2,517 research outputs found
A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
Several transiting super-Earths are expected to be discovered in the coming
few years. While tools to model the interior structure of transiting planets
exist, inferences about the composition are fraught with ambiguities. We
present a framework to quantify how much we can robustly infer about
super-Earth and Neptune-size exoplanet interiors from radius and mass
measurements. We introduce quaternary diagrams to illustrate the range of
possible interior compositions for planets with four layers (iron core,
silicate mantles, water layers, and H/He envelopes). We apply our model to
CoRoT-7b, GJ 436b, and HAT-P-11b. Interpretation of planets with H/He envelopes
is limited by the model uncertainty in the interior temperature, while for
CoRoT-7b observational uncertainties dominate. We further find that our planet
interior model sharpens the observational constraints on CoRoT-7b's mass and
radius, assuming the planet does not contain significant amounts of water or
gas. We show that the strength of the limits that can be placed on a
super-Earth's composition depends on the planet's density; for similar
observational uncertainties, high-density super-Mercuries allow the tightest
composition constraints. Finally, we describe how techniques from Bayesian
statistics can be used to take into account in a formal way the combined
contributions of both theoretical and observational uncertainties to
ambiguities in a planet's interior composition. On the whole, with only a mass
and radius measurement an exact interior composition cannot be inferred for an
exoplanet because the problem is highly underconstrained. Detailed quantitative
ranges of plausible compositions, however, can be found.Comment: 20 pages, 10 figures, published in Ap
Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b
Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than
Mercury's are thought to have experienced extensive atmospheric evolution. Here
we propose that a potential outcome of this atmospheric evolution is the
formation of helium-dominated atmospheres. The hydrodynamic escape rates of
Neptune- and sub-Neptune-sized exoplanets are comparable to the
diffusion-limited escape rate of hydrogen, and therefore the escape is heavily
affected by diffusive separation between hydrogen and helium. A helium
atmosphere can thus be formed -- from a primordial hydrogen-helium atmosphere
-- via atmospheric hydrodynamic escape from the planet. The helium atmosphere
has very different abundances of major carbon and oxygen species from those of
a hydrogen atmosphere, leading to distinctive transmission and thermal emission
spectral features. In particular, the hypothesis of a helium-dominated
atmosphere can explain the thermal emission spectrum of GJ 436 b, a warm
Neptune-sized exoplanet, while also consistent with the transmission spectrum.
This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen,
with the predominance of CO over CH4 as the main form of carbon. With our
atmospheric evolution model, we find that if the mass of the initial atmosphere
envelope is 1E-3 planetary mass, hydrodynamic escape can reduce the hydrogen
abundance in the atmosphere by several orders of magnitude in ~10 billion
years. Observations of exoplanet transits may thus detect signatures of helium
atmospheres and probe the evolutionary history of small exoplanets.Comment: ApJ, accepte
Infrared Observations During the Secondary Eclipse of HD 209458 b II. Strong Limits on the Infrared Spectrum Near 2.2 Microns
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 figure
Theoretical Spectra of Terrestrial Exoplanet Surfaces
We investigate spectra of airless rocky exoplanets with a theoretical
framework that self-consistently treats reflection and thermal emission. We
find that a silicate surface on an exoplanet is spectroscopically detectable
via prominent Si-O features in the thermal emission bands of 7 - 13 \mu m and
15 - 25 \mu m. The variation of brightness temperature due to the silicate
features can be up to 20 K for an airless Earth analog, and the silicate
features are wide enough to be distinguished from atmospheric features with
relatively high-resolution spectra. The surface characterization thus provides
a method to unambiguously identify a rocky exoplanet. Furthermore,
identification of specific rocky surface types is possible with the planet's
reflectance spectrum in near-infrared broad bands. A key parameter to observe
is the difference between K band and J band geometric albedos (A_g (K)-A_g
(J)): A_g (K)-A_g (J) > 0.2 indicates that more than half of the planet's
surface has abundant mafic minerals, such as olivine and pyroxene, in other
words primary crust from a magma ocean or high-temperature lavas; A_g (K)-A_g
(J) < -0.09 indicates that more than half of the planet's surface is covered or
partially covered by water ice or hydrated silicates, implying extant or past
water on its surface. Also, surface water ice can be specifically distinguished
by an H-band geometric albedo lower than the J-band geometric albedo. The
surface features can be distinguished from possible atmospheric features with
molecule identification of atmospheric species by transmission spectroscopy. We
therefore propose that mid-infrared spectroscopy of exoplanets may detect rocky
surfaces, and near-infrared spectrophotometry may identify ultramafic surfaces,
