Search

Extrasolar super-Earths (1-10 M$_{\earth}$) are likely to exist with a wide range of atmospheres. Some super-Earths may be able to retain massive hydrogen-rich atmospheres. Others might never accumulate hydrogen or experience significant escape of lightweight elements, resulting in atmospheres more like those of the terrestrial planets in our Solar System. We examine how an observer could differentiate between hydrogen-rich and hydrogen-poor atmospheres by modeling super-Earth emission and transmission spectra, and we find that discrimination is possible by observing the transmission spectrum alone. An Earth-like atmosphere, composed of mostly heavy elements and molecules, will have a very weak transmission signal due to its small atmospheric scale height (since the scale height is inversely proportional to molecular weight). On the other hand, a large hydrogen-rich atmosphere reveals a relatively large transmission signal. The super Earth emission spectrum can additionally contrain the atmospheric composition and temperature structure. Super-Earths with massive hydrogen atmospheres will reveal strong spectral features due to water, whereas those that have lost most of their hydrogen (and have no liquid ocean) will be marked by CO$_2$ features and a lack of H$_2$O. We apply our study specifically to the low-mass planet orbiting an M star, Gl 581c ($M sin i$ = 5 M$_{\earth}$), although our conclusions are relevant for super-Earths in general. The ability to distinguish hydrogen-rich atmospheres might be essential for interpreting mass and radius observations of planets in the transition between rocky super-Earths and Neptune-like planets., 28 pages, 6 figures, accepted to ApJ

The NASA Discovery mission EPOXI, utilizing the Deep Impact flyby spacecraft, comprises two phases: EPOCh (Extrasolar Planet Observation and Characterization) and DIXI (Deep Impact eXtended Investigation). With EPOCh, we use the 30-cm high resolution visible imager to obtain ultraprecise photometric light curves of known transiting planet systems. We will analyze these data for evidence of additional planets, via transit timing variations or transits; for planetary moons or rings; for detection of secondary eclipses and the constraint of geometric planetary albedos; and for refinement of the system parameters. Over a period of four months, EPOCh observed four known transiting planet systems, with each system observed continuously for several weeks. Here we present an overview of EPOCh, including the spacecraft and science goals, and preliminary photometry results., 7 pages, 5 figures. To appear in the Proceedings of the 253rd IAU Symposium: "Transiting Planets", May 2008, Cambridge, MA

The Canadian MOST satellite is a unique platform for observations of bright transiting exoplanetary systems. Providing nearly continuous photometric observations for up to 8 weeks, MOST can produce important observational data to help us learn about the properties of exosolar planets. We review our current observations of HD 209458, HD 189733 with implications towards the albedo and our progress towards detecting reflected light from an exoplanet., 7 pages, 3 figures. To appear in the Proceedings of IAU Symposium 253, "Transiting Planets", May 2008, Cambridge, MA

We present radiation transfer models that demonstrate that reflected light levels from three dimensional (3D) exoplanetary atmospheres can be more than 50% lower than those predicted by models of homogeneous or smooth atmospheres. Compared to smooth models, 3D atmospheres enable starlight to penetrate to larger depths resulting in a decreased probability for the photons to scatter back out of the atmosphere before being absorbed. The increased depth of penetration of starlight in a 3D medium is a well known result from theoretical studies of molecular clouds and planetary atmospheres. For the first time we study the reflectivity of 3D atmospheres as a possible explanation for the apparent low geometric albedos inferred for extrasolar planetary atmospheres. Our models indicate that 3D atmospheric structure may be an important contributing factor to the non-detections of scattered light from exoplanetary atmospheres. We investigate the self-shadowing radiation transfer effects of patchy cloud cover in 3D scattered light simulations of the atmosphere of HD209458b. We find that, for a generic planet, geometric albedos can be as high as 0.45 in some limited situations, but that in general the geometric albedo is much lower. We conclude with some explanations on why extrasolar planets are likely dark at optical wavelengths., Accepted to MNRAS

This report is a comprehensive study of the search for and study of planets around other stars (exoplanets). The young but maturing field of exoplanets is perhaps one of the most compelling fields of study in science today--both because of the discoveries made to date on giant planets around other stars, and because the detection of planets just like our Earth ('Earth analogs') is at last within reach technologically. In the Report we outline the need for a vigorous research program in exoplanets to understand our place in the cosmos: whether planets like our home Earth are a common or rare outcome of cosmic evolution. The strategy we developed is intended to address the following fundamental questions, in priority order, within three distinct 5-yr long phases, over a 15 year period: (1) What are the physical characteristics of planets in the habitable zones around bright, nearby stars? (2) What is the architecture of planetary systems? (3) When, how and in what environments are planets formed? The Report recommends a two-pronged strategy for the detection and characterization of planets the size of the Earth. For stars much less massive and cooler than our Sun (M-dwarfs), existing ground-based techniques including radial velocitymore » and transit searches, and space-based facilities both existing and under development such as Spitzer and JWST, are adequate for finding and studying planets close to the mass and size of the Earth. Conducted in parallel with the M-dwarf strategy is one for the more challenging observations of the hotter and brighter F, G, and K stars, some of which are very close in properties to our Sun, in which the frequency of Earth-sized planets is assessed with Corot and Kepler, but new space missions are required for detection and study of specific Earth-mass and Earth-sized objects. Our Task Force concludes that the development of a space-based astrometric mission, narrowly-focused to identify specific nearby stars with Earth-mass planets, followed by direct detection and study via a spaceborne coronagraph/occulter or interferometric mission, is the most robust approach to pursue. Ground and space-based microlensing programs pursued in parallel would provide complementary information on planetary system architectures on galactic scales. The program for F, G, and K stars must be preceded, at the beginning of the strategy, by broad yet detailed technical assessments to determine whether the astrometric and direct detection technologies will be ready in the time frames envisioned (the second and third 5-yr periods, respectively). Also measurement of dust around nearby candidate stars must be undertaken early to determine whether typical systems are clean enough to make direct detection feasible. Alternative strategies are discussed should problems arise in any of these areas. Finally, the Task Force lays out recommended programs in ground-based observations of larger planets, of planet-forming disks, and theoretical and laboratory studies crucial to interpreting and understanding the outcome of the planet search and characterization observations.« less

