Constraining Exoplanet Mass from Transmission Spectroscopy

Determination of an exoplanet's mass is a key to understanding its basic properties, including its potential for supporting life. To date, mass constraints for exoplanets are predominantly based on radial velocity (RV) measurements, which are not suited for planets with low masses, large semi-major axes, or those orbiting faint or active stars. Here, we present a method to extract an exoplanet's mass solely from its transmission spectrum. We find good agreement between the mass retrieved for the hot Jupiter HD189733b from transmission spectroscopy with that from RV measurements. Our method will be able to retrieve the masses of Earth-sized and super-Earth planets using data from future space telescopes that were initially designed for atmospheric characterization.Comment: 66 pages, 25 figures, published in the December 20, 2013 edition of Science Magazine. Includes supplementary material

Towards consistent mapping of distant worlds: secondary-eclipse scanning of the exoplanet HD189733b

Mapping distant worlds is the next frontier for exoplanet infrared photometry studies. Ultimately, constraining spatial and temporal properties of an exoplanet atmosphere will provide further insight into its physics. For tidally-locked hot Jupiters that transit and are eclipsed by their host star, the first steps are now possible. Our aim is to constrain an exoplanet's shape, brightness distribution (BD) and system parameters from its light curve. Notably, we rely on the eclipse scanning. We use archived Spitzer 8-{\mu}m data of HD189733 (6 transits, 8 secondary eclipses, and a phase curve) in a global MCMC procedure for mitigating systematics. We also include HD189733's out-of-transit radial velocity measurements. We find a 6-{\sigma} deviation from the expected occultation of a uniformly-bright disk. This deviation emerges mainly from HD189733b's thermal pattern, not from its shape. We indicate that the correlation of the orbital eccentricity, e, and BD (uniform time offset) does also depend on the stellar density, \rho*, and the impact parameter, b (e-b-\rho*-BD correlation). For HD189733b, we find that relaxing the e-constraint and using more complex BDs lead to lower stellar/planetary densities and a more localized and latitudinally-shifted hot spot. We obtain an improved constraint on the upper limit of HD189733b's orbital eccentricity, e<0.011 (95%), when including the RV measurements. Our study provides new insights into the analysis of exoplanet light curves and a proper framework for future eclipse-scanning observations. Observations of the same exoplanet at different wavelengths will improve the constraints on its system parameters while ultimately yielding a large-scale time-dependent 3D map of its atmosphere. Finally, we discuss the perspective of extending our method to observations in the visible, in particular to better understand exoplanet albedos.Comment: Accepted for publication in A&A. Final version will be available soon at http://www.aanda.org by Free Open Acces

Towards robust corrections for stellar contamination in JWST exoplanet transmission spectra

Transmission spectroscopy is still the preferred characterization technique for exoplanet atmospheres, although it presents unique challenges which translate into characterization bottlenecks when robust mitigation strategies are missing. Stellar contamination is one of such challenges that can overpower the planetary signal by up to an order of magnitude, and thus not accounting for stellar contamination can lead to significant biases in the derived atmospheric properties. Yet, accounting for stellar contamination may not be straightforward, as important discrepancies exist between state-of-the-art stellar models and measured spectra and between models themselves. Here we explore the extent to which stellar models can be used to reliably correct for stellar contamination and yield a planet's uncontaminated transmission spectrum. We find that (1) discrepancies between stellar models can dominate the noise budget of JWST transmission spectra of planets around stars with heterogeneous photospheres; (2) the true number of unique photospheric spectral components and their properties can only be accurately retrieved when the stellar models have a sufficient fidelity; and (3) under such optimistic circumstances the contribution of stellar contamination to the noise budget of a transmission spectrum is considerably below that of the photon noise for the standard transit observation setup. Therefore, we suggest (1) increased efforts towards development of model spectra of stars and their active regions in a data-driven manner; and (2) the development of empirical approaches for deriving spectra of photospheric components using the observatories with which the atmospheric explorations are carried out.Comment: 15 pages, 8 figures, 2 table

GPU-based framework for detecting small Solar System bodies in targeted exoplanet surveys

Small Solar System bodies are pristine remnants of Solar System formation, which provide valuable insights for planetary science and astronomy. Their discovery and cataloging also have strong practical implications to life on Earth as the nearest asteroids could pose a serious impact threat. Concurrently with dedicated observational projects, searches for small bodies have been performed on numerous archival data sets from different facilities. Here, we present a framework to increase the scientific return of an exoplanet transit-search survey by recovering serendipitous detections of small bodies in its daily and archival data using a GPU-based synthetic tracking algorithm. As a proof of concept, we analysed $12 \times 12 \mathrm{arcmin^2}$ sky fields observed by the 1-m telescopes of the SPECULOOS survey. We analysed 90 sky fields distributed uniformly across the sky as part of the daily search for small bodies and 21 archival fields located within 5 deg from the ecliptic plane as part of the archival search (4.4 deg$^2$ in total). Overall, we identified 400 known objects of different dynamical classes from Inner Main-belt Asteroids to Jupiter Trojans and 43 potentially new small bodies with no priors on their motion. We were able to reach limiting magnitude for unknown objects of $V$=23.8 mag, and a retrieval rate of $\sim$80% for objects with $V<$22 mag and $V<$23.5 mag for the daily and archival searches, respectively. SPECULOOS and similar exoplanet surveys can thus serve as pencil-beam surveys for small bodies and probe parameter space beyond $V$=22 mag.Comment: Accepted for publication in MNRAS (Monthly Notices of the Royal Astronomical Society), 11 pages, 11 figure

Unveiling the atmospheres of giant exoplanets with an EChO-class mission

More than a thousand exoplanets have been discovered over the last decade. Perhaps more excitingly, probing their atmospheres has become possible. With current data we have glimpsed the diversity of exoplanet atmospheres that will be revealed over the coming decade. However, numerous questions concerning their chemical composition, thermal structure, and atmospheric dynamics remain to be answered. More observations of higher quality are needed. In the next years, the selection of a space-based mission dedicated to the spectroscopic characterization of exoplanets would revolutionize our understanding of the physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic vision program in 2014. Our paper is therefore based on the planned capabilities of the Exoplanet Characterization Observatory (EChO), but it should equally apply to any future mission with similar characteristics. With its large spectral coverage (0.4 − 16 μm), high spectral resolution (λ/Δλ > 300 below 5 μm and λ/Δλ > 30 above 5 μm) and 1.5m mirror, a future mission such as EChO will provide spectrally resolved transit lightcurves, secondary eclipