  We propose to search for biosignatures in the spectra of reflected light from
about 100 Earth-sized planets that are already known to be orbiting in their
habitable zones (HZ). For a sample of G and K type hosts, most of these planets
will be between 25 and 50 milli-arcsec (mas) from their host star and 1 billion
to 10 billion times fainter. To separate the planet's image from that of its
host star at the wavelength (763nm) of the oxygen biosignature we need a
telescope with an aperture of 16 metres. Furthermore, the intensity of the
light from the host star at the position in the image of the exoplanet must be
suppressed otherwise the exoplanet will be lost in the glare.
  This presents huge technical challenges. The Earth's atmosphere is turbulent
which makes it impossible to achieve the required contrast from the ground at
763nm. The telescope therefore needs to be in space and to fit the telescope in
the rocket fairing it must be a factor of 4 or more times smaller when folded
than when operational. To obtain spectroscopy of the planet's biosignature at
763nm we need to use an integral field spectrometer (IFS) with a field of view
(FOV) of 1000 x 1000 milli-arcsec (mas) and a spectral resolution of 100. This
is a device that simultaneously takes many pictures of the exoplanet each at a
slightly different wavelength which are then recorded as a data cube with two
spatial dimensions and one wavelength dimension. In every data cube wavelength
slice, the background light from the host star at the location of the planet
image must be minimised. This is achieved via a coronagraph which blocks the
light from the host star and active/adaptive optics techniques which
continuously maintain very high accuracy optical alignment to make the images
as sharp as possible. These are the technical challenges to be addressed in a
design study.
