573 research outputs found
Evaporation of extrasolar planets
Atomic hydrogen escaping from the extrasolar giant planet HD209458b provides
the largest observational signature ever detected for an extrasolar planet
atmosphere. In fact, the upper atmosphere of this planet is evaporating.
Observational evidences and interpretations coming from various models are
reviewed. Implications for exoplanetology are discussed.Comment: 12 pages, 4 figures. To appear in the proceedings of the Les Houches
Winter School "Physics and Astrophysics of Planetary Systems" (EDP Sciences:
EAS Publications Series
Radiative braking in the extended exosphere of GJ436b
The recent detection of a giant exosphere surrounding the warm Neptune GJ436
b has shed new light on the evaporation of close-in planets, revealing that
moderately irradiated, low-mass exoplanets could make exceptional targets for
studying this mechanism and its impact on the exoplanet population. Three
HST/STIS observations were performed in the Lyman- line of GJ436 at
different epochs, showing repeatable transits with large depths and extended
durations. Here, we study the role played by stellar radiation pressure on the
structure of the exosphere and its transmission spectrum. We found that the
neutral hydrogen atoms in the exosphere of GJ436 b are not swept away by
radiation pressure as shown to be the case for evaporating hot Jupiters.
Instead, the low radiation pressure from the M-dwarf host star only brakes the
gravitational fall of the escaping hydrogen toward the star and allows its
dispersion within a large volume around the planet, yielding radial velocities
up to about -120 km s that match the observations. We performed
numerical simulations with the EVaporating Exoplanets code (EVE) to study the
influence of the escape rate, the planetary wind velocity, and the stellar
photoionization. While these parameters are instrumental in shaping the
exosphere and yield simulation results in general agreement with the
observations, the spectra observed at the different epochs show specific,
time-variable features that require additional physics.Comment: 10 pages, 5 figure
Observability of hydrogen-rich exospheres in Earth-like exoplanets
(Abridged) The existence of an extended neutral hydrogen exosphere around
small planets can be used as an evidence for the presence of water in their
lower atmosphere but, to date, such feature has not been securely detected in
rocky exoplanets. Planetary exospheres can be observed using transit
spectroscopy of the Lyman- line, which is limited mainly by
interstellar medium absorption in the core of the line, and airglow
contamination from the geocorona when using low-orbit space telescopes. Our
objective is to assess the detectability of the neutral hydrogen exosphere of
an Earth-like planet transiting a nearby M dwarf using Lyman-
spectroscopy and provide the necessary strategies to inform future
observations. The spatial distribution in the upper atmosphere is provided by
an empirical model of the geocorona, and we assume a velocity distribution
based on radiative pressure as the main driver in shaping the exosphere. We
compute the excess absorption in the stellar Lyman- line while in
transit, and use realistic estimates of the uncertainties involved in
observations to determine the observability of the signal. We found that the
signal in Lyman- of the exosphere of an Earth-like exoplanet transiting
M dwarfs with radii between 0.1 and 0.6 R produces an excess absorption
between 50 and 600 ppm. The Lyman- flux of stars decays exponentially
with distance because of interstellar medium absorption, which is the main
observability limitation. Other limits are related to the stellar radial
velocity and instrumental setup. The excess absorption in Lyman- is
observable using LUVOIR/LUMOS in M dwarfs up to a distance of 15 pc. The
analysis of noise-injected data suggests that it would be possible to detect
the exosphere of an Earth-like planet transiting TRAPPIST-1 within 20 transits.Comment: 12 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
A Non-isothermal Theory for Interpreting Sodium Lines in Transmission Spectra of Exoplanets
We present a theory for interpreting the sodium lines detected in
transmission spectra of exoplanetary atmospheres. Previous analyses employed
the isothermal approximation and dealt only with the transit radius. By
recognising the absorption depth and the transit radius as being independent
observables, we develop a theory for jointly interpreting both quantities,
which allows us to infer the temperatures and number densities associated with
the sodium lines. We are able to treat a non-isothermal situation with a
constant temperature gradient. Our novel diagnostics take the form of
simple-to-use algebraic formulae and require measurements of the transit radii
(and their corresponding absorption depths) at line center and in the line wing
for both sodium lines. We apply our diagnostics to the HARPS data of HD
189733b, confirm the upper atmospheric heating reported by Huitson et al.
