1,032 research outputs found
Constraining High Speed Winds in Exoplanet Atmospheres Through Observations of Anomalous Doppler Shifts During Transit
Three-dimensional (3-D) dynamical models of hot Jupiter atmospheres predict
very strong wind speeds. For tidally locked hot Jupiters, winds at high
altitude in the planet's atmosphere advect heat from the day side to the cooler
night side of the planet. Net wind speeds on the order of 1-10 km/s directed
towards the night side of the planet are predicted at mbar pressures, which is
the approximate pressure level probed by transmission spectroscopy. These winds
should result in an observed blue shift of spectral lines in transmission on
the order of the wind speed. Indeed, Snellen et al. (2010) recently observed a
2 +/- 1 km/s blue shift of CO transmission features for HD 209458b, which has
been interpreted as a detection of the day-to-night winds that have been
predicted by 3-D atmospheric dynamics modeling. Here we present the results of
a coupled 3-D atmospheric dynamics and transmission spectrum model, which
predicts the Doppler-shifted spectrum of a hot Jupiter during transit resulting
from winds in the planet's atmosphere. We explore four different models for the
hot Jupiter atmosphere using different prescriptions for atmospheric drag via
interaction with planetary magnetic fields. We find that models with no
magnetic drag produce net Doppler blue shifts in the transmission spectrum of
~2 km/s and that lower Doppler shifts of ~1 km/s are found for the higher drag
cases, results consistent with -- but not yet strongly constrained by -- the
Snellen et al. (2010) measurement. We additionally explore the possibility of
recovering the average terminator wind speed as a function of altitude by
measuring Doppler shifts of individual spectral lines and spatially resolving
wind speeds across the leading and trailing terminators during ingress and
egress.Comment: Submitted to ApJ, 28 pages including 9 figures in manuscript forma
Broadband Transmission Spectroscopy of the super-Earth GJ 1214b suggests a Low Mean Molecular Weight Atmosphere
We used WIRCam on CFHT to observe four transits of the super-Earth GJ 1214b
in the near-infrared. For each transit we observed in two bands
nearly-simultaneously by rapidly switching the WIRCam filter wheel back and
forth for the duration of the observations. By combining all our J-band (~1.25
microns) observations we find a transit depth in this band of 1.338\pm0.013% -
a value consistent with the optical transit depth reported by Charbonneau and
collaborators. However, our best-fit combined Ks-band (~2.15 microns) transit
depth is deeper: 1.438\pm0.019%. Formally our Ks-band transits are deeper than
the J-band transits observed simultaneously by a factor of 1.072\pm0.018 - a
4-sigma discrepancy. The most straightforward explanation for our deeper
Ks-band depth is a spectral absorption feature from the limb of the atmosphere
of the planet; for the spectral absorption feature to be this prominent the
atmosphere of GJ 1214b must have a large scale height and a low mean molecular
weight. That is, it would have to be hydrogen/helium dominated and this planet
would be better described as a mini-Neptune. However, recently published
observations from 0.78 - 1.0 microns, by Bean and collaborators, show a lack of
spectral features and transit depths consistent with those obtained by
Charbonneau and collaborators. The most likely atmospheric composition for GJ
1214b that arises from combining all these observations is less clear; if the
atmosphere of GJ 1214b is hydrogen/helium dominated then it must have either a
haze layer that is obscuring transit depth differences at shorter wavelengths,
or significantly different spectral features than current models predict. Our
observations disfavour a water-world composition, but such a composition will
remain a possibility until observations reconfirm our deeper Ks-band transit
depth or detect features at other wavelengths. [Abridged]Comment: ApJ accepted. 12 pages, 6 figures, in EmulateApJ forma
The Atmospheric Chemistry of GJ 1214b: Photochemistry and Clouds
Recent observations of the transiting super-Earth GJ 1214b reveal that its
atmosphere may be hydrogen-rich or water-rich in nature, with clouds or hazes
potentially affecting its transmission spectrum in the optical and
very-near-IR. Here we further examine the possibility that GJ 1214b does indeed
possess a hydrogen-dominated atmosphere, which is the hypothesis that is
favored by models of the bulk composition of the planet. We study the effects
of non-equilibrium chemistry (photochemistry, thermal chemistry, and mixing) on
the planet's transmission spectrum. We furthermore examine the possibility that
