174 research outputs found
A ~5 M_earth Super-Earth Orbiting GJ 436?: The Power of Near-Grazing Transits
Most of the presently identified exoplanets have masses similar to that of
Jupiter and therefore are assumed to be gaseous objects. With the
ever-increasing interest in discovering lower-mass planets, several of the
so-called super-Earths (1 M_earth<M<10 M_earth), which are predicted to be
rocky, have already been found. Here we report the possible discovery of a
planet around the M-type star GJ 436 with a minimum mass of 4.7+/-0.6 M_earth
and a true mass of ~5 M_earth, which would make it the least massive planet
around a main-sequence star found to date. The planet is identified from its
perturbations on an inner Neptune-mass transiting planet (GJ 436b), by pumping
eccentricity and producing variations in the orbital inclination. Analysis of
published radial velocity measurements indeed reveals a significant signal
corresponding to an orbital period that is very close to the 2:1 mean motion
resonance with the inner planet. The near-grazing nature of the transit makes
it extremely sensitive to small changes in the inclination.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical
Journal Letter
Phase curves of WASP-33b and HD 149026b and a New Correlation Between Phase Curve Offset and Irradiation Temperature
We present new 3.6 and 4.5 Spitzer phase curves for the highly
irradiated hot Jupiter WASP-33b and the unusually dense Saturn-mass planet HD
149026b. As part of this analysis, we develop a new variant of pixel level
decorrelation that is effective at removing intrapixel sensitivity variations
for long observations (>10 hours) where the position of the star can vary by a
significant fraction of a pixel. Using this algorithm, we measure eclipse
depths, phase amplitudes, and phase offsets for both planets at 3.6 and
4.5 . We use a simple toy model to show that WASP-33b's phase offset,
albedo, and heat recirculation efficiency are largely similar to those of other
hot Jupiters despite its very high irradiation. On the other hand, our fits for
HD 149026b prefer a very high albedo and an unusually high recirculation
efficiency. We also compare our results to predictions from general circulation
models, and find that while neither are a good match to the data, the
discrepancies for HD 149026b are especially large. We speculate that this may
be related to its high bulk metallicity, which could lead to enhanced
atmospheric opacities and the formation of reflective cloud layers in localized
regions of the atmosphere. We then place these two planets in a broader context
by exploring relationships between the temperatures, albedos, heat transport
efficiencies, and phase offsets of all planets with published thermal phase
curves. We find a striking relationship between phase offset and irradiation
temperature--the former drops with increasing temperature until around 3400 K,
and rises thereafter. Although some aspects of this trend are mirrored in the
circulation models, there are notable differences that provide important clues
for future modeling efforts
3.6 and 4.5 μm Phase Curves and Evidence for Non-equilibrium Chemistry in the Atmosphere of Extrasolar Planet HD 189733b
We present new, full-orbit observations of the infrared phase variations of the canonical hot Jupiter HD 189733b obtained in the 3.6 and 4.5 μm bands using the Spitzer Space Telescope. When combined with previous phase curve observations at 8.0 and 24 μm, these data allow us to characterize the exoplanet's emission spectrum as a function of planetary longitude and to search for local variations in its vertical thermal profile and atmospheric composition. We utilize an improved method for removing the effects of intrapixel sensitivity variations and robustly extracting phase curve signals from these data, and we calculate our best-fit parameters and uncertainties using a wavelet-based Markov Chain Monte Carlo analysis that accounts for the presence of time-correlated noise in our data. We measure a phase curve amplitude of 0.1242% ± 0.0061% in the 3.6 μm band and 0.0982% ± 0.0089% in the 4.5 μm band, corresponding to brightness temperature contrasts of 503 ± 21 K and 264 ± 24 K, respectively. We find that the times of minimum and maximum flux occur several hours earlier than predicted for an atmosphere in radiative equilibrium, consistent with the eastward advection of gas by an equatorial super-rotating jet. The locations of the flux minima in our new data differ from our previous observations at 8 μm, and we present new evidence indicating that the flux minimum observed in the 8 μm is likely caused by an overshooting effect in the 8 μm array. We obtain improved estimates for HD 189733b's dayside planet-star flux ratio of 0.1466% ± 0.0040% in the 3.6 μm band and 0.1787% ± 0.0038% in the 4.5 μm band, corresponding to brightness temperatures of 1328 ± 11 K and 1192 ± 9 K, respectively; these are the most accurate secondary eclipse depths obtained to date for an extrasolar planet. We compare our new dayside and nightside spectra for HD 189733b to the predictions of one-dimensional radiative transfer models from Burrows et al. and conclude that fits to this planet's dayside spectrum provide a reasonably accurate estimate of the amount of energy transported to the night side. Our 3.6 and 4.5 μm phase curves are generally in good agreement with the predictions of general circulation models for this planet from Showman et al., although we require either excess drag or slower rotation rates in order to match the locations of the measured maxima and minima in the 4.5, 8.0, and 24 μm bands. We find that HD 189733b's 4.5 μm nightside flux is 3.3σ smaller than predicted by these models, which assume that the chemistry is in local thermal equilibrium. We conclude that this discrepancy is best explained by vertical mixing, which should lead to an excess of CO and correspondingly enhanced 4.5 μm absorption in this region. This result is consistent with our constraints on the planet's transmission spectrum, which also suggest excess absorption in the 4.5 μm band at the day-night terminator
A Spitzer Transmission Spectrum for the Exoplanet GJ 436b, Evidence for Stellar Variability, and Constraints on Dayside Flux Variations
In 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 micron bands using the IRAC instrument on the Spitzer Space Telescope
between UT 2007 June 29 and UT 2009 Feb 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
(4.7 sigma 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 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. 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. Our measured 8 micron secondary eclipse
depths are consistent with a constant value, and we place a 1 sigma upper limit
of 17% on changes in the planet's dayside flux in this band. Averaging over the
eleven visits gives us an improved estimate of 0.0452% +/- 0.0027% for the
secondary eclipse depth. 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. [ABRIDGED]Comment: 26 pages, 18 figures, 7 tables in emulateapj format. Accepted for
publication in Ap
Secondary Eclipse Photometry of WASP-4b with Warm Spitzer
We present photometry of the giant extrasolar planet WASP-4b at 3.6 and 4.5
micron taken with the Infrared Array Camera on board the Spitzer Space
Telescope as part of Spitzer's extended warm mission. We find secondary eclipse
depths of 0.319+/-0.031% and 0.343+/-0.027% for the 3.6 and 4.5 micron bands,
respectively and show model emission spectra and pressure-temperature profiles
for the planetary atmosphere. These eclipse depths are well fit by model
emission spectra with water and other molecules in absorption, similar to those
used for TrES-3 and HD 189733b. Depending on our choice of model, these results
indicate that this planet has either a weak dayside temperature inversion or no
inversion at all. The absence of a strong thermal inversion on this highly
irradiated planet is contrary to the idea that highly irradiated planets are
expected to have inversions, perhaps due the presence of an unknown absorber in
the upper atmosphere. This result might be explained by the modestly enhanced
activity level of WASP-4b's G7V host star, which could increase the amount of
UV flux received by the planet, therefore reducing the abundance of the unknown
stratospheric absorber in the planetary atmosphere as suggested in Knutson et
al. (2010). We also find no evidence for an offset in the timing of the
secondary eclipse and place a 2 sigma upper limit on |ecos(omega)| of 0.0024,
which constrains the range of tidal heating models that could explain this
planet's inflated radius.Comment: 8 pages, 7 figures (some in color), accepted for publication in Ap
Spitzer Secondary Eclipses of the Dense, Modestly-irradiated, Giant Exoplanet HAT-P-20b Using Pixel-Level Decorrelation
HAT-P-20b is a giant exoplanet that orbits a metal-rich star. The planet
itself has a high total density, suggesting that it may also have a high
metallicity in its atmosphere. We analyze two eclipses of the planet in each of
the 3.6- and 4.5 micron bands of Warm Spitzer. These data exhibit intra-pixel
detector sensitivity fluctuations that were resistant to traditional
decorrelation methods. We have developed a simple, powerful, and radically
different method to correct the intra-pixel effect for Warm Spitzer data, which
we call pixel-level decorrelation (PLD). PLD corrects the intra-pixel effect
very effectively, but without explicitly using - or even measuring - the
fluctuations in the apparent position of the stellar image. We illustrate and
validate PLD using synthetic and real data, and comparing the results to
previous analyses. PLD can significantly reduce or eliminate red noise in
Spitzer secondary eclipse photometry, even for eclipses that have proven to be
intractable using other methods. Our successful PLD analysis of four HAT-P-20b
eclipses shows a best-fit blackbody temperature of 1134 +/-29K, indicating
inefficient longitudinal transfer of heat, but lacking evidence for strong
molecular absorption. We find sufficient evidence for variability in the 4.5
micron band that the eclipses should be monitored at that wavelength by
Spitzer, and this planet should be a high priority for JWST spectroscopy. All
four eclipses occur about 35 minutes after orbital phase 0.5, indicating a
slightly eccentric orbit. A joint fit of the eclipse and transit times with
extant RV data yields e(cos{omega}) = 0.01352 (+0.00054, -0.00057), and
establishes the small eccentricity of the orbit to high statistical confidence.
