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"From Cosmic Birth to Living Earths" advocates a 12-meter optical/near-IRspace telescope for launch ~2035. The goal that sets this large size is thedetection of biosignatures from Earth-like planets in their habitable zonesaround G-stars. The discovery of a single instance of life elsewhere in theuniverse would be a profound event for humanity. But not at any cost. At 8-9BUSD this High Definition Space Telescope (HDST) would take all the NASAastrophysics budget for nearly 20 years, unless new funds are found. For ageneration NASA could build no "Greater Observatories" matching JWST in therest of the spectrum. This opportunity cost prompted me to study the drivingexobiosphere detection case for HDST. I find that: (1) the focus on G-stars isnot well justified; (2) only G-stars require the use of direct imaging; (3) inthe chosen 0.5 - 2.5 micron band, the available biosignatures are ambiguous anda larger sample does not help; (4) the expected number of exobiospheres is 1,with a 5% chance of zero; (5) the accessible sample size is too small to showthat exobiospheres are rare; (6) a sufficiently large sample would require amuch larger telescope; (7) the great progress in M-star planet spectroscopy -both now and with new techniques, instruments and telescopes already planned -means that a biosignature will likely be found before HDST could complete itssearch in ~2045. For all these reasons I regretfully conclude that HDST, whilecommendably ambitious, is not the right choice for NASA Astrophysics at thistime. The first exobiosphere discovery is likely to be such a major event thatscientific and public pressure will produce new funding across a range ofdisciplines, not just astrophysics, to study the nature of Life in theUniverse. Then will be the time when a broader science community can advocatefor a mission that will make definitive exobiosphere measurements.

Issues with the High Definition Space Telescope (HDST) ExoEarth  Biosignature Case: A Critique of the 2015 AURA Report "From Cosmic Birth to  Living Earths: the future of UVOIR Astronomy"