With the recent discoveries of hundreds of extrasolar planets, the search for planets like Earth and life in the universe, is quickly gaining momentum. In the future, large space observatories could directly detect the light scattered from rocky planets, but they would not be able to spatially resolve a planet's surface. Using reflectance models and real cloud data from satellite observations, here we show that, despite Earth's dynamic weather patterns, the light scattered by the Earth to a hypothetical distant observer as a function of time contains sufficient information to accurately measure Earth's rotation period. This is because ocean currents and continents result in relatively stable averaged global cloud patterns. The accuracy of these measurements will vary with the viewing geometry and other observational constraints. If the rotation period can be measured with accuracy, data spanning several months could be coherently combined to obtain spectroscopic information about individual regions of the planetary surface. Moreover, deviations from a periodic signal can be used to infer the presence of relatively short-live structures in its atmosphere (i.e., clouds). This could provide a useful technique for recognizing exoplanets that have active weather systems, changing on a timescale comparable to their rotation. Such variability is likely to be related to the atmospheric temperature and pressure being near a phase transition and could support the possibility of liquid water on the planet's surface.

EPOXI (EPOCh + DIXI) is a NASA Discovery Program Mission of Opportunity using the Deep Impact flyby spacecraft. The EPOCh (Extrasolar Planet Observation and Characterization) Science Investigation will gather photometric time series of known transiting exoplanet systems from January through August 2008. Here we describe the steps in the photometric extraction of the time series and present preliminary results of the first four EPOCh targets., Comment: 4 pages, 2 figures. To appear in the Proceedings of the 253rd IAU Symposium: "Transiting Planets", May 2008, Cambridge, MA

The prospects for finding transiting exoplanets in the range of a few to 20 Earth masses is growing rapidly with both ground-based and spaced-based efforts. We describe a publicly available computer code to compute and quantify the compositional ambiguities for differentiated solid exoplanets with a measured mass and radius, including the mass and radius uncertainties., Comment: PASP, in press. 20 pages including 5 figures. Try the MATLAB code and send us comments

Differentiation in terrestrial planets is expected to include the formation of a metallic iron core. We predict the existence of terrestrial planets that have differentiated but have no metallic core--planets that are effectively a giant silicate mantle. We discuss two paths to forming a coreless terrestrial planet, whereby the oxidation state during planetary accretion and solidification will determine the size or existence of any metallic core. Under this hypothesis, any metallic iron in the bulk accreting material is oxidized by water, binding the iron in the form of iron oxide into the silicate minerals of the planetary mantle. The existence of such silicate planets has consequences for interpreting the compositions and interior density structures of exoplanets based on their mass and radius measurements., Comment: ApJ, in press. 22 pages, 5 figures

Terrestrial-like exoplanets may obtain atmospheres from three primary sources: Capture of nebular gases, degassing during accretion, and degassing from subsequent tectonic activity. Here we model degassing during accretion to estimate the range of atmospheric mass and composition on exoplanets ranging from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and differentiated meteorite compositions. Degassing alone can create a wide range of masses of planetary atmospheres, ranging from less than a percent of the planet's total mass up to ~6 mass% of hydrogen, ~20 mass% of water, and/or ~5 mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a result of oxidizing metallic iron with water, and excess water and carbon can produce atmospheres through simple degassing. As a byproduct of our atmospheric outgassing models we find that modest initial water contents (10 mass% of the planet and above) create planets with deep surface liquid water oceans soon after accretion is complete., Comment: ApJ, in press. 32 pages, 6 figures

In this chapter we examine what can be learned about extrasolar planet atmospheres by concentrating on a class of planets that transit their parent stars. As discussed in the previous chapter, one way of detecting an extrasolar planet is by observing the drop in stellar intensity as the planet passes in front of the star. A transit represents a special case in which the geometry of the planetary system is such that the planet s orbit is nearly edge-on as seen from Earth. As we will explore, the transiting planets provide opportunities for detailed follow-up observations that allow physical characterization of extrasolar planets, probing their bulk compositions and atmospheres.

The bulk composition of an exoplanet is commonly inferred from its average density. For small planets, however, the average density is not unique within the range of compositions. Variations of a number of important planetary parameters--which are difficult or impossible to constrain from measurements alone--produce planets with the same average densities but widely varying bulk compositions. We find that adding a gas envelope equivalent to 0.1%-10% of the mass of a solid planet causes the radius to increase 5-60% above its gas-free value. A planet with a given mass and radius might have substantial water ice content (a so-called ocean planet) or alternatively a large rocky-iron core and some H and/or He. For example, a wide variety of compositions can explain the observed radius of GJ 436b, although all models require some H/He. We conclude that the identification of water worlds based on the mass-radius relationship alone is impossible unless a significant gas layer can be ruled out by other means., 5 pages, 3 figures, accepted to ApJ

We compute theoretical infrared light curves for several known extrasolar planets. We have constructed a set of routines to calculate the orbital parameters for a given planet and integrate over the planetary disk to determine the total flux density of the planet as it orbits the parent star. We have further developed a spectral synthesis routine to calculate theoretical spectra of extrasolar giant planets from 3–24 $\mu$ m. The code requires a temperature-pressure profile as input, calculated by solving the radiative transfer equation; it then calculates continuous opacities and line opacities for water, carbon monoxide, and methane, and finally integrates over the layers of the atmosphere to determine the emergent flux. By integrating the theoretical spectrum over the bandpass of a particular instrument and including realistic instrument noise, we produce a set of multi-wavelength, infrared light curves. Using these light curves, we predict whether a particular known planet can be observed and characterized using the Spitzer Space Telescope, as well as other proposed space-based instruments, such as the Fourier-Kelvin Stellar Interferometer (FKSI) and the James Webb Space Telescope (JWST).

We use new interior models of cold planets to investigate the mass-radius relationships of solid exoplanets, considering planets made primarily of iron, silicates, water, and carbon compounds. We find that the mass-radius relationships for cold terrestrial-mass planets of all compositions we considered follow a generic functional form that is not a simple power law: $\log_{10} R_s = k_1 + 1/3 \log_{10}(M_s) - k_2 M_s^{k_3}$ for up to $M_p \approx 20 M_{\oplus}$, where $M_s$ and $R_s$ are scaled mass and radius values. This functional form arises because the common building blocks of solid planets all have equations of state that are well approximated by a modified polytrope of the form $\rho = \rho_0 + c P^n$. We find that highly detailed planet interior models, including temperature structure and phase changes, are not necessary to derive solid exoplanet bulk composition from mass and radius measurements. For solid exoplanets with no substantial atmosphere we have also found that: with 5% fractional uncertainty in planet mass and radius it is possible to distinguish among planets composed predominantly of iron or silicates or water ice but not more detailed compositions; with $\sim$~5% uncertainty water ice planets with $\gtrsim 25%$ water by mass may be identified; the minimum plausible planet size for a given mass is that of a pure iron planet; and carbon planet mass-radius relationships overlap with those of silicate and water planets due to similar zero-pressure densities and equations of state. We propose a definition of "super Earths'' based on the clear distinction in radii between planets with significant gas envelopes and those without., Comment: ApJ, in press, 33 pages including 16 figures

Of the over 200 known extrasolar planets, just 14 pass in front of and behind their parent stars as seen from Earth. This fortuitous geometry allows direct determination of many planetary properties. Previous reports of planetary thermal emission give fluxes that are roughly consistent with predictions based on thermal equilibrium with the planets' received radiation, assuming a Bond albedo of approximately 0.3. Here we report direct detection of thermal emission from the smallest known transiting planet, HD 149026b, that indicates a brightness temperature (an expression of flux) of 2,300 +/- 200 K at 8 microm. The planet's predicted temperature for uniform, spherical, blackbody emission and zero albedo (unprecedented for planets) is 1,741 K. As models with non-zero albedo are cooler, this essentially eliminates uniform blackbody models, and may also require an albedo lower than any measured for a planet, very strong 8 microm emission, strong temporal variability, or a heat source other than stellar radiation. On the other hand, an instantaneous re-emission blackbody model, in which each patch of surface area instantly re-emits all received light, matches the data. This planet is known to be enriched in heavy elements, which may give rise to novel atmospheric properties yet to be investigated.

We report the results of infrared (8 micron) transit and secondary eclipse photometry of the hot Neptune exoplanet, GJ436b using Spitzer. The nearly photon-limited precision of these data allow us to measure an improved radius for the planet, and to detect the secondary eclipse. The transit (centered at HJD = 2454280.78149 +/-0.00016) shows the flat-bottomed shape typical of infrared transits, and it precisely defines the planet-to-star radius ratio (0.0839 +/-0.0005), independent of the stellar properties. However, we obtain the planetary radius, as well as the stellar mass and radius, by fitting to the transit curve simultaneously with an empirical mass-radius relation for M-dwarfs (M=R). We find Rs=Ms=0.47 +/-0.02 in solar units, and Rp=27,600 +/-1170 km (4.33 +/-0.18 Earth radii). This radius significantly exceeds the radius of a naked ocean planet, and requires a gasesous hydrogen-helium envelope. The secondary eclipse occurs at phase 0.587 +/-0.005, proving a significant orbital eccentricity (e=0.15 +/-0.012). The amplitude of the eclipse (5.7 +/-0.8e-4) indicates a brightness temperature for the planet of T=712 +/-36K. If this is indicative of the planet's physical temperature, it suggests the occurrence of tidal heating in the planet. An uncharacterized second planet likely provides ongoing gravitational perturbations, to maintain GJ436b's orbit eccentricity over long time scales., Comment: 15 pages, 2 figures, submitted to ApJ Letters

Recent Spitzer infrared measurements of hot Jupiter eclipses suggest that eclipse mapping techniques could be used to spatially resolve the day-side photospheric emission of these planets using partial occultations. As a first step in this direction, we simulate ingress/egress lightcurves for the three brightest known eclipsing hot Jupiters and evaluate the degree to which parameterized photospheric emission models can be distinguished from each other with repeated, noisy eclipse measurements. We find that the photometric accuracy of Spitzer is insufficient to use this tool effectively. On the other hand, the level of photospheric details that could be probed with a few JWST eclipse measurements could greatly inform hot Jupiter atmospheric modeling efforts. A JWST program focused on non-parametric eclipse map inversions for hot Jupiters should be actively considered., 32 pages, 6 figures, 3 tables, accepted for publication in ApJ

