22 research outputs found
Molecular Detectability in Exoplanetary Emission Spectra
Of the many recently discovered worlds orbiting distant stars, very little is
yet known of their chemical composition. With the arrival of new transit
spectroscopy and direct imaging facilities, the question of molecular
detectability as a function of signal-to-noise (SNR), spectral resolving power
and type of planets has become critical. In this paper, we study the
detectability of key molecules in the atmospheres of a range of planet types,
and report on the minimum detectable abundances at fixed spectral resolving
power and SNR. The planet types considered - hot Jupiters, hot super-Earths,
warm Neptunes, temperate Jupiters and temperate super-Earths - cover most of
the exoplanets characterisable today or in the near future. We focus on key
atmospheric molecules, such as CH4, CO, CO2, NH3, H2O, C2H2, C2H6, HCN, H2S and
PH3. We use two methods to assess the detectability of these molecules: a
simple measurement of the deviation of the signal from the continuum, and an
estimate of the level of confidence of a detection through the use of the
likelihood ratio test over the whole spectrum (from 1 to 16). We find
that for most planetary cases, SNR=5 at resolution R=300 ()
and R=30 () is enough to detect the very strongest spectral
features for the most abundant molecules, whereas an SNR comprised between 10
and 20 can reveal most molecules with abundances 10^-6 or lower, often at
multiple wavelengths. We test the robustness of our results by exploring
sensitivity to parameters such as vertical thermal profile, mean molecular
weight of the atmosphere and relative water abundances. We find that our main
conclusions remain valid except for the most extreme cases. Our analysis shows
that the detectability of key molecules in the atmospheres of a variety of
exoplanet cases is within realistic reach, even with low SNR and spectral
resolving power.Comment: ICARUS Accepte
The atmospheric chemistry of the warm Neptune GJ 3470b: influence of metallicity and temperature on the CH4/CO ratio
Current observation techniques are able to probe the atmosphere of some giant
exoplanets and get some clues about their atmospheric composition. However, the
chemical compositions derived from observations are not fully understood, as
for instance in the case of the CH4/CO abundance ratio, which is often inferred
different from what has been predicted by chemical models. Recently, the warm
Neptune GJ3470b has been discovered and because of its close distance from us
and high transit depth, it is a very promising candidate for follow up
characterisation of its atmosphere. We study the atmospheric composition of
GJ3470b in order to compare with the current observations of this planet, to
prepare the future ones, but also as a typical case study to understand the
chemical composition of warm (sub-)Neptunes. The metallicity of such
atmospheres is totally uncertain, and vary probably to values up to 100x solar.
We explore the space of unknown parameters to predict the range of possible
atmospheric compositions. Within the parameter space explored we find that in
most cases methane is the major carbon-bearing species. We however find that in
some cases, typically for high metallicities with a sufficiently high
temperature the CH4/CO abundance ratio can become lower than unity, as
suggested by some multiwavelength photometric observations of other warm
(sub-)Neptunes, such as GJ1214b and GJ436b. As for the emission spectrum of
GJ3470b, brightness temperatures at infrared wavelengths may vary between 400
and 800K depending on the thermal profile and metallicity. Combined with a hot
temperature profile, a substantial enrichment in heavy elements by a factor of
100 with respect to the solar composition can shift the carbon balance in
favour of carbon monoxide at the expense of CH4. Nevertheless, current
observations of this planet do not allow yet to determine which model is more
accurate.Comment: 12 pages, 8 figures, accepted in Astronomy & Astrophysic
Remote-sensing Characterisation of Major Solar System Bodies with the Twinkle Space Telescope
Remote-sensing observations of Solar System objects with a space telescope
offer a key method of understanding celestial bodies and contributing to
planetary formation and evolution theories. The capabilities of Twinkle, a
space telescope in a low Earth orbit with a 0.45m mirror, to acquire
spectroscopic data of Solar System targets in the visible and infrared are
assessed. Twinkle is a general observatory that provides on demand observations
of a wide variety of targets within wavelength ranges that are currently not
accessible using other space telescopes or that are accessible only to
oversubscribed observatories in the short-term future. We determine the periods
for which numerous Solar System objects could be observed and find that Solar
System objects are regularly observable. The photon flux of major bodies is
determined for comparison to the sensitivity and saturation limits of Twinkle's
instrumentation and we find that the satellite's capability varies across the
three spectral bands (0.4-1, 1.3-2.42, and 2.42-4.5{\mu}m). We find that for a
number of targets, including the outer planets, their large moons, and bright
asteroids, the model created predicts that with short exposure times,
high-resolution spectra (R~250, {\lambda}
2.42{\mu}m) could be obtained with signal-to-noise ratio (SNR) of >100 with
exposure times of <300s
Small Bodies Science with Twinkle
Twinkle is an upcoming 0.45m space-based telescope equipped with a visible
and two near-infrared spectrometers covering the spectral range 0.4 to
4.5{\mu}m with a resolving power R~250 ({\lambda}<2.42{\mu}m) and R~60
({\lambda}>2.42{\mu}m). We explore Twinkle's capabilities for small bodies
science and find that, given Twinkle's sensitivity, pointing stability, and
spectral range, the mission can observe a large number of small bodies. The
sensitivity of Twinkle is calculated and compared to the flux from an object of
a given visible magnitude. The number, and brightness, of asteroids and comets
that enter Twinkle's field of regard is studied over three time periods of up
to a decade. We find that, over a decade, several thousand asteroids enter
Twinkle's field of regard with a brightness and non-sidereal rate that will
allow Twinkle to characterise them at the instrumentation's native resolution
with SNR > 100. Hundreds of comets can also be observed. Therefore, Twinkle
offers researchers the opportunity to contribute significantly to the field of
Solar System small bodies research.Comment: Published in JATI
Generation of an optimal target list for the Exoplanet Characterisation Observatory (EChO)
The Exoplanet Characterisation Observatory (EChO) has been studied as a space
mission concept by the European Space Agency in the context of the M3 selection
process. Through direct measurement of the atmospheric chemical composition of
hundreds of exoplanets, EChO would address fundamental questions such as: What
are exoplanets made of? How do planets form and evolve? What is the origin of
exoplanet diversity?
