Abstract
A three-color, extremely high signal to noise, precisely pointed,
sub-arcsecond photometer would enable us to distinguish Earth-like
exoplanets from other rocky, gassy, or icy worlds - if we had the right
three wavelengths, and the ability to block out the primary star’s
glare. Color-color discrimination of Earth-like planets has been posited
for quite some time. Broadband filters are not seen to be precise
diagnostics of planets with life, but rather broad but critical
similarity or difference to the one habitable planet we know. Visible
wavelengths (Vis) are advantageous due to relatively greater abundance
of photons of that wavelength range from planets around Sun-like stars.
Near-ultraviolet (UV) wavelengths may be advantageous due to ozone and
scattering properties in Earth’s atmosphere that make Earth truly stand
out from other known and modeled planets. Near infrared wavelengths (IR)
help discriminate potential biological color contributions. We conducted
an optimization exercise to arrive at three broadband filters that
reliably separate modeled Earth-like, nominally habitable planets from
other possible exoplanets. Criteria we use included: - Atmosphere/clouds
of habitable worlds - Sea/Land proportions (ocean, granite, basalt,
other surface material) - Vegetation (chlorophyll spectra and fictional
other photosynthetic materials depending on wavelengths of parent stars
etc.) - Other factors that can be spectrally modeled (planet-covering
cities, world oceans, etc.) The optimized bands resemble previous work
for exoplanets and the solar system, but underscore the advantage of UV
wavelengths and indicate their potential utility for exoplanet
identification and/or discrimination in concert with other exoplanet
observations. An exoplanet survey that could quickly identify such
planets could then be followed up by more detailed, longer term study by
more intensive campaigns with more capable, but more resource or time
constrained.