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Title: SPITZER TRANSITS OF THE SUPER-EARTH GJ1214b AND IMPLICATIONS FOR ITS ATMOSPHERE

Abstract

We observed the transiting super-Earth exoplanet GJ1214b using warm Spitzer at 4.5 {mu}m wavelength during a 20 day quasi-continuous sequence in 2011 May. The goals of our long observation were to accurately define the infrared transit radius of this nearby super-Earth, to search for the secondary eclipse, and to search for other transiting planets in the habitable zone of GJ1214. We here report results from the transit monitoring of GJ1214b, including a reanalysis of previous transit observations by Desert et al. In total, we analyze 14 transits of GJ1214b at 4.5 {mu}m, 3 transits at 3.6 {mu}m, and 7 new ground-based transits in the I+z band. Our new Spitzer data by themselves eliminate cloudless solar composition atmospheres for GJ1214b, and methane-rich models from Howe and Burrows. Using our new Spitzer measurements to anchor the observed transit radii of GJ1214b at long wavelengths, and adding new measurements in I+z, we evaluate models from Benneke and Seager and Howe and Burrows using a {chi}{sup 2} analysis. We find that the best-fit model exhibits an increase in transit radius at short wavelengths due to Rayleigh scattering. Pure water atmospheres are also possible. However, a flat line (no atmosphere detected) remains among the bestmore » of the statistically acceptable models, and better than pure water atmospheres. We explore the effect of systematic differences among results from different observational groups, and we find that the Howe and Burrows tholin-haze model remains the best fit, even when systematic differences among observers are considered.« less

Authors:
;  [1]; ;  [2]; ; ;  [3];  [4];  [5];  [6]
  1. Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
  2. Institute d'Astrophysique et de Geophysique, Universite de Liege, Liege (Belgium)
  3. Department of Earth, Atmospheric and Planetary Sciences, and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
  4. Department of Planetary Sciences and Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 (United States)
  5. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 (United States)
  6. Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)
Publication Date:
OSTI Identifier:
22126983
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 765; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; ECLIPSE; INFRARED RADIATION; METHANE; MONITORING; PLANETARY ATMOSPHERES; PLANETS; RAYLEIGH SCATTERING; WATER; WAVELENGTHS

