) oxygen (B) oxygen (  ) RED EDGE?

Bad News  Detecting light from planets beyond solar system is hard:  Planet signal is weak but detectable (few photons/sec/m 2)  Star emits million to billion more than planet  Planet within 1 AU of star  Dust in target solar system  300 brighter than planet  Finding a firefly next to a searchlight on a foggy night >10 9 >10 6

The Diffraction Problem (Visible) Focal plane Diameter (D) The image in the focal plane is the spatial Fourier transform of the entrance field Wavelength  Entrance Pupil 1 1e-10

Airy Rings Linear Scale Log Scale (1e-10 is black)

The Angular Resolution Challenge + Coronagraphs at >3 /D Interferometers at > 1 /B 10  m, 28 m Coronagraph Cost ($$), Launch Date

Control of Star Light  Control diffracted light with various apodizing pupil and/or coronagraph masks  Square masks  Graded aperture  Multiple Gaussian masks  Band limited masks Control scattered light – Deformable mirror with 10,000 actuators for final /3000 wavefront (<1 Å)

Wavefront Sensing and Control

What is the biggest problem? Wavefront Error! Phase AberrationsAmplitude Aberrations

Coronagraph Status Rapid progress in past year HCIT (Trauger at JPL) is now achieving contasts of ~ few with band-limited masks and active optics Significant progress in understanding control and imaging

Can a VLST be Life Finder?  Life Finder will have special requirements  Stability (or sensing)  Uniform amplitude or control  Need to evaluate m in space versus 100m on ground  Requirements:  Ability to do 5-10% photometry within an hour (size depends on distance to Earth-like planet)  Small field, spectral resolution improves ability to remove speckle  Wave front sensing and control

Planet Finding Is A Decades-Long Undertaking  Like cosmology, the search for planets and life will motivate broad research areas and utilize many telescopes for decades to come