The Astrophysical Journal, 757:7 (6pp), 2012 September 20 doi:10.1088/0004-637X/757/1/7 C 2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A. WISE DETECTIONS OF DUST IN THE HABITABLE ZONES OF PLANET-BEARING STARS Farisa Y. Morales1, D. L. Padgett2, G. Bryden1, M. W. Werner1, and E. Furlan3,4 1 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA; [email protected] 2 Goddard Space Flight Center, Greenbelt, MD 20771, USA 3 National Optical Astronomy Observatory, Tucson, AZ 85719, USA Received 2012 May 5; accepted 2012 July 17; published 2012 August 28 ABSTRACT We use data from the Wide-field Infrared Survey Explorer (WISE) all-sky release to explore the incidence of warm dust in the habitable zones around exoplanet-host stars. Dust emission at 12 and/or 22 μm(Tdust ∼ 300 and/or ∼150 K) traces events in the terrestrial planet zones; its existence implies replenishment by evaporation of comets or collisions of asteroids, possibly stirred by larger planets. Of the 591 planetary systems (728 extrasolar planets) in the Exoplanet Encyclopaedia as of 2012 January 31, 350 are robustly detected by WISE at 5σ level. We perform detailed photosphere subtraction using tools developed for Spitzer data and visually inspect all the WISE images to confirm bona fide point sources. We find nine planet-bearing stars show dust excess emission at 12 and/or 22 μm at 3σ level around young, main-sequence, or evolved giant stars. Overall, our results yield an excess incidence of ∼2.6% for stars of all evolutionary stages, but ∼1% for planetary debris disks around main-sequence stars. Besides recovering previously known warm systems, we identify one new excess candidate around the young star UScoCTIO 108. Key words: circumstellar matter – infrared: planetary systems – planets and satellites: formation – stars: individual (UScoCTIO 108) Online-only material: color figures 1. INTRODUCTION Bryden et al. 2009). The relationship between planets and debris is unclear except in the general sense that the debris is indicative The recent release of the Wide-field Infrared Survey Explorer of planetary system formation, so one might expect to see some (WISE; Wright et al. 2010) all-sky survey provides an opportu- association. Several prominent A-type stars with imaged planets nity to explore the incidence of warm circumstellar dust for a also have bright debris disks (Fomalhaut, HR 8799, and β Pic; large sample of stars. The WISE data set covers 100% of the sky Kalas et al. 2008; Marois et al. 2008; Lagrange et al. 2009), at four infrared bands W1,...,W4 centered at 3.4, 4.6, 12, and further suggesting a link between the two phenomena. The 22 μm wavelengths that are sensitive to thermal emission from question is whether the presence of planets enhances, depresses, objects at temperatures similar to our Earth (∼300 K), asteroid or is neutral in terms of the frequency or observability of dust belt, and the interior zodiacal cloud (165–250 K). In this work, systems. we use this all-sky data set to search for dust in the habitable Here, we use WISE data to attempt to find evidence of zones around exoplanet-bearing stars. comet/asteroid activity in the habitable zones of stars known to In the last two decades, over 700 exoplanets have been harbor exoplanets, and thereby examine the link between planets revealed and confirmed, primarily from radial velocity (RV) and debris. Several groups have already searched for similar and transit studies, plus ∼2300 announced candidates for the planet–debris relationships using WISE.Krivovetal.(2011) Kepler mission (Batalha et al. 2012) awaiting confirmation.5 used data from the preliminary WISE release, covering 57% The majority of these planets are in systems very different from of the sky, to search for dust around 52 systems with transiting our own, with Neptune-size or bigger planets, too large and exoplanets finding zero 3σ excess detections in any single WISE often too close to the host star to be habitable. On the other band (where σ refers to the fractional infrared excess statistical hand, a significant fraction of planetary debris disks have warm significance). But by combining <3σ excesses in W3 and W4, dust (Tdust ∼ 200 K). Following up on results from Infrared Krivov et al. (2011) identify two candidate excess detections Astronomical Satellite and Infrared Space Observatory, surveys that remain to be confirmed. With the Kepler Objects of by the Spitzer Space Telescope (Werner et al. 2004) found that Interest (KOI) Catalogue of candidate transiting exoplanets (not many nearby stars are surrounded by dust that is thought to be necessarily planet bearing), Lawler & Gladman (2012) find the generated by collisions between asteroids and/or sublimation fraction of stars with warm excesses to be ∼3% using the of comets. Of the ∼350 debris