Of the over 200 known extrasolar planets, 14 exhibit transits in front of their parent stars as seen from Earth. Spectroscopic observations of the transiting planets can probe the physical conditions of their atmospheres. One such technique can be used to derive the planetary spectrum by subtracting the stellar spectrum measured during eclipse (planet hidden behind star) from the combined-light spectrum measured outside eclipse (star + planet). Although several attempts have been made from Earth-based observatories, no spectrum has yet been measured for any of the established extrasolar planets. Here we report a measurement of the infrared spectrum (7.5--13.2 micron) of the transiting extrasolar planet HD209458b. Our observations reveal a hot thermal continuum for the planetary spectrum, with approximately constant ratio to the stellar flux over this wavelength range. Superposed on this continuum is a broad emission peak centered near 9.65 micron that we attribute to emission by silicate clouds. We also find a narrow, unidentified emission feature at 7.78 micron. Models of these ``hot Jupiter'' planets predict a flux peak near 10 micron, where thermal emission from the deep atmosphere emerges relatively unimpeded by water absorption, but models dominated by water fit the observed spectrum poorly., Comment: To appear in Nature 22 Feb 2007; 17 pages, 7 figures, includes supplementary information

During the last few years, considerable effort has been directed towards large-scale (> $1 Billion) missions to detect and characterize earth-like planets around nearby stars, such as the Terrestrial Planet Finder Interferometer and Darwin missions. However, technological issues such as formation flying, cryocooling, obtaining sufficient null depth for broadband signals, and control of systematic noise sources will likely prevent these missions from entering Phase A until at least the end of the present decade. Futhermore, a large mission like TPF-I will also need the endorsement of the next Astronomical Decadal Survey to obtain a Phase A start in the next decade. Thus, given the present circumstances, we can expect TPF-I to launch no earlier than about 2020 or even as late as 2025. Presently more than 168 planets have been discovered by precision radial velocity survey techniques, and little is known about the majority of them. A simplified nulling interferometer operating in the near- to mid-infrared (e.g. ~ 3-8 microns), like the Fourier Kelvin Stellar Interferometer (FKSI), can characterize the atmospheres of a large sample of the known planets. Many other scientific problems can be addressed with a system like FKSI, including the imaging of debris disks, active galactic nuclei, and low mass companions around nearby stars. We discuss the rationale, both scientific and technological, for a competed mission in the $450-600 Million range, of which FKSI is an example. Such a mission is essential to develop our community and keep the larger community, including young scientists, engaged in the long-term effort towards the detection of Earth-like planets.

The nulling coronagraph is one of 5 instrument concepts selected by NASA for study for potential use in the TPF-C mission. This concept for extreme starlight suppression has two major components, a nulling interferometer to suppress the starlight to ~10-10 per airy spot within 2 λ/D of the star, and a calibration interferometer to measure the residual scattered starlight. The ability to work at 2 λ/D dramatically improves the science throughput of a space based coronagraph like TPF-C. The calibration interferometer is an equally important part of the starlight suppression system. It measures the measures the wavefront of the scattered starlight with very high SNR, to 0.05nm in less than 5 minutes on a 5mag star. In addition, the post coronagraph wavefront sensor will be used to measure the residual scattered light after the coronagraph and subtract it in post processing to 1~2x10-11 to enable detection of an Earthlike planet with a SNR of 5~10.

The CorECam Instrument Concept Study (ICS) addressed the requirements and science program for the Terrestrial Planet Finder Coronagraph's (TPF-C) primary camera. CorECam provides a simple interface to TPF-C's Starlight Suppression System (SSS) which would be provided by the TPF-C Program, and comprises camera modules providing visible, and near-infrared (NIR) camera focal plane imaging. In its primary operating mode, CorECam will conduct the core science program of TPF-C, detecting terrestrial planets at visible wavelengths. CorECam additionally provides the imaging capabilities to characterize terrestrial planets, and conduct an extended science program focused on investigating the nature of the exosolar systems in which terrestrial planets are detected. In order to evaluate the performance of CorECam we developed a comprehensive, end-to-end model using OSCAR which has provided a number of key conclusions on the robustness of the TPF-C baseline design, and allows investigation of alternative techniques for wavefront sensing and control. CorECam recommends photon counting detectors be baselined for imaging with TPF-C since they provide mitigations against the background radiation environment, improved sensitivity and facilitate alternative WFSC approaches.

The Terrestrial Planet Finder Coronagraph (TPF-C) is a deep space mission designed to detect and characterize Earth-like planets around nearby stars. TPF-C will be able to search for signs of life on these planets. TPF-C will use spectroscopy to measure basic properties including the presence of water or oxygen in the atmosphere; powerful signatures in the search for habitable worlds. This capability to characterize planets is what allows TPF-C to transcend other astronomy projects and become an historical endeavor on a par with the discovery voyages of the great navigators.

