104 research outputs found
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SuperWASP: wide angle search for planets
No full textSuperWASP is a fully robotic, ultra-wide angle survey for planetary transits. Currently under construction, it will consist of 5 cameras, each monitoring a 9.5 by 9.5 degree field of view. The Torus mount and enclosure will be fully automated and linked to a built-in weather station. We aim to begin observations at the beginning of 2003
Astrophysical False Positives Encountered in Wide-Field Transit Searches
No full textWide-field photometric transit surveys for Jupiter-sized planets are inundated by astrophysical false positives, namely systems that contain an eclipsing binary and mimic the desired photometric signature. We discuss several examples of such false alarms. These systems were initially identified as candidates by the PSST instrument at Lowell Observatory. For three of the examples, we present follow-up spectroscopy that demonstrates that these systems consist of (1) an M-dwarf in eclipse in front of a larger star, (2) two main-sequence stars presenting grazing-incidence eclipses, and (3) the blend of an eclipsing binary with the light of a third, brighter star. For an additional candidate, we present multi-color follow-up photometry during a subsequent time of eclipse, which reveals that this candidate consists of a blend of an eclipsing binary and a physically unassociated star. We discuss a couple indicators from publicly-available catalogs that can be used to identify which candidates are likely giant stars, a large source of the contaminants in such surveys
Illusion and reality in the atmospheres of exoplanets
No full textThe atmospheres of exoplanets reveal all their properties beyond mass, radius, and orbit. Based on bulk densities, we know that exoplanets larger than 1.5 Earth radii must have gaseous envelopes and, hence, atmospheres. We discuss contemporary techniques for characterization of exoplanetary atmospheres. The measurements are difficult, because—even in current favorable cases—the signals can be as small as 0.001% of the host star's flux. Consequently, some early results have been illusory and not confirmed by subsequent investigations. Prominent illusions to date include polarized scattered light, temperature inversions, and the existence of carbon planets. The field moves from the first tentative and often incorrect conclusions, converging to the reality of exoplanetary atmospheres. That reality is revealed using transits for close-in exoplanets and direct imaging for young or massive exoplanets in distant orbits. Several atomic and molecular constituents have now been robustly detected in exoplanets as small as Neptune. In our current observations, the effects of clouds and haze appear ubiquitous. Topics at the current frontier include the measurement of heavy element abundances in giant planets, detection of carbon-based molecules, measurement of atmospheric temperature profiles, definition of heat circulation efficiencies for tidally locked planets, and the push to detect and characterize the atmospheres of super-Earths. Future observatories for this quest include the James Webb Space Telescope and the new generation of extremely large telescopes on the ground. On a more distant horizon, NASA's study concepts for the Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/Infrared Surveyor (LUVOIR) missions could extend the study of exoplanetary atmospheres to true twins of Earth
Extrasolar Planets Observed with JWST and the ELTs
No full textThe advent of cryogenic space-borne infrared observatories such as the Spitzer Space Telescope has lead to a revolution in the study of planets and planetary systems orbiting sun-like stars. Already Spitzer has characterized the emergent infrared spectra of close-in giant exoplanets using transit and eclipse techniques. The James Webb Space Telescope (JWST) will be able to extend these studies to superEarth exoplanets orbiting in the habitable zones of M-dwarf stars in the near solar neighborhood. The forthcoming ground-based Extremely Large Telescopes (ELTs) will playa key role in these studies, being especially valuable for spectroscopy at higher spectral resolving powers where large photon fluxes are needed. The culmination of this work within the next two decades will be the detection and spectral characterization of the major molecular constituents in the atmosphere of a habitable superEarth orbiting a nearby lower main sequence star
A Statistical Characterization of the Atmospheres of Sub-Saturn Planet Candidates in the Kepler Archive
No full textExoplanet atmospheric characterization is still in its early stages. Large surveys like the Kepler Mission provide thousands of planet candidates, but follow-up observations to characterize the individual candidates are often difficult to obtain. In this thesis, I develop a method to detect small atmospheric signals in Kepler’s planet candidate light curves by averaging light curves for multiple candidates with similar orbital and physical characteristics. I also consider two applications of this method: at secondary eclipse, to determine the average albedo of groups of planet candidates, and near transit, to determine whether on average the planets have cloud-free, low- mean-molecular-weight atmospheres, or cloudy/hazy/high-mean-molecular-weight atmospheres. This approach allows the measurement of properties that are un- measurable for candidates individually, because of their low signal-to-noise, and it prevents biasing of the results by false positives (candidates that are not actually planets) and outliers by not depending on only a few measurable candidates.