hydrated surfaces and water ice.Comment: Accepted for publication on the Ap
Recommended from our members
Diagnostic Computation of Moisture Budgets in the ERA-Interim Reanalysis with Reference to Analysis of CMIP-Archived Atmospheric Model Data
The diagnostic evaluation of moisture budgets in archived atmosphere model data is examined. Sources of error in diagnostic computation can arise from the use of numerical methods different from those used in the atmosphere model, the time and vertical resolution of the archived data, and data availability. These sources of error are assessed using the climatological moisture balance in the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) that archives vertically integrated moisture fluxes and convergence. The largest single source of error arises from the diagnostic evaluation of divergence. The chosen second-order accurate centered finite difference scheme applied to the actual vertically integrated moisture fluxes leads to significant differences from the ERA-Interim reported moisture convergence. Using daily data, instead of 6-hourly data, leads to an underestimation of the patterns of moisture divergence and convergence by midlatitude transient eddies. A larger and more widespread error occurs when the vertical resolution of the model data is reduced to the 8 levels that is quite common for daily data archived for the Coupled Model Intercomparison Project (CMIP). Dividing moisture divergence into components due to the divergent flow and advection requires bringing the divergence operator inside the vertical integral, which introduces a surface term for which a means of accurate evaluation is developed. The analysis of errors is extended to the case of the spring 1993 Mississippi valley floods, the causes of which are discussed. For future archiving of data (e.g., by CMIP), it is recommended that monthly means of time-step-resolution flowâhumidity covariances be archived at high vertical resolution
The Male Psychology Conference, University College London, June 2014
The Male Psychology conference is taking place at University College, London, on June 20th 2014. This article describes the context of the conference, for example, the reasons why it is necessary and the types of challenges facing men and boys today. Conference presentations will span the topics of: suicide and male help-seeking behaviour, improving therapy for men, steroid use, empathy, emotional intelligence, and the perennial topic of masculinity. Abstracts of the presentations are in the appendix of this article. The conference will have keynote speeches from highly respected speakers on the subjects of government policy on menâs psychological needs (David Wilkins), sexual misconduct (Professor Andrew Samuels), and getting beyond notions like âmanning upâ (Glen Poole). There will also be poetry readings straight from the heart of Rob Mackerill, and posters on subjects ranging from male asylum seekers to The Sopranos. It is hoped that this conference â the first Male Psychology conference â will improve awareness of the importance of this area to psychologists and others. Full details of the conference can be found here http://www.malepsychology.org.uk/index.html
Changing Face of the Extrasolar Giant Planet, HD 209458b
High-resolution atmospheric flow simulations of the tidally-locked extrasolar
giant planet, HD 209458b, show large-scale spatio-temporal variability. This is
in contrast to the simple, permanent day/night (i.e., hot/cold) picture. The
planet's global circulation is characterized by a polar vortex in motion around
each pole and a banded structure corresponding to ~3 broad zonal (east-west)
jets. For very strong jets, the circulation-induced temperature difference
between moving hot and cold regions can reach up to ~1000 K, suggesting that
atmospheric variability could be observed in the planet's spectral and
photometric signatures.Comment: 6 pages, 1 ps figure, 2 low-res color figures (JPEG). Figure 3
updated. Contact authors for hi-res versions of color figures. Accepted for
publication in ApJ
On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets
The close-in extrasolar giant planets (CEGPs) reside in irradiated
environments much more intense than that of the giant planets in our solar
system. The high UV irradiance strongly influences their photochemistry and the
general current view believed that this high UV flux will greatly enhance
photochemical production of hydrocarbon aerosols. In this letter, we
investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find
that the abundances of hydrocarbons in the atmospheres of CEGPs are
significantly less than that of Jupiter except for models in which the CH
abundance is unreasonably high (as high as CO) for the hot (effective
temperatures K) atmospheres. Moreover, the hydrocarbons will be
condensed out to form aerosols only when the temperature-pressure profiles of
the species intersect with the saturation profiles--a case almost certainly not
realized in the hot CEGPs atmospheres. Hence our models show that photochemical
hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In
contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon
aerosols in their atmospheres which are responsible for strong absorption
shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon
aerosols in the atmospheres of CEGPs rules out one class of models with low
albedos and featureless spectra shortward of 600 nm.Comment: ApJL accepte
- âŠ