clouds could play a significant role in attenuating GJ 1214b's transmission
spectrum at short wavelengths. We find that non-equilibrium chemistry can have
a large effect on the overall chemical composition of GJ 1214b's atmosphere,
however these changes mostly take place above the height in the atmosphere that
is probed by transmission spectroscopy. The effects of non-equilibrium
chemistry on GJ 1214b's transmission spectrum are therefore minimal, with the
largest effects taking place if the planet's atmosphere has super-solar
metallicity and a low rate of vertical mixing. Interestingly, we find that the
best fit to the observations of GJ 1214b's atmosphere in transmission occur if
the planet's atmosphere is deficient in CH4, and possesses a cloud layer at a
pressure of ~200 mbar. This is consistent with a picture of efficient methane
photolysis, accompanied by formation of organic haze that obscures the lower
atmosphere of GJ 1214b at optical wavelengths. However, for methane to be
absent from GJ 1214b's transmission spectrum, UV photolysis of this molecule
must be efficient at pressures of greater than ~1 mbar, whereas we find that
methane only photolyzes to pressures less than 0.1 mbar, even under the most
optimistic assumptions. (Abridged)Comment: Accepted to ApJ; 32 pages, 8 figures, 1 tabl
On the Emergent Spectra of Hot Protoplanet Collision Afterglows
We explore the appearance of terrestrial planets in formation by studying the
emergent spectra of hot molten protoplanets during their collisional formation.
While such collisions are rare, the surfaces of these bodies may remain hot at
temperatures of 1000-3000 K for up to millions of years during the epoch of
their formation. These object are luminous enough in the thermal infrared to be
observable with current and next generation optical/IR telescopes, provided
that the atmosphere of the forming planet permits astronomers to observe
brightness temperatures approaching that of the molten surface. Detectability
of a collisional afterglow depends on properties of the planet's atmosphere --
primarily on the mass of the atmosphere. A planet with a thin atmosphere is
more readily detected, because there is little atmosphere to obscure the hot
surface. Paradoxically, a more massive atmosphere prevents one from easily
seeing the hot surface, but also keeps the planet hot for a longer time. In
terms of planetary mass, more massive planets are also easier to detect than
smaller ones because of their larger emitting surface areas. We present
preliminary calculations assuming a range of protoplanet masses (1-10
M_\earth), surface pressures (1-1000 bar), and atmospheric compositions, for
molten planets with surface temperatures ranging from 1000 to 1800 K, in order
to explore the diversity of emergent spectra that are detectable. While current
8- to 10-m class ground-based telescopes may detect hot protoplanets at wide
orbital separations beyond 30 AU (if they exist), we will likely have to wait
for next-generation extremely large telescopes or improved diffraction
suppression techniques to find terrestrial planets in formation within several
AU of their host stars.Comment: 28 pages, 6 figures, ApJ manuscript format, accepted into the Ap
The Optical and Near-Infrared Transmission Spectrum of the Super-Earth GJ1214b: Further Evidence for a Metal-Rich Atmosphere
We present an investigation of the transmission spectrum of the 6.5 M_earth
planet GJ1214b based on new ground-based observations of transits of the planet
in the optical and near-infrared, and on previously published data.
Observations with the VLT+FORS and Magellan+MMIRS using the technique of
multi-object spectroscopy with wide slits yielded new measurements of the
planet's transmission spectrum from 0.61 to 0.85 micron, and in the J, H, and K
atmospheric windows. We also present a new measurement based on narrow-band
photometry centered at 2.09 micron with the VLT+HAWKI. We combined these data
with results from a re-analysis of previously published FORS data from 0.78 to
1.00 micron using an improved data reduction algorithm, and previously reported
values based on Spitzer data at 3.6 and 4.5 micron. All of the data are
consistent with a featureless transmission spectrum for the planet. Our K-band
data are inconsistent with the detection of spectral features at these
wavelengths reported by Croll and collaborators at the level of 4.1 sigma. The
planet's atmosphere must either have at least 70% water by mass or optically
thick high-altitude clouds or haze to be consistent with the data.Comment: (v2) ApJ in press, no major changes from v
The Atmospheric Signatures of Super-Earths: How to Distinguish Between Hydrogen-Rich and Hydrogen-Poor Atmospheres
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 features and a lack of HO.