I first develop the method and apply it to the secondary eclipses of 32 close-in Kepler planet candidates between 1 and 6 R⊕ with short cadence data available, in
order to determine their average albedo. I then adapt the method to the long cadence data, accounting for the effects of the longer integration time. The increase in the number of candidates available in long cadence allows for finer radius groupings of 1 to 2 R⊕, 2 to 4 R⊕, and 4 to 6 R⊕. The short cadence average includes 6,238 individual eclipses, while the long cadence averages contain 80,492 eclipses from 56 candidates in the 1 to 2 R⊕ bin, 22,677 eclipses from 38 candidates in the 2 to 4 R⊕ bin, and 4,572 eclipses from 16 candidates in the 4 to 6 R⊕ bin. In both studies, I find that these planet candidates are generally dark, though there are bright outliers like Kepler-10b, and I discuss the implications of these results for understanding the atmospheres of these planets.
Finally, I apply the method to Kepler planet candidates in short cadence near transit, looking for a brief brightening due to light that is refracted through the atmospheres of the planets and directed toward the observer just before and just after transit. Refracted light is strongest in planetary atmospheres that are cloud-free and have a low mean molecular weight. Preliminary results suggest this strong refraction effect is not present in the selected group of 10 candidates with radii between 0.8 and 3 R⊕, but I begin to develop a more detailed model and sketch out future plans to improve the model and to continue testing for the presence of refracted light with greater sensitivity
OPTIMIZING JWST EXOPLANETARY ATMOSPHERIC CHARACTERIZATION THROUGH PRIORITIZATION AND VALIDATION OF TESS-DISCOVERED EXOPLANETS AND PANCHROMATIC STUDIES
No full textThe approaching launch of the James Webb Space Telescope (JWST), coupled with the recent all-sky search of the Transiting Exoplanet Survey Satellite (TESS), heralds a new era in exoplanetary atmospheric characterization, with TESS projected to detect over one thousand transiting exoplanets smaller than Neptune, and JWST offering unprecedented spectroscopic capabilities.
My work focuses on optimizing future observations in three ways. First, JWST time and resources will not allow observations of all TESS discoveries, so we must prioritize exoplanets for atmospheric characterization. I simulated JWST transmission spectroscopy observations of the anticipated TESS planet yield and compared the results to simulated transmission spectroscopy observations of already known exoplanets. My most significant finding is that several hundred TESS 1.5 to 2.5 Earth radii sub-Neptunes can be observed at higher signal-to-noise than currently known similarly-sized exoplanets. My work was used as the basis in developing the Kempton et al. (2018) Transmission Spectroscopy Metric (TSM), which is widely used by the exoplanet atmosphere community in prioritizing which exoplanets to observe with JWST.
Second, predictions show that TESS will detect thousands of astrophysical false positives that mimic exoplanet discoveries by also producing periodic decreases in starlight. A common scenario occurs when light from the target star blends with that of nearby eclipsing binary stars. Thus, TESS discoveries must be validated as true exoplanets using additional instruments or techniques. I designed software codes to predict how well two multi-band photometry instruments can discriminate between blended eclipsing binary false positives and true exoplanets. I found that the instruments can validate hundreds of candidate exoplanets smaller in size than Neptune.
Finally, previous atmospheric characterization studies have shown that observations using only infrared instruments---such as those used by JWST---may produce ambiguous atmospheric compositions. An exoplanet atmosphere may be more clearly understood by analyzing observations across multiple wavelength regimes. I analyzed Hubble Space Telescope (HST) transmission spectroscopy data for the hot Jupiter KELT-7b across wavelengths from the near ultraviolet to near infrared. This panchromatic analysis helps us better understand observations we can use to complement the near and mid-infrared observations of JWST, which is particularly important while HST is still operational
Alien Earth: Glint observations of a remote planet
No full textAbstractWe give a preliminary report on a multi-wavelength study of specular reflections from the oceans and clouds of Earth. We use space-borne observations from a distance sufficient to ensure that light rays reflected from all parts of Earth are closely parallel, as they will be when studying exoplanets. We find that the glint properties of Earth in this far-field vantage point are surprising - in the sense that some of the brightest reflections are not from conventional ocean-glints, but appear to arise from cirrus cloud crystals. The Earth observations discussed here were acquired with the High Resolution Instrument (HRI) - a 0.3 m f/35 telescope on the Deep Impact (DI) spacecraft during the Extrasolar Planet Observation and Characterization (EPOCh) investigation.</jats:p
3.8-mu m photometry during the secondary eclipse of the extrasolar planet HD 209458b
No full textWe report infrared photometry of the extrasolar planet HD 209458b during the time of secondary eclipse (planet passing behind the star). Observations were acquired during two secondary eclipses at the NASA Infrared Telescope Facility (IRTF) in 2003 September. We used a circular variable filter (1.5 per cent bandpass) centred at 3.8 mu m to isolate the predicted flux peak of the planet at this wavelength. Residual telluric absorption and instrument variations were removed by offsetting the telescope to nearby bright comparison stars at a high temporal cadence. Our results give a secondary eclipse depth of 0.0013 +/- 0.0011, not yet sufficient precision to detect the eclipse, whose expected depth is similar to 0.002 -0.003. We here elucidate the current observational limitations to this technique, and discuss the approach needed to achieve detections of hot Jupiter secondary eclipses at 3.8 mu m from the ground
