Identifying Beach-front Property on Earth-like

Worlds

Peter R. McCullough, STScIPeter R. McCullough, STScI

Youth for Astronomy and Engineering Program Youth for Astronomy and Engineering Program

Jan 23, 2007Jan 23, 2007

Exoplanets are relevant to these Satellites:

HubbleHubble SpitzerSpitzer MOSTMOST COROTCOROT KeplerKepler JWSTJWST 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!

Unpolarized S polarizedP polarized

Imag

es f

rom

elt

onop

tics

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Glint and Rayleigh-Scattered Light both are bright at 90

degrees

Earth

Planet

Star

Unpolarized S polarizedP polarized

Unpolarized

Unpolarized S polarizedP polarized

P 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 The image of a star can be designed to be nearly identical in the two polarizations, so the glare of the identical in the two polarizations, so the glare of the star can be suppressed not only by coronagraphy but star can be suppressed not only by coronagraphy but also by subtracting one polarized image from the also by subtracting one polarized image from the other. The unpolarized star cancels out; the polarized other. The unpolarized star cancels out; the polarized planet doesn’t.planet doesn’t.

Spectra imply dividing light into 100+ bins; linear Spectra imply dividing light into 100+ bins; linear polarization implies two bins.polarization implies two bins.

Rayleigh scattering is very blue; glint from oceans is Rayleigh scattering is very blue; glint from oceans is achromatic. (so four bins: 2 polarization; 2 achromatic. (so four bins: 2 polarization; 2 wavelengths)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 The glint’s flux difference in the two polarizations is 0.15 photons per second for a 10-m telescope 0.15 photons per second for a 10-m telescope observing Earth-Sun system at 10 pc. Long integrations observing Earth-Sun system at 10 pc. Long integrations (days) can pick out the polarized light from an oceanic (days) can pick out the polarized light from an oceanic planet in the glare of the star. One hour integration planet in the glare of the star. One hour integration gives Poisson S/N ~ 20 if star light can be suppressed gives Poisson S/N ~ 20 if star light can be suppressed entirely by a superb technology.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…To learn more, download…

Speech by McCullough at the Conference,Speech by McCullough at the Conference,

““Astrophysics Enabled by the Return to the Moon”Astrophysics Enabled by the Return to the Moon”

Held at STScI on Nov 30, 2006Held at STScI on Nov 30, 2006

AndAndhttp://xxx.lanl.gov/abs/astro-ph/0610518