More specifically, EChO is a dedicated survey mission for transit and eclipse
spectroscopy capable of observing a large, diverse and well-defined planetary
sample within its four to six year mission lifetime.
In this paper we use the end-to-end instrument simulator EChOSim to model the
currently discovered targets, to gauge which targets are observable and assess
the EChO performances obtainable for each observing tier and time. We show that
EChO would be capable of observing over 170 relativity diverse planets if it
were launched today, and the wealth of optimal targets for EChO expected to be
discovered in the next 10 years by space and ground-based facilities is simply
overwhelming.
In addition, we build on previous molecular detectability studies to show
what molecules and abundances will be detectable by EChO for a selection of
real targets with various molecular compositions and abundances.
EChO's unique contribution to exoplanetary science will be in identifying the
main constituents of hundreds of exoplanets in various mass/temperature
regimes, meaning that we will be looking no longer at individual cases but at
populations. Such a universal view is critical if we truly want to understand
the processes of planet formation and evolution in various environments.
In this paper we present a selection of key results. The full results are
available online (http://www.ucl.ac.uk/exoplanets/echotargetlist/).Comment: Accepted for publication in Experimental Astronomy, 20 pages, 10
figures, 3 table
Bringing pupils into the ORBYTS of research
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Exoplanet spectroscopy and photometry with the Twinkle space telescope
The Twinkle space telescope has been designed for the characterisation of exoplanets and Solar System objects. Operating in a low Earth, Sun-synchronous orbit, Twinkle is equipped with a 45 cm telescope and visible (0.4 – 1 μm) and infrared (1.3 – 4.5 μm) spectrometers which can be operated simultaneously. Twinkle is a general observatory which will provide on-demand observations of a wide variety of targets within wavelength ranges that are currently not accessible using other space telescopes or accessible only to oversubscribed observatories in the short-term future. Here we explore the ability of Twinkle’s spectrometers to characterise the currently-known exoplanets. We study the spectral resolution achievable by combining multiple observations for various planetary and stellar types. We also simulate spectral retrievals for some well-known planets (HD 209458 b, GJ 3470 b and 55 Cnc e). From the exoplanets known today, we find that with a single transit or eclipse, Twinkle could probe 89 planets at low spectral resolution (R 20) in channel 1 (1.3 – 4.5 μm). With 10 observations, the atmospheres of 144 planets could be characterised with R 20. By stacking 10 transits, there are 1185 potential targets for study at R < 20 as well as 388 planets at higher resolutions. The majority of targets are found to be large gaseous planets although by stacking multiple observations smaller planets around bright stars (e.g. 55 Cnc e) could be observed with Twinkle. Photometry and low resolution spectroscopy with Twinkle will be useful to refine planetary, stellar and orbital parameters, monitor stellar activity through time and search for transit time and duration variations (TTVs and TDVs). Refinement of these parameters could be used to in the planning of observations with larger space-based observatories such as JWST and ARIEL. For planets orbiting very bright stars, Twinkle observations at higher spectral resolution will enable us to probe the chemical and thermal properties of an atmosphere. Simultaneous coverage across a wide wavelength range will reduce the degeneracies seen with Hubble and provide access to detections of a wide range molecules. There is the potential to revisit them many times over the mission lifetime to detect variations in cloud cover
Probing the extreme planetary atmosphere of WASP-12b
We report near-infrared measurements of the terminator region transmission
spectrum and dayside emission spectrum of the exoplanet WASP-12b obtained using
the HST WFC3 instrument. The disk-average dayside brightness temperature
averages about 2900 K, peaking to 3200 K around 1.46 microns. We modeled a
range of atmospheric cases for both the emission and transmission spectrum and
confirm the recent finding by Crossfield et al. (2012b) that there is no
evidence for C/O >1 in the atmosphere of WASP-12b. Assuming a physically
plausible atmosphere, we find evidence that the presence of a number of
molecules is consistent with the data, but the justification for inclusion of
these opacity sources based on the Bayesian Information Criterion (BIC) is
marginal. We also find the near-infrared primary eclipse light curve is
consistent with small amounts of prolate distortion. As part of the calibration
effort for these data, we conducted a detailed study of instrument systematics
using 65 orbits of WFC3-IR grims observations. The instrument systematics are
dominated by detector-related affects, which vary significantly depending on
the detector readout mode. The 256x256 subarray observations of WASP 12
produced spectral measurements within 15% of the photon-noise limit using a
simple calibration approach. Residual systematics are estimated to be less than
70 parts per million.Comment: Accepted for publication in Icaru