A Response to Elvis' 2015 Critique of the AURA Report "From Cosmic Birth to Living Earths"

A Response to Elvis' 2015 Critique of the AURA Report "From Cosmic Birth to Living Earths"

Authors:

Seager et al

Abstract:

To ensure progress in astronomy over the coming decades, the key questions are "what facilities will we build, and when?" Toward this end, the Association of Universities for Research in Astronomy (AURA) recently commissioned a study on future space-based options for UV and optical astronomy. The resulting study - "From Cosmic Births to Living Earths" - concluded that a space telescope equipped with a 12-meter class primary mirror would make fundamental advances across virtually all of astrophysics, including finding and characterizing the atmospheres of dozens of Earth-like planets. This ambitious telescope concept is referred to as the High Definition Space Telescope (HDST). In a recent arXiv white paper, Elvis (2015) critiqued a subset of the findings of the AURA study, focusing on the detection and characterization of rocky exoplanets in the habitable zone. In this response, we clarify these issues to confirm that HDST would play a transformative role in the study of terrestrial worlds. Its capabilities for studying exoplanets would be truly unique, even in 2035, and would complement HDST's broad and deep range of exciting astrophysics.

Problems With Detecting ExoEarths With the Proposed High Definition Space Telescope

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"

Author:

Elvis

Abstract:

"From Cosmic Birth to Living Earths" advocates a 12-meter optical/near-IR space telescope for launch ~2035. The goal that sets this large size is the detection of biosignatures from Earth-like planets in their habitable zones around G-stars. The discovery of a single instance of life elsewhere in the universe would be a profound event for humanity. But not at any cost. At 8-9B USD this High Definition Space Telescope (HDST) would take all the NASA astrophysics budget for nearly 20 years, unless new funds are found. For a generation NASA could build no "Greater Observatories" matching JWST in the rest of the spectrum. This opportunity cost prompted me to study the driving exobiosphere detection case for HDST. I find that: (1) the focus on G-stars is not well justified; (2) only G-stars require the use of direct imaging; (3) in the chosen 0.5 - 2.5 micron band, the available biosignatures are ambiguous and a 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 show that exobiospheres are rare; (6) a sufficiently large sample would require a much 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 its search in ~2045. For all these reasons I regretfully conclude that HDST, while commendably ambitious, is not the right choice for NASA Astrophysics at this time. The first exobiosphere discovery is likely to be such a major event that scientific and public pressure will produce new funding across a range of disciplines, not just astrophysics, to study the nature of Life in the Universe. Then will be the time when a broader science community can advocate for a mission that will make definitive exobiosphere measurements.

The AURA Concept for the High Definition Space Telescope

From Cosmic Birth to Living Earths: The Future of UVOIR Space Astronomy

Authors:

Dalcanton et al

Abstract:

For the first time in history, humans have reached the point where it is possible to construct a revolutionary space-based observatory that has the capability to find dozens of Earth-like worlds, and possibly some with signs of life. This same telescope, designed as a long-lived facility, would also produce transformational scientific advances in every area of astronomy and astrophysics from black hole physics to galaxy formation, from star and planet formation to the origins of the Solar System. The Association of Universities for Research in Astronomy (AURA) commissioned a study on a next-generation UVOIR space observatory with the highest possible scientific impact in the era following JWST. This community-based study focuses on the future space-based options for UV and optical astronomy that significantly advance our understanding of the origin and evolution of the cosmos and the life within it. The committee concludes that a space telescope equipped with a 12-meter class primary mirror can find and characterize dozens of Earth-like planets and make fundamental advances across nearly all fields of astrophysics. The concept is called the High Definition Space Telescope (HDST). The telescope would be located at the Sun-Earth L2 point and would cover a spectral range that, at a minimum, runs from 0.1 to 2 microns. Unlike JWST, HDST will not need to operate at cryogenic temperatures. HDST can be made to be serviceable on orbit but does not require servicing to complete its primary scientific objectives. We present the scientific and technical requirements for HDST and show that it could allow us to determine whether or not life is common outside the Solar System. We do not propose a specific design for such a telescope, but show that designing, building and funding such a facility is feasible beginning in the next decade - if the necessary strategic investments in technology begin now.