(2012), derive a temperature gradient of K km and
find densities to cm.Comment: Accepted by ApJ Letters. 6 pages, 3 figure
Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an
ultracool dwarf star recently discovered to host three transiting and temperate
Earth-sized planets. We find the star is a relatively strong and variable
coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun,
despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with
the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation
of the planets that is many times stronger than experienced by the present-day
Earth. Using a simple energy-limited model we show that the relatively close-in
Earth-sized planets, which span the classical habitable zone of the star, are
subject to sufficient X-ray and EUV irradiation to significantly alter their
primary and any secondary atmospheres. Understanding whether this high-energy
irradiation makes the planets more or less habitable is a complex question, but
our measured fluxes will be an important input to the necessary models of
atmospheric evolution.Comment: 5 pages, published as a letter in MNRAS (accepted 16 September 2016
Genetic assimilation: a review of its potential proximate causes and evolutionary consequences
Background Most, if not all, organisms possess the ability to alter their phenotype in direct response to changes in their environment, a phenomenon known as phenotypic plasticity. Selection can break this environmental sensitivity, however, and cause a formerly environmentally induced trait to evolve to become fixed through a process called genetic assimilation. Essentially, genetic assimilation can be viewed as the evolution of environmental robustness in what was formerly an environmentally sensitive trait. Because genetic assimilation has long been suggested to play a key role in the origins of phenotypic novelty and possibly even new species, identifying and characterizing the proximate mechanisms that underlie genetic assimilation may advance our basic understanding of how novel traits and species evolve
A Spitzer Search for Water in the Transiting Exoplanet HD189733b
We present Spitzer Space Telescope observations of the extrasolar planet
HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with
the Infrared Array Camera. The system parameters, including planetary radius,
stellar radius, and impact parameter are derived from fits to the transit light
curves at both wavelengths. We measure two consistent planet-to-star radius
ratios, (Rp/Rs)[3.6m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and
(Rp/Rs)[5.8m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include
both the random and systematic errors in the transit baseline. Although planet
radii are determined at 1%-accuracy, if all uncertainties are taken into
account the resulting error bars are still too large to allow for the detection
of atmospheric constituants like water vapour. This illustrates the need to
observe multiple transits with the longest possible out-of-transit baseline, in
order to achieve the precision required by transmission spectroscopy of giant
extrasolar planets.Comment: Accepted in The Astrophysical Journal Letter
A Spitzer Search for Water in the Transiting Exoplanet HD189733b
We present Spitzer Space Telescope observations of the extrasolar planet
HD189733b primary transit, obtained simultaneously at 3.6 and 5.8 microns with
the Infrared Array Camera. The system parameters, including planetary radius,
stellar radius, and impact parameter are derived from fits to the transit light
curves at both wavelengths. We measure two consistent planet-to-star radius
ratios, (Rp/Rs)[3.6m] = 0.1560 +/- 0.0008(stat) +/- 0.0002(syst) and
(Rp/Rs)[5.8m] = 0.1541 +/- 0.0009(stat) +/- 0.0009(syst), which include
both the random and systematic errors in the transit baseline. Although planet
radii are determined at 1%-accuracy, if all uncertainties are taken into
account the resulting error bars are still too large to allow for the detection
of atmospheric constituants like water vapour. This illustrates the need to
observe multiple transits with the longest possible out-of-transit baseline, in
order to achieve the precision required by transmission spectroscopy of giant
extrasolar planets.Comment: Accepted in The Astrophysical Journal Letter
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