Citation Formats

Fraine, Jonathan D., Deming, Drake, Gillon, Michaeel, Jehin, Emmanueel, Demory, Brice-Olivier, Benneke, Bjoern, Seager, Sara, Lewis, Nikole K., Knutson, Heather, and Desert, Jean-Michel, E-mail: jfraine@astro.umd.edu. SPITZER TRANSITS OF THE SUPER-EARTH GJ1214b AND IMPLICATIONS FOR ITS ATMOSPHERE. United States: N. p., 2013. Web. doi:10.1088/0004-637X/765/2/127.
Fraine, Jonathan D., Deming, Drake, Gillon, Michaeel, Jehin, Emmanueel, Demory, Brice-Olivier, Benneke, Bjoern, Seager, Sara, Lewis, Nikole K., Knutson, Heather, & Desert, Jean-Michel, E-mail: jfraine@astro.umd.edu. SPITZER TRANSITS OF THE SUPER-EARTH GJ1214b AND IMPLICATIONS FOR ITS ATMOSPHERE. United States. doi:10.1088/0004-637X/765/2/127.
Fraine, Jonathan D., Deming, Drake, Gillon, Michaeel, Jehin, Emmanueel, Demory, Brice-Olivier, Benneke, Bjoern, Seager, Sara, Lewis, Nikole K., Knutson, Heather, and Desert, Jean-Michel, E-mail: jfraine@astro.umd.edu. Sun . "SPITZER TRANSITS OF THE SUPER-EARTH GJ1214b AND IMPLICATIONS FOR ITS ATMOSPHERE". United States. doi:10.1088/0004-637X/765/2/127.
@article{osti_22126983,
title = {SPITZER TRANSITS OF THE SUPER-EARTH GJ1214b AND IMPLICATIONS FOR ITS ATMOSPHERE},
author = {Fraine, Jonathan D. and Deming, Drake and Gillon, Michaeel and Jehin, Emmanueel and Demory, Brice-Olivier and Benneke, Bjoern and Seager, Sara and Lewis, Nikole K. and Knutson, Heather and Desert, Jean-Michel, E-mail: jfraine@astro.umd.edu},
abstractNote = {We observed the transiting super-Earth exoplanet GJ1214b using warm Spitzer at 4.5 {mu}m wavelength during a 20 day quasi-continuous sequence in 2011 May. The goals of our long observation were to accurately define the infrared transit radius of this nearby super-Earth, to search for the secondary eclipse, and to search for other transiting planets in the habitable zone of GJ1214. We here report results from the transit monitoring of GJ1214b, including a reanalysis of previous transit observations by Desert et al. In total, we analyze 14 transits of GJ1214b at 4.5 {mu}m, 3 transits at 3.6 {mu}m, and 7 new ground-based transits in the I+z band. Our new Spitzer data by themselves eliminate cloudless solar composition atmospheres for GJ1214b, and methane-rich models from Howe and Burrows. Using our new Spitzer measurements to anchor the observed transit radii of GJ1214b at long wavelengths, and adding new measurements in I+z, we evaluate models from Benneke and Seager and Howe and Burrows using a {chi}{sup 2} analysis. We find that the best-fit model exhibits an increase in transit radius at short wavelengths due to Rayleigh scattering. Pure water atmospheres are also possible. However, a flat line (no atmosphere detected) remains among the best of the statistically acceptable models, and better than pure water atmospheres. We explore the effect of systematic differences among results from different observational groups, and we find that the Howe and Burrows tholin-haze model remains the best fit, even when systematic differences among observers are considered.},
doi = {10.1088/0004-637X/765/2/127},
journal = {Astrophysical Journal},
number = 2,
volume = 765,
place = {United States},
year = {Sun Mar 10 00:00:00 EST 2013},
month = {Sun Mar 10 00:00:00 EST 2013}
}
  • We report observations of two consecutive transits of the warm super-Earth exoplanet GJ 1214b at 3.6 and 4.5 {mu}m with the Infrared Array Camera instrument on board the Spitzer Space Telescope. The two transit light curves allow for the determination of the transit parameters for this system. We find these parameters to be consistent with the previously determined values and no evidence for transit timing variations. The main investigation consists of measuring the transit depths in each bandpass to constrain the planet's transmission spectrum. Fixing the system scale and impact parameters, we measure R{sub p} /R{sub *} = 0.1176{sup +0.0008}{submore » -0.0009} and 0.1163{sup +0.0010}{sub -0.0008} at 3.6 and 4.5 {mu}m, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical allows us to rule out a cloud-free, solar composition (i.e., hydrogen-dominated) atmosphere at 4.5{sigma} confidence. This independently confirms a recent finding that was based on a measurement of the planet's transmission spectrum using the Very Large Telescope (VLT). The Spitzer, MEarth, and VLT observations together yield a remarkably flat transmission spectrum over the large wavelength domain spanned by the data. Consequently, cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H{sub 2}O) by volume to be consistent with all the observations.« less
  • Unlike hot Jupiters or other gas giants, super-Earths are expected to have a wide variety of compositions, ranging from terrestrial bodies like our own to more gaseous planets like Neptune. Observations of transiting systems, which allow us to directly measure planet masses and radii and constrain atmospheric properties, are key to understanding the compositional diversity of the planets in this mass range. Although Kepler has discovered hundreds of transiting super-Earth candidates over the past 4 yr, the majority of these planets orbit stars that are too far away and too faint to allow for detailed atmospheric characterization and reliable massmore » estimates. Ground-based transit surveys focus on much brighter stars, but most lack the sensitivity to detect planets in this size range. One way to get around the difficulty of finding these smaller planets in transit is to start by choosing targets that are already known to host super-Earth sized bodies detected using the radial velocity (RV) technique. Here we present results from a Spitzer program to observe six of the most favorable RV-detected super-Earth systems, including HD 1461, HD 7924, HD 156668, HIP 57274, and GJ 876. We find no evidence for transits in any of their 4.5 μm flux light curves, and place limits on the allowed transit depths and corresponding planet radii that rule out even the most dense and iron-rich compositions for these objects. We also observed HD 97658, but the observation window was based on a possible ground-based transit detection that was later ruled out; thus the window did not include the predicted time for the transit detection recently made by the Microvariability and Oscillations of Stars space telescope.« less
  • Capitalizing on the observational advantage offered by its tiny M dwarf host, we present Hubble Space Telescope/Wide Field Camera 3 (WFC3) grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 {mu}m. Inconsistent with a cloud-free solar composition atmosphere at 8.2{sigma}, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. Amore » dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 {mu}m in diameter.« less
  • We report recent ground-based photometry of the transiting super-Earth exoplanet GJ 1214b at several wavelengths, including the infrared near 1.25 {mu}m (J band). We observed a J-band transit with the FLAMINGOS infrared imager and the 2.1 m telescope on Kitt Peak, and we observed several optical transits using a 0.5 m telescope on Kitt Peak and the 0.36 m Universidad de Monterrey Observatory telescope. Our high-precision J-band observations exploit the brightness of the M dwarf host star at this infrared wavelength as compared with the optical and are significantly less affected by stellar activity and limb darkening. We fit themore » J-band transit to obtain an independent determination of the planetary and stellar radii. Our radius for the planet (2.61{sup +0.30} {sub -0.11} R {sub +}) is in excellent agreement with the discovery value reported by Charbonneau et al. based on optical data. We demonstrate that the planetary radius is insensitive to degeneracies in the fitting process. We use all of our observations to improve the transit ephemeris, finding P = 1.5804043 {+-} 0.0000005 days and T {sub 0} = 2454964.94390 {+-} 0.00006 BJD.« less
  • We present optical photometry of 16 transits of the super-Earth GJ 1214b, allowing us to refine the system parameters and search for additional planets via transit timing. Starspot-crossing events are detected in two light curves, and the star is found to be variable by a few percent. Hence, in our analysis, special attention is given to systematic errors that result from starspots. The planet-to-star radius ratio is 0.11610 {+-} 0.00048, subject to a possible upward bias by a few percent due to the unknown spot coverage. Even assuming this bias to be negligible, the mean density of the planet canmore » be either 3.03 {+-} 0.50 g cm{sup -3} or 1.89 {+-} 0.33 g cm{sup -3}, depending on whether the stellar radius is estimated from evolutionary models, or from an empirical mass-luminosity relation combined with the light curve parameters. One possible resolution is that the orbit is eccentric (e {approx} 0.14), which would favor the higher density, and hence a much thinner atmosphere for the planet. The transit times were found to be periodic within about 15 s, ruling out the existence of any other super-Earths with periods within a factor of two of the known planet.« less