Identifying Beach-front Property on Earth-like Worlds Peter R. McCullough, STScI Youth for Astronomy and Engineering Program Jan 23, 2007

Exoplanets are relevant to these Satellites: Hubble Hubble Spitzer Spitzer MOST MOST COROT COROT Kepler Kepler JWST JWST TPF * TPF * *Flight Opportunity for TPF is TBD.

Habitable worlds? 70 Vir, available from Extrasolar Visions Inc.

Water is important for life…

That glint tells you that surface is water!

UnpolarizedS polarizedP polarized Images from eltonoptics.com

Glint and Rayleigh-Scattered Light both are bright at 90 degrees Earth Planet Star

UnpolarizedS polarizedP polarized Unpolarized

S polarizedP polarized

S polarized

Unpolarized Light Satellite images Simulations

Polarized Light S-pol P-pol

10-m or 20-m telescope required. Total flux 2x flux difference

What’s great about this? The image of a star can be designed to be nearly identical in the two polarizations, so the glare of the star can be suppressed not only by coronagraphy but also by subtracting one polarized image from the other. The unpolarized star cancels out; the polarized planet doesn’t. The image of a star can be designed to be nearly identical in the two polarizations, so the glare of the star can be suppressed not only by coronagraphy but also by subtracting one polarized image from the other. The unpolarized star cancels out; the polarized planet doesn’t. Spectra imply dividing light into 100+ bins; linear polarization implies two bins. Spectra imply dividing light into 100+ bins; linear polarization implies two bins. Rayleigh scattering is very blue; glint from oceans is achromatic. (so four bins: 2 polarization; 2 wavelengths) Rayleigh scattering is very blue; glint from oceans is achromatic. (so four bins: 2 polarization; 2 wavelengths) Glint is very localized (~15 degrees of “longitude”). Glint is very localized (~15 degrees of “longitude”). The glint’s flux difference in the two polarizations is 0.15 photons per second for a 10-m telescope observing Earth-Sun system at 10 pc. Long integrations (days) can pick out the polarized light from an oceanic planet in the glare of the star. One hour integration gives Poisson S/N ~ 20 if star light can be suppressed entirely by a superb technology. The glint’s flux difference in the two polarizations is 0.15 photons per second for a 10-m telescope observing Earth-Sun system at 10 pc. Long integrations (days) can pick out the polarized light from an oceanic planet in the glare of the star. One hour integration gives Poisson S/N ~ 20 if star light can be suppressed entirely by a superb technology.

No atmospheric absorption

Earth-like clear atmospheric absorption

Light curve for Earth

Ocean planet with clear atmosphere; Rayleigh suppressed (long wavelength)

With clouds too

Summary 1)Polarization may be more practical than spectra for physical characterization of exo-earths. 2)Polarization’s unique signature helps identify oceanic planets, i.e. the beach-front property we all desire! 3)A 10-m diameter space telescope with imaging polarization capability can detect oceans if they exist on terrestrial exoplanets and nearly map continental boundaries. To learn more, download… Speech by McCullough at the Conference, “Astrophysics Enabled by the Return to the Moon” Held at STScI on Nov 30, 2006 And