Exoplanetary Systems with SAFARI:A Far Infrared Imaging Spectrometer for SPICA

Exoplanetary Systems with SAFARI:
A Far Infrared Imaging Spectrometer for SPICA

J. R. Goicoechea and B. Swinyard
on behalf of the SPICA/SAFARI science teams
Abstract

The far-infrared (far-IR) spectral window plays host to a wide range of spectroscopic diagnostics with which to study planetary disk systems and exoplanets at wavelengths completely blocked by the Earth atmosphere. These include the thermal emission of dusty belts in debris disks, the water ice features in the “snow lines” of protoplanetary disks, as well as many key chemical species (O, OH, HO, NH, HD, etc). These tracers play a critical diagnostic role in a number of key areas including the early stages of planet formation and potentially, exoplanets. The proposed Japanese-led IR space telescope SPICA, with its 3m–class cooled mirror (5 K) will be the next step in sensitivity after ESA’s Herschel Space Observatory (successfully launched in May 2009). SPICA is a candidate “M-mission” in ESA’s Cosmic Vision 2015-2025 process. We summarize the science possibilities of SAFARI: a far-IR imaging-spectrometer (covering the 34 – 210 m band) that is one of a suite of instruments for SPICA.

Centro de Astrobiología (CSIC-INTA), Madrid, Spain

Rutherford Appleton Laboratory, Chilton, Didcot, UK

1. A New Window in Exoplanet Research

The study of exo-planets (EPs) requires many different approaches across the full wavelength spectrum to both discover and characterize the newly discovered objects in order that we might fully understand the prevalence, formation, and evolution of planetary systems. The mid–IR and far–IR spectral regions are especially important in the study of planetary atmospheres as it spans both the peak of thermal emission from the majority of EPs thus far discovered (up to 1000 K) and is particularly rich in molecular features that can uniquely identify the chemical composition, from protoplanetary disks to planetary atmospheres and trace the fingerprints of primitive biological activity. In the coming decades many space- and ground-based facilities are planned that are designed to search for EPs on all scales from massive, young “hot Jupiters”, through large rocky super-Earths down to the detection of exo-Earths. Few of the planned facilities, however, will have the ability to characterize the planetary atmospheres which they discover through the application of mid-IR and far-IR spectroscopy. SPICA will be realized within 10 years and has a suite of instruments that can be applied to the detection and characterization of EPs over the 5–210 m spectral range (see e.g. our White Paper; Goicoechea et al. 2008).

Figure 1.: Left: Fit to HD 209458b “hot Jupiter” (T 1000 K) fluxes inferred from a secondary transit with Spitzer (Swain et al. 2008) around a G0 star ( 47 pc, in black) and interpolation to  = 10 pc (gray). The emission of a cooler Jupiter-like planet at 5 pc is shown in dashed (reflected emission neglected). Thick horizontal lines are the 5-1hr photometric sensitivities of SPICA mid-IR instruments and SAFARI. Dotted lines show sensitivities in spectrophotometric mode (). SPICA will observe similar transits of inner “hot Jupiter” routinely and will potentially extract their IR spectrum (rich in HO, O, CH, NH and HD features as in Solar System planets). Right: Increasing planet-to-star contrast at longer mid– and far–infrared wavelengths (Goicoechea et al. 2008).

The SAFARI instrument (Swinyard et al. 2009) will provide capabilities to complement SPICA studies in the mid-IR (either coronagraphic or transit studies). Indeed, SAFARI could be the only planned instrument able to study EPs in a completely new wavelength domain (for SAFARI’s band 1) not covered by JWST nor by Herschel (Fig. 1). This situation is often associated with unexpected discoveries. Since cool EPs show much higher contrast in the far-IR than in the near–/mid–IR (e.g. Jupiter’s effective temperature is  110 K), if such EPs are found in the next 10 years, their transit studies with SPICA will help to constrain their main properties, which are much more difficult to infer at shorter wavelengths. Note that following Infrared Space Observatory (ISO) observations, Jupiter seen at 5 pc will produce a flux of  35 Jy at 37 m, but less than  1 Jy at 15 m. SAFARI band1 ( 34 –60 m) hosts a variety of interesting atmospheric molecular features (e.g. HO at 39 m, HD at 37 m and NH at 40 and 42 m). Strong emission/absorption of these features was first detected by ISO in the atmospheres of Jupiter, Saturn, Titan, Uranus and Neptune (Feuchtgruber et al. 1999).

Acknowledgments.

We warmly thank the SPICA/SAFARI science teams for fruitful discussions. JRG is supported by a Ramón y Cajal research contract.

References

  • Feuchtgruber et al. (1999) Feuchtgruber, H., Lellouch, E., Bezard, B. et al. 1999, A&A, 341, L17.
  • Goicoechea et al. (2008) Goicoechea, J. R., Swinyard, B., Tinetti, G. et al. 2008, A White Paper for ESA’s Exo-Planet Roadmap Advisory Team (2008 July 29), astro-ph/0809.0242.
  • Swain et al. (2008) Swain, M. R., Bouwman, J., Akeson, R. L. et al. 2008, ApJ, 674, 482.
  • Swinyard et al. (2009) Swinyard, B., Nakagawa, T., et al. 2009, Exp. Astron., 23, 193
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