A Ground-based Search for Thermal Emission from the Exoplanet TrES-1
No full textEclipsing planetary systems give us an important window on extrasolar planet atmospheres. By measuring the depth of the secondary eclipse, when the planet moves behind the star, we can estimate the strength of the thermal emission from the day side of the planet. Obtaining a ground‐based detection of one of these eclipses has proven to be a significant challenge, as time‐dependent variations in instrument throughput and atmospheric seeing and absorption overwhelm the small signal of the eclipse at infrared wavelengths. We gathered a series of simultaneous L grism spectra of the transiting planet system TrES‐1 and a nearby comparison star of comparable brightness, allowing us to correct for these effects, in principle. Combining the data from two eclipses, we demonstrate a detection sensitivity of 0.15% in the eclipse depth relative to the stellar flux. This approaches the sensitivity required to detect the planetary emission, which theoretical models predict should lie between 0.05% and 0.1% of the stellar flux in our 2.9–4.3 μm bandpass. We explore the factors that ultimately limit the precision of this technique, and discuss potential avenues for future improvements
A Spitzer Transmission Spectrum for the Exoplanet Gj 436b, Evidence for Stellar Variability, and Constraints on Dayside Flux Variations
No full textIn this paper, we describe a uniform analysis of eight transits and eleven secondary eclipses of the extrasolar planet GJ 436b obtained in the 3.6, 4.5, and 8.0 μm bands using the IRAC instrument on the Spitzer Space Telescope between UT 2007 June 29 and UT 2009 February 4. We find that the best-fit transit depths for visits in the same bandpass can vary by as much as 8% of the total (4.7σ significance) from one epoch to the next. Although we cannot entirely rule out residual detector effects or a time-varying, high-altitude cloud layer in the planet's atmosphere as the cause of these variations, we consider the occultation of active regions on the star in a subset of the transit observations to be the most likely explanation. We find that for the deepest 3.6 μm transit the in-transit data have a higher standard deviation than the out-of-transit data, as would be expected if the planet occulted a star spot. We also compare all published transit observations for this object and find that transits observed in the infrared typically have smaller timing offsets than those observed in visible light. In this case, the three deepest Spitzer transits are all measured within a period of five days, consistent with a single epoch of increased stellar activity. We reconcile the presence of magnetically active regions with the lack of significant visible or infrared flux variations from the star by proposing that the star's spin axis is tilted with respect to our line of sight and that the planet's orbit is therefore likely to be misaligned. In contrast to the results reported by Beaulieu et al., we find no convincing evidence for methane absorption in the planet's transmission spectrum. If we exclude the transits that we believe to be most affected by stellar activity, we find that we prefer models with enhanced CO and reduced methane, consistent with GJ 436b's dayside composition from Stevenson et al. It is also possible that all transits are significantly affected by this activity, in which case it may not be feasible to characterize the planet's transmission spectrum using broadband photometry obtained over multiple epochs. These observations serve to illustrate the challenges associated with transmission spectroscopy of planets orbiting late-type stars; we expect that other systems, such as GJ 1214, may display comparably variable transit depths. We compare the limb-darkening coefficients predicted by PHOENIX and ATLAS stellar atmosphere models and discuss the effect that these coefficients have on the measured planet-star radius ratios given GJ 436b's near-grazing transit geometry. Our measured 8 μm secondary eclipse depths are consistent with a constant value, and we place a 1σ upper limit of 17% on changes in the planet's dayside flux in this band. These results are consistent with predictions from general circulation models for this planet, which find that the planet's dayside flux varies by a few percent or less in the 8 μm band. Averaging over the eleven visits gives us an improved estimate of 0.0452% ± 0.0027% for the secondary eclipse depth; we also examine residuals from the eclipse ingress and egress and place an upper limit on deviations caused by a non-uniform surface brightness for GJ 436b. We combine timing information from our observations with previously published data to produce a refined orbital ephemeris and determine that the best-fit transit and eclipse times are consistent with a constant orbital period. We find that the secondary eclipse occurs at a phase of 0.58672 ± 0.00017, corresponding to ecos (ω) = 0.13754 ± 0.00027, where e is the planet's orbital eccentricity and ω is the longitude of pericenter. We also present improved estimates for other system parameters, including the orbital inclination, a/R sstarf, and the planet-star radius ratio
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