We apply our study specifically to the low-mass planet orbiting an M star, Gl
581c ( = 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.Comment: 28 pages, 6 figures, accepted to Ap
MOST Spacebased Photometry of the Transiting Exoplanet System HD 189733: Precise Timing Measurements for Transits Across an Active Star
We have measured transit times for HD 189733b passing in front of its bright
(V = 7.67) chromospherically active and spotted parent star. Nearly continuous
broadband optical photometry of this system was obtained with the MOST
(Microvariability & Oscillations of STars) space telescope during 21 days in
August 2006, monitoring 10 consecutive transits. We have used these data to
search for deviations from a constant orbital period which can indicate the
presence of additional planets in the system that are as yet undetected by
Doppler searches. There are no transit timing variations above the level of
s, ruling out super-Earths (of masses 1 - 4 M_{\earth}) in the 1:2
and 2:3 inner resonances and planets of 20 M_{\earth} in the 2:1 outer
resonance of the known planet. We also discuss complications in measuring
transit times for a planet that transits an active star with large star spots,
and how the transits can help constrain and test spot models. This has
implications for the large number of such systems expected to be discovered by
the CoRoT and Kepler missions.Comment: 26 pages, 7 figures, accepted to Ap
MOST Spacebased Photometry of the Transiting Exoplanet System HD 209458: Transit Timing to Search for Additional Planets
We report on the measurement of transit times for the HD 209458 planetary
system from photometry obtained with the MOST (Microvariability & Oscillations
of STars) space telescope. Deviations from a constant orbital period can
indicate the presence of additional planets in the system that are yet
undetected, potentially with masses approaching an Earth mass. The MOST data
sets of HD 209458 from 2004 and 2005 represent unprecedented time coverage with
nearly continuous observations spanning 14 and 43 days and monitoring 3
transits and 12 consecutive transits, respectively. The transit times we obtain
show no variations on three scales: (a) no long-term change in P since before
2004 at the 25 ms level, (b) no trend in transit timings during the 2005 run,
and (c) no individual transit timing deviations above 80 sec level. Together
with previously published transit times from Agol & Steffen (2007), this allows
us to place limits on the presence of additional close-in planets in the
system, in some cases down to below an Earth mass. This result, along with
previous radial velocity work, now eliminates the possibility that a perturbing
planet could be responsible for the additional heat source needed to explain HD
209458b's anomalous low density.Comment: 21 pages, 6 figures, accepted to the Ap
The Very Low Albedo of an Extrasolar Planet: MOST Spacebased Photometry of HD 209458
Measuring the albedo of an extrasolar planet provides insights into its
atmospheric composition and its global thermal properties, including heat
dissipation and weather patterns. Such a measurement requires very precise
photometry of a transiting system sampling fully many phases of the secondary
eclipse. Spacebased optical photometry of the transiting system HD 209458 from
the MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 and
44 days in 2004 and 2005 respectively, allows us to set a sensitive limit on
the optical eclipse of the hot exosolar giant planet in this system. Our best
fit to the observations yields a flux ratio of the planet and star of 7 9
ppm (parts per million), which corresponds to a geometric albedo through the
MOST bandpass (400-700 nm) of = 0.038 0.045. This gives a 1
upper limit of 0.08 for the geometric albedo and a 3 upper limit of
0.17. HD 209458b is significantly less reflective than Jupiter (for which
would be about 0.5). This low geometric albedo rules out the presence of bright
reflective clouds in this exoplanet's atmosphere. We determine refined
parameters for the star and exoplanet in the HD 209458 system based on a model
fit to the MOST light curve.Comment: 23 pages, 8 figures, accepted to the ApJ. v2: minor changes to mirror
accepted ApJ versio
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