We report the development of the first high resolution cross-dispersed silicon immersion grating spectrometer. This instrument is called the Florida IR Silicon immersion grating specTrometer (FIRST). FIRST can produce R = 50,000 under a 0.6 arcsec seeing and simultaneously cover 1.3-1.8 μm with a 1kx1k HgCdTe array at the Apache Point Observatory 3.5 meter telescope. FIRST has a 50 mm diameter collimated beam and the overall instrument is within a volume of 0.8x0.5x0.5 m3. The high dispersion, large wavelength coverage and small instrument volume become possible due to the use of a silicon immersion grating (54.7 deg blaze angle and 50 mm diameter entrance pupil) with extremely high dispersion power (3.4 times dispersion power of a conventional echelle) and coarse grooves (16.1 l/mm, coarser than the commercially available echelles). The silicon immersion grating used in a lab bench mounted Czeney-Turner spectrograph with an only 25 mm diameter collimated beam and a 100 um core fiber has produced R = 55,000 cross-dispersed solar spectra. This instrument is designed to precisely measure radial velocities of low mass stars, M dwarfs for detecting 5-10 Earth mass planets. The estimated Doppler precision is ~ 3 m/s for a J = 9 M5V dwarf in 15 min at the APO 3.5m telescope.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have measured the infrared transit of the extrasolar planet HD209458b using the Spitzer Space Telescope. We observed two primary eclipse events (one partial and one complete transit) using the 24 micron array of the Multiband Imaging Photometer for Spitzer (MIPS). We analyzed a total of 2392 individual images (10-second integrations) of the planetary system, recorded before, during, and after transit. We perform optimal photometry on the images and use the local zodiacal light as a short-term flux reference. At this long wavelength, the transit curve has a simple box-like shape, allowing robust solutions for the stellar and planetary radii independent of stellar limb darkening, which is negligible at 24 microns. We derive a stellar radius of R$_*$ = 1.06 $\pm$ 0.07 R$_\sun$, a planetary radius of R$_p$ = 1.26 $\pm$ 0.08 R$_J$, and a stellar mass of 1.17 M$_\sun$. Within the errors, our results agree with the measurements at visible wavelengths. The 24-micron radius of the planet therefore does not differ significantly compared to the visible result. We point out the potential for deriving extrasolar transiting planet radii to high accuracy using transit photometry at slightly shorter IR wavelengths where greater photometric precision is possible., 18 pages, 3 figures, Accepted to Astrophysical Journal

We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses. The HD 209458 photometry was obtained in MOST's lower-precision Direct Imaging mode, which is used for targets in the brightness range $6.5 < V < 13$. We describe the photometric reduction techniques for this mode of observing, in particular the corrections for stray Earthshine. We do not detect the secondary eclipse in the MOST data, to a limit in depth of 0.053 mmag (1 \sigma). We set a 1 \sigma upper limit on the planet-star flux ratio of 4.88 x 10^-5 corresponding to a geometric albedo upper limit in the MOST bandpass (400 to 700 nm) of 0.25. The corresponding numbers at the 3 \sigma level are 1.34 x 10^-4 and 0.68 respectively. HD 209458b is half as bright as Jupiter in the MOST bandpass. This low geometric albedo value is an important constraint for theoretical models of the HD209458b atmosphere, in particular ruling out the presence of reflective clouds. A second MOST campaign on HD 209458 is expected to be sensitive to an exoplanet albedo as low as 0.13 (1 sigma), if the star does not become more intrinsically variable in the meantime., Comment: 29 pages, 9 figures. Accepted for publication in the Astrophysical Journal (July 2006, v645n1)

We report detection of strong infrared thermal emission from the nearby (d=19 pc) transiting extrasolar planet HD189733b, by measuring the flux decrement during its prominent secondary eclipse. A 6-hour photometric sequence using Spitzer's infrared spectrograph in peak-up imaging mode at 16-microns shows the secondary eclipse depth to be 0.551 +/-0.030%, with accuracy limited by instrumental baseline uncertainties, but with 32-sigma precision (0.017%) on the detection. The 16-micron brightness temperature of this planet (1117+/-42K) is very similar to the Spitzer detections of TrES-1 and HD209458b, but the observed planetary flux (660 micro-Jy) is an order of magnitude greater. This large signal will allow a detailed characterization of this planet in the infrared. Our photometry has sufficient signal-to-noise (~400 per point) to motivate a search for structure in the ingress/egress portions of the eclipse curve, caused by putative thermal structure on the disk of the planet. We show that by binning our 6-second sampling down to 6-minute resolution, we detect the modulation in the intensity derivative during ingress/egress due to the overall shape of the planet, but our sensitivity is not yet sufficient to distinguish between realistic models of the temperature distribution across the planet's disk. We point out the potential for extending Spitzer secondary eclipse detections down to the regime of transiting hot Neptunes, if such systems are discovered among nearby lower main sequence stars., Comment: 14 pages, 3 figures, accepted for ApJ

The star upsilon Andromeda is orbited by three known planets, the innermost of which has an orbital period of 4.617 days and a mass at least 0.69 that of Jupiter. This planet is close enough to its host star that the radiation it absorbs overwhelms its internal heat losses. Here we present the 24 micron light curve of this system, obtained with the Spitzer Space Telescope. It shows a clear variation in phase with the orbital motion of the innermost planet. This is the first demonstration that such planets possess distinct hot substellar (day) and cold antistellar (night) faces., Comment: "Director's cut" of paper to appear in Science, 27 October, 2006