Given the existence of a bound stellar companion, HAT-P-20b is another
excellent candidate for orbital evolution via Kozai migration or other
three-body mechanism.Comment: version published in ApJ, minor text and figure revision
The atmospheres of the hot-Jupiters Kepler-5b and Kepler-6b observed during occultations with Warm-Spitzer and Kepler
This paper reports the detection and the measurements of occultations of the
two transiting hot giant exoplanets Kepler-5b and Kepler-6b by their parent
stars. The observations are obtained in the near infrared with Spitzer Space
Telescope and at optical wavelengths by combining more than a year of Kepler
photometry. The investigation consists of constraining the eccentricities of
these systems and of obtaining broad band emergent spectra for individual
planets. For both targets, the occultations are detected at 3 sigma level at
each wavelength with mid-occultation times consistent with circular orbits. The
brightness temperatures of these planets are deduced from the infrared
observations and reach T=1930+/-100K and T=1660+/-120K for Kepler-5b and
Kepler-6b respectively. We measure optical geometric albedos A_g in the Kepler
bandpass and find A_g=0.12+/-0.04 for Kepler-5b and A_g=0.11+/-0.04 for
Kepler-6b leading to an upper limit for the Bond albedo of A_B < 0.17 in both
cases. The observations for both planets are best described by models for which
most of the incident energy is redistributed on the dayside, with only less
than 10% of the absorbed stellar flux redistributed to the night side of these
planets. The data for Kepler-5b favor a model without a temperature inversion,
whereas for Kepler-6b they do not allow distinguishing between models with and
without inversion.Comment: 26 pages, 18 figures, 3 tables, submitted to Ap
Low False-Positive Rate of Kepler Candidates Estimated From A Combination Of Spitzer And Follow-Up Observations
(Abridged) NASA's Kepler mission has provided several thousand transiting
planet candidates, yet only a small subset have been confirmed as true planets.
Therefore, the most fundamental question about these candidates is the fraction
of bona fide planets. Estimating the rate of false positives of the overall
Kepler sample is necessary to derive the planet occurrence rate. We present the
results from two large observational campaigns that were conducted with the
Spitzer telescope during the the Kepler mission. These observations are
dedicated to estimating the false positive rate (FPR) amongst the Kepler
candidates. We select a sub-sample of 51 candidates, spanning wide ranges in
stellar, orbital and planetary parameter space, and we observe their transits
with Spitzer at 4.5 microns. We use these observations to measures the
candidate's transit depths and infrared magnitudes. A bandpass-dependent depth
alerts us to the potential presence of a blending star that could be the source
of the observed eclipse: a false-positive scenario. For most of the candidates
(85%), the transit depths measured with Kepler are consistent with the depths
measured with Spitzer as expected for planetary objects, while we find that the
most discrepant measurements are due to the presence of unresolved stars that
dilute the photometry. The Spitzer constraints on their own yield FPRs between
5-40%, depending on the KOIs. By considering the population of the Kepler field
stars, and by combining follow-up observations (imaging) when available, we
find that the overall FPR of our sample is low. The measured upper limit on the
FPR of our sample is 8.8% at a confidence level of 3 sigma. This observational
result, which uses the achromatic property of planetary transit signals that is
not investigated by the Kepler observations, provides an independent indication
that Kepler's false positive rate is low.Comment: 33 pages, 16 figures, 3 tables; accepted for publication in ApJ on
February 7, 201
- …