Asantha Cooray, University of California, Irvine, and Margaret Meixner, Space Telescope Science Institute, of the Origins Space Telescope Science and Technology Definition Team
The Origins Space Telescope (OST) is the mission concept for the Far-Infrared Surveyor, one of the four large-mission science and technology definition studies sponsored by NASA for the 2020 Astronomy and Astrophysics Decadal survey. The mission has four science themes: (i) Characterizing small bodies in the outer solar system; (ii) Tracing the signatures of life and the ingredients of habitable worlds; (iii) Unveiling the growth of black holes and galaxies over cosmic time; and (iv) Charting the rise of metals, dust and the first galaxies. We outline the science drivers related to extra-solar planets.
Figure 1: Modeled secondary eclipse spectra in the thermal infrared, shown as the planet-to-star flux ratio in parts per million, for Earth (blue solid), Venus (yellow), Mars (red), and a super-Earth (blue dashed) with an identical atmospheric composition and structure to Earth but with a planetary radius of 1.5 Earth radii. Key molecular features are indicated (credit: Tyler Robinson).
Formation and characterization of exoplanets
OST will open a vast discovery space in the study of cool exoplanets in habitable zones and in following the trail of ingredients needed to foster life. Targeting exoplanets with transit spectroscopy in the 6-40 micron wavelength range where planet-star contrast is maximized, OST will measure the temperatures and search for basic chemical ingredients for life on rocky planets. The OST will be able to detect the thermal emission peak of cool exoplanets at habitable temperatures (about 300 Kelvin) around late-type stars, and measure their atmospheric composition. This may be accomplished by a combination of transit spectroscopy and secondary eclipse observations (Figure 1). Important atmospheric diagnostics of planets in the 6-40 micron wavelength range include spectral bands due to ammonia (a bio-signature), the 9.6-micron ozone band (another bio-signature), the 7.7-micron methane band (also key for life detection), and the 15-micron carbon dioxide band (an important greenhouse gas). Initial calculations indicate that a 10 meter OST can detect these features against M-dwarfs at 10 parsecs in about 100 hours of total transit, provided that systematic detector noise can be kept below about 10 parts per million.
The mid-infrared technical/instrumental capabilities of OST will include a coronagraph for high-contrast imaging. At an inner working angle of 2λ/D (roughly twice the diffraction limit), it will be capable of imaging planets at distances from five astronomical units and above (at about 25 parsecs for a 10-meter OST at 6 microns). At a contrast ratio of 10-7 (a factor of 10 improvement beyond laboratory demonstrations to date in the mid-infrared), OST will be sensitive to Jupiter-like exoplanets at comparable distances from their host stars (Figure 2). The Origins Space Telescope will also reveal the path of water as both ice and gas from the interstellar medium to the inner regions of planet-forming disks, and determine the total masses of disks via the HD 112 microns rotational ground-state line around stars across the stellar mass range out to distances of 500 parsecs. The Origins Space Telescope will also address fundamental questions on how planetary systems evolve after they initially form by determining the true frequency of Kuiper-belt analogs and measuring the mineral and volatile composition of typical debris disks.
Figure 2: Spectra of the sun and Jupiter seen from 10 parsecs. OST's unique combination of high angular resolution and mid-infrared wavelength coverage will allow us to study the thermal emission of true Jupiter analogs around the nearest stars (Jupiter spectrum from Michael Cushing and Pat Irwin; credit: Eric Nielsen).
The Mission Study Status:
Over the past nine months, the science and technology definition team (STDT) engaged the community to develop compelling science investigation proposals for our mission study. More than 32 community-driven science proposals were reviewed from five science working groups. The scientific requirements of the 14 top-ranked proposals were used to select a baseline architecture to best address the key scientific questions of the next decade. The selected architecture is an actively cooled, filled-aperture telescope with effective diameter around 9 meters. The five instruments under study are: three imaging spectrometers for low (R=300-10000), medium (R=105), and high (R=107) spectral resolving power, a wide-field, broad-band imager with polarization capability, and a mid-infrared (6-40 micrometer) imager/integrated field unit with a coronagraph for direct imaging of exoplanets and transit/secondary eclipse/phase curve studies. For exoplanet spectroscopy, stability will be the most important requirement, and we are considering a dedicated ultra-stable mid-infrared photon-counting capability for this application.
The OST mission concept study team will continue to develop scientific requirements and detailed designs for the Origins Space Telescope and instruments. The exoplanet community is especially invited to join our working group on exoplanet direct imaging and characterization. The current effort in this area is coordinated by Eric Nielsen (direct imaging) and Tyler Robinson (transit spectroscopy). Members of the community interested in contributing to the study should contact any member of the STDT (http://origins.ipac.caltech.edu) or send email to ost_info@lists.ipac.caltech.edu.