disks measured individually with recently released all-sky WISE survey. Also using the all-sky Spitzer (Chen et al. 2005;Suetal.2006; Beichman et al. 2006; WISE release and Kepler’s KOIs, Ribas et al. (2012) identify Trilling et al. 2008; Carpenter et al. 2009; Plavchan et al. 2009), 13 transiting planet systems with IR excess (>2σ ) at 12 and/or ∼70 are known to have warm components with Tdust > 150 K 22 μm, out of a 546 star sample. Only four are 3σ detections (Morales et al. 2011). for a corresponding detection rate of ∼0.7%. Among the Spitzer discoveries are the first systems known to Unlike the previous work, our study makes use of the WISE have both orbiting dust and exoplanets (Beichman et al. 2005; all-sky release and focuses on the set of confirmed exoplanets, including not just transiting planets, but also those detected by 4 Visitor, Infrared Processing and Analysis Center, Caltech, Pasadena, CA RV measurements or by direct imaging. We do not consider 91125, USA. 5 Extrasolar Planets Encyclopaedia, http://exoplanet.eu/ the Kepler candidate objects, but our sample does overlap with 1 The Astrophysical Journal, 757:7 (6pp), 2012 September 20 Morales et al. Figure 1. Color–magnitude diagram for planet-bearing stars in our sample, color-coded by planet discovery method. The 202 out of 350 planet-bearing stars in our sample plotted here, mostly from transiting (“Tran”) and RV studies, have WISE W4 photometry with S/NTotal 3. In the legend, “Ima” stands for the direct imaging method used to identify substellar-mass companions, and “Ast” for astrometry. Most are main-sequence stars without excess emission and found between the vertical dashed lines ([3.4] − [22] ≈ 0.2), while interesting excess candidates populate the region with [3.4] − [22] > ∼0.2. Also included are the well-known “Fab Four” debris disks, Vega, Fomalhaut, β Pic, Eri (asterisks), and 66 well-characterized Spitzer warm debris disk (triangles) from Morales et al. (2011) labeled “M11” in the legend. A visual inspection of WISE images of the non-labeled green triangles with [3.4] − [22] > ∼0.2 reveals that their W1 data are contaminated by excess flux from diffraction halo artifacts and/or the W4 data are contaminated by background emission. 2M1207 and UScoCTIO108 are substantially fainter than the rest of the planet–star–dust systems; this “limit” is due to the apparent magnitudes of the confirmed companions rather than the WISE detection limits. (A color version of this figure is available in the online journal.) Krivov et al. (2011) and Ribas et al. (2012) for a handful of et al. 2010). Figure 1 is a color–magnitude diagram color-coded confirmed transiting systems. by planet discovery method, to illustrate how stars with infrared For comparison, Padgett et al. (2012) searched for circumstel- excess emission can be initially identified. Most stars do not lar warm dust using the WISE all-sky data set, while considering have excess (i.e., [3.4] − [22] ∼ 0), but the WISE fluxes reveal all Hipparcos stars within 120 pc, as well as Tycho stars with an interesting sample of objects with [3.4] − [22] > ∼ 0.2. proper motions >30 mas yr−1. Their results suggest that the fre- quency of warm excess emission associated with stellar sources 3. ANALYSIS is 2% at the WISE sensitivity limits. In Section 2, we describe the data for our chosen sample of 3.1. SED Fitting target stars. Based on this data, we next (Section 3) determine which planet-bearing stars have evidence for warm dust by To measure excess emission at the WISE wavelengths, we fitting the spectral energy distribution (SED) and visually require a precise method to subtract off the stellar photosphere. inspecting the WISE images. In Section 4, we interpret the results We start by compiling near-IR Two Micron All Sky Survey and summarize our findings in Section 5. (2MASS) data (cross-correlated with the WISE catalog) and optical Tycho photometry (Høg et al. 2000) from the NASA 7 2. SAMPLE SELECTION AND THE WISE DATA Exoplanet Archive. For the SED fitting, we use previously de- veloped advanced tools built to study large samples of debris Our starting target list consists of 591 planetary systems (with disks as seen by Spitzer (Morales et al. 2009, 2011). These tools 728 extrasolar planets) given in the Exoplanet Encyclopaedia as χ 2 fit the SEDs of each of the 350 exoplanet-bearing stars to of 2012 January 31 (Schneider et al. 2011). The Schneider et al. derive the photosphere and disk contributions. The latter can be compilation includes planetary-mass companions detected us- approximated by a blackbody corresponding to the characteris- ing all methods presently utilized, i.e., RV or astrometry, tran- tic temperature of the dust grains, and NextGen models are used sit studies, micro-lensing, pulsar timing, and direct imaging.