The recombination of hydrogen and helium at z~1000-7000 gives unavoidable distortions to the Cosmic Microwave Background (CMB) spectrum. We present a detailed calculation of the line intensities arising from the Ly-alpha (2p-1s) and two-photon (2s-1s) transitions for the recombination of hydrogen, as well as the corresponding lines from helium. We give an approximate formula for the strength of the main recombination line distortion on the CMB in different cosmologies, this peak occurring at about 170 microns. We also find a previously undescribed long wavelength peak (which we call the pre-recombination peak) from the lines of the 2p-1s transitions, which are formed before significant recombination of the corresponding atoms occurred. Detailed calculations of the two-photon emission line shapes are presented here for the first time. The frequencies of the photons emitted from the two-photon transition have a wide spectrum and this causes the location of the peak of the two-photon line of hydrogen to be located almost at the same wavelength as the main Ly-alpha peak. The helium lines also give distortions at similar wavelengths, so that the combined distortion has a complex shape. The detection of this distortion would provide direct supporting evidence that the Universe was indeed once a plasma. Moreover, the distortions are a sensitive probe of physics during the time of recombination. Although the spectral distortion is overwhelmed by dust emission from the Galaxy, and is maximum at wavelengths roughly where the cosmic far-infrared background peaks, it may be able to tailor an experiment to detect its non-trivial shape., 12 pages, 12 figures; Minor corrections in text and references; MNRAS in press

The New World Observer has the potential to discover and study planets around other stars without expensive and risky technical heroics. We describe the starshade, a large, deployable sheet on a separate spacecraft that is flown into position along the line of sight to a nearby star. We show how a starshade can be designed and built in a practical and affordable manner to fully remove starlight and leave only planet light entering a telescope. The simulations demonstrate That NWI can detect planetary system features as faint as comets, perform spectroscopy to look for water and life signs, and perform photometry to search for oceans, continents, clouds and polar caps.

The New Worlds Observer, currently studied under a NASA Institute for Advanced Concepts grant, will be a pinhole camera in space designed to directly detect and study extrasolar terrestrial planets. An apodized occultor or pinhole creates an image of the planetary system in the focal plane far away, where a second telescope craft orbits to detect the light. In this study we simulate the expected signal of NWO to find the optimal configuration and specifications of the two craft. The efficiency of direct detection through photometric imaging depends strongly on occulter and telescope size, while preliminary studies on absorption biomarker detection and photometric variability measurements are summarized.

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.

A class of extrasolar giant planets - the so-called `hot Jupiters' - orbit within 0.05 AU of their primary stars. These planets should be hot and so emit detectable infrared radiation. The planet HD 209458b is an ideal candidate for the detection and characterization of this infrared light because it is eclipsed by the star. This planet has an anomalously large radius (1.35 times that of Jupiter), which may be the result of ongoing tidal dissipation, but this explanation requires a non-zero orbital eccentricity (~0.03), maintained by interaction with a hypothetical second planet. Here we report detection of infrared (24 micron) radiation from HD 209458b, by observing the decrement in flux during secondary eclipse, when the planet passes behind the star. The planet's 24 micron flux is 55 +/- 10 micro-Jy (1 sigma), with a brightness temperature of 1130 +/- 150 Kelvins, confirming the predicted heating by stellar irradiation. The secondary eclipse occurs at the midpoint between transits of the planet in front of the star (to within +/- 7 min, 1 sigma), which means that a dynamically significant orbital eccentricity is unlikely., to appear in Nature April 7, posted to Nature online March 23 (11 pages, 3 figures)

Earth's deciduous plants have a sharp order-of-magnitude increase in leaf reflectance between approximately 700 and 750 nm wavelength. This strong reflectance of Earth's vegetation suggests that surface biosignatures with sharp spectral features might be detectable in the spectrum of scattered light from a spatially unresolved extrasolar terrestrial planet. We assess the potential of Earth's step-function-like spectroscopic feature, referred to as the "red edge", as a tool for astrobiology. We review the basic characteristics and physical origin of the red edge and summarize its use in astronomy: early spectroscopic efforts to search for vegetation on Mars and recent reports of detection of the red edge in the spectrum of Earthshine (i.e., the spatially integrated scattered light spectrum of Earth). We present Earthshine observations from Apache Point Observatory to emphasize that time variability is key to detecting weak surface biosignatures such as the vegetation red edge. We briefly discuss the evolutionary advantages of vegetation's red edge reflectance, and speculate that while extraterrestrial "light harvesting organisms" have no compelling reason to display the exact same red edge feature as terrestrial vegetation, they might have similar spectroscopic features at different wavelengths than terrestrial vegetation. This implies that future terrestrial-planet-characterizing space missions should obtain data that allow time-varying, sharp spectral features at unknown wavelengths to be identified. We caution that some mineral reflectance edges are similar in slope and strength to vegetation's red edge (albeit at different wavelengths); if an extrasolar planet reflectance edge is detected care must be taken with its interpretation., 19 pages, 6 figures, to appear in Astrobiology

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near-to-mid-infrared spectral region (3-8µm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets: Σ What are the characteristics of the known extrasolar giant planets? Σ What are the characteristics of the extrasolar zodiacal clouds around nearby stars? Σ Are there giant planets around classes of stars other than those already studied? We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5m telescopes on a 12.5m boom feeding a Mach-Zender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution.

The Extrasolar Planetary Imaging Coronagraph (EPIC) is a proposed NASA Discovery mission to image and characterize extrasolar giant planets in orbits with semi-major axes between 2 and 10 AU. EPIC will provide insights into the physical nature of a variety of planets in other solar systems complimenting radial velocity (RV) and astrometric planet searches. It will detect and characterize the atmospheres of planets identified by radial velocity surveys, determine orbital inclinations and masses, characterize the atmospheres around A and F type stars which cannot be found with RV techniques, and observe the inner spatial structure and colors of debris disks. EPIC has a proposed launch date of 2012 to heliocentric Earth trailing drift-away orbit, with a 3 year mission lifetime (5 year goal), and will revisit planets at least three times at intervals of 9 months. The robust mission design is simple and flexible ensuring mission success while minimizing cost and risk. The science payload consists of a heritage optical telescope assembly (OTA), and visible nulling coronagraph (VNC) instrument. The instrument achieves a contrast ratio of 109 over a 4.84 arcsecond field-of-view with an unprecedented inner working angle of 0.14 arcseconds over the spectral range of 440-880 nm, with spectral resolutions from 10 - 150. The telescope is a 1.5 meter offaxis Cassegrain with an OTA wavefront error of λ/9, which when coupled to the VNC greatly reduces the requirements on the large scale optics, compressing them to stability requirements within the relatively compact VNC optical chain. The VNC features two integrated modular nullers, a spatial filter array (SFA), and an E2V-L3 photon counting CCD. Direct null control is accomplished from the science focal mitigating against complex wavefront and amplitude sensing and control strategies.

The New Worlds Observer (NWO) is a proposed space mission to provide high resolution spectroscopy from the far UV to the near IR of extra-solar terrestrial sized planets. The design of NWO is based on the concept of a large, space-based, pinhole camera made up of two spacecraft flying in formation. The first spacecraft is a large, thin occulting shield (perhaps hundreds of meters in diameter) with a shaped "pinhole" aperture about 10m in diameter. The second spacecraft is a conventional-quality space telescope (possibly with a 10m primary mirror) which "flies" through the pinhole image of the planetary system to observe the extra-solar planets free from stellar background. In this paper we describe the design of the two spacecraft system. In particular, the shaped-pinhole design utilizes the shaped-pupil coronagraph pioneered for the Terrestrial Planet Finder. In this paper we describe some of the NWO's technology challenges and science opportunities. Additionally, we describe an extension of the design to provide 100km resolution images of extra-solar planets.

Transit (TR) surveys for extrasolar planets have recently uncovered a population of ``very hot Jupiters,'' planets with orbital periods of P< 3 d. At first sight this may seem surprising, given that radial velocity (RV) surveys have found a dearth of such planets, despite the fact that their sensitivity increases with decreasing P. We examine the confrontation between RV and TR survey results, paying particular attention to selection biases that favor short-period planets in transit surveys. We demonstrate that, when such biases and small-number statistics are properly taken into account, the period distribution of planets found by RV and TR surveys are consistent at better than the 1-sigma level. This consistency holds for a large range of reasonable assumptions. In other words, there are not enough planets detected to robustly conclude that the RV and TR short-period planet results are inconsistent. Assuming a logarithmic distribution of periods, we find that the relative frequency of very hot Jupiters (VHJ: P=1-3 d) to hot Jupiters (HJ: P=3-9 d) is 10-20%. Given an absolute frequency of HJ of ~1%, this implies that approximately one star in ~500-1000 has a VHJ. We also note that VHJ and HJ appear to be distinct in terms of their upper mass limit. We discuss the implications of our results for planetary migration theories, as well as present and future TR and RV surveys., 11 pages, 4 figures, 2 tables. Minor changes. Accepted to ApJ, to appear in the April 20, 2005 issue (v623)

Open clusters potentially provide an ideal environment for the search for transiting extrasolar planets since they feature a relatively large number of stars of the same known age and metallicity at the same distance. With this motivation, over a dozen open clusters are now being monitored by four different groups. We review the motivations and challenges for open cluster transit surveys for short-period giant planets. Our photometric monitoring survey (EXPLORE/OC) of Galactic southern open clusters was designed with the goals of maximizing the chance of finding and characterizing planets, and of providing for a statistically valuable astrophysical result in the case of no detections. We use the EXPLORE/OC data from two open clusters NGC 2660 and NGC 6208 to illustrate some of the largely unrecognized issues facing open cluster surveys including severe contamination by Galactic field stars ($>$ 80%) and relatively low number of cluster members for which high precision photometry can be obtained. We discuss how a careful selection of open cluster targets under a wide range of criteria such as cluster richness, observability, distance, and age can meet the challenges, maximizing chances to detect planet transits. In addition, we present the EXPLORE/OC observing strategy to optimize planet detection which includes high-cadence observing and continuously observing individual clusters rather than alternating between targets., 20 pages, 11 figures, this version to appear in Feb 2005 PASP

We predict how a remote observer would see the brightness variations of giant planets similar to Jupiter and Saturn as they orbit their central stars. We model the geometry of Jupiter, Saturn and Saturn's rings for varying orbital and viewing parameters. Scattering properties for the planets and rings at wavelenghts 0.6-0.7 microns follow Pioneer and Voyager observations, namely, planets are forward scattering and rings are backward scattering. Images of the planet with or without rings are simulated and used to calculate the disk-averaged luminosity varying along the orbit, that is, a light curve is generated. We find that the different scattering properties of Jupiter and Saturn (without rings) make a substantial difference in the shape of their light curves. Saturn-size rings increase the apparent luminosity of the planet by a factor of 2-3 for a wide range of geometries. Rings produce asymmetric light curves that are distinct from the light curve of the planet without rings. If radial velocity data are available for the planet, the effect of the ring on the light curve can be distinguished from effects due to orbital eccentricity. Non-ringed planets on eccentric orbits produce light curves with maxima shifted relative to the position of the maximum planet's phase. Given radial velocity data, the amount of the shift restricts the planet's unknown orbital inclination and therefore its mass. Combination of radial velocity data and a light curve for a non-ringed planet on an eccentric orbit can also be used to constrain the surface scattering properties of the planet. To summarize our results for the detectability of exoplanets in reflected light, we present a chart of light curve amplitudes of non-ringed planets for different eccentricities, inclinations, and the viewing azimuthal angles of the observer., Comment: 40 pages, 13 figures, submitted to Ap.J

The Fourier‐Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near‐to‐mid‐infrared spectral region (3 – 8 μm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets:• What are the characteristics of the known extrasolar giant planets?• What are the characteristics of the extrasolar zodiacal clouds around nearby stars?• Are there giant planets around classes of stars other than those already studied?We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5 m telescopes on a 12.5 m boom feeding a Mach‐Zend...

The EXPLORE Project expanded in 2003 to include a campaign to monitor rich southern open clusters for transits of extrasolar planets (EXPLORE/OC). In May and June 2003, we acquired precise, high-cadence photometry of the second open cluster in our campaign, NGC 6208. Here, we present preliminary results from our I-band survey of over 60000 stars in the field of NGC 6208, around 5000 of which were monitored with photometric precision better than 1%., 4 pages, 4 figures; to appear in the AIP Conf. Series "The Search for Other Worlds," Univ. of MD, October 2003

Atomic hydrogen loss at the top of HD 209458b's atmosphere has been recently detected Vidal-Madjar et al. 2003. We have developed a 1-dimensional model to study the chemistry in the upper atmosphere of this extrasolar "hot jupiter". The 3 most abundant elements (other than He), as well as 4 parent molecules are included in this model, viz., H, C, O, H2, CO, H2O, and CH4. The higher temperatures (~ 1000 K) and higher stellar irradiance (~6x10^5 W m^{-2}) strongly enhance and modify the chemical reaction rates in this atmosphere. Our two main results are that (a) the production of atomic hydrogen in the atmosphere is mainly driven by H2O photolysis and reaction of OH with H2, and is not sensitive to the exact abundances of CO, H2O, and CH4, and (b) H2O and CH4 can be produced via the photolysis of CO followed by the reactions with H2., Comment: submitted to ApJL

Recent advances in deformable mirror technology for correcting wavefront errors and in pupil shapes and masks for coronagraphic suppression of diffracted starlight enable a powerful approach to detecting extrasolar planets in reflected (scattered) starlight at visible wavelengths. We discuss the planet-finding performance of Hubble-like telescopes using these technical advances. A telescope of aperture of at least 4 meters could accomplish the goals of the Terrestrial Planet Finder (TPF) mission. The '4mTPF' detects an Earth around a Sun at five parsecs in about one hour of integration time. It finds molecular oxygen, ozone, water vapor, the 'red edge' of chlorophyll-containing land-plant leaves, and the total atmospheric column density -- all in forty hours or less. The 4mTPF has a strong science program of discovery and characterization of extrasolar planets and planetary systems, including other worlds like Earth. With other astronomical instruments sharing the focal plane, the 4mTPF could also continue and expand the general program of astronomical research of the Hubble Space Telescope.

We evaluate the direct detection of extrasolar giant planets with a two-aperture nulling infrared interferometer, working at angles ${\theta}, Comment: Accepted for publication in ApJ Letters

The ultra-precise photometric space satellite MOST (Microvariability and Oscillations of STars) will provide the first opportunity to measure the albedos and scattered light curves from known short-period extrasolar planets. Due to the changing phases of an extrasolar planet as it orbits its parent star, the combined light of the planet-star system will vary on the order of tens of micromagnitudes. The amplitude and shape of the resulting light curve is sensitive to the planet's radius and orbital inclination, as well as the composition and size distribution of the scattering particles in the planet's atmosphere. To predict the capabilities of MOST and other planned space missions, we have constructed a series of models of such light curves, improving upon earlier work by incorporating more realistic details such as: limb darkening of the star, intrinsic granulation noise in the star itself, tidal distortion and back-heating, higher angular resolution of the light scattering from the planet, and exploration of the significance of the angular size of the star as seen from the planet. We use photometric performance simulations of the MOST satellite, with the light curve models as inputs, for one of the mission's primary targets, $\tau$ Bo\"otis. These simulations demonstrate that, even adopting a very conservative signal detection limit of 4.2 $\mu$mag in amplitude (not power), we will be able to either detect the $\tau$ Bo\"otis planet light curve or put severe constraints on possible extrasolar planet atmospheric models., Comment: Accepted to ApJ, 24 pages, 8 figures

We report observations of the transiting extrasolar planet, HD 209458 b, designed to detect the secondary eclipse. We employ the method of `occultation spectroscopy', which searches in combined light (star and planet) for the disappearance and reappearance of weak infrared spectral features due to the planet as it passes behind the star and reappears. Our observations cover two predicted secondary eclipse events, and we obtained 1036 individual spectra of the HD 209458 system using the SpeX instrument at the NASA IRTF in September 2001. Our spectra extend from 1.9 to 4.2 microns with a spectral resolution of 1500. We have searched for a continuum peak near 2.2 microns (caused by CO and water absorption bands), as predicted by some models of the planetary atmosphere to be approximately 6E-4 of the stellar flux, but no such peak is detected at a level of about 3E-4 of the stellar flux. Our results represent the strongest limits on the infrared spectrum of the planet to date and carry significant implications for understanding the planetary atmosphere. In particular, some models that assume the stellar irradiation is re-radiated entirely on the sub-stellar hemisphere predict a flux peak inconsistent with our observations. Several physical mechanisms can improve agreement with our observations, including the re-distribution of heat by global circulation, a nearly isothermal atmosphere, and/or the presence of a high cloud., Comment: Accepted to the Astrophysical Journal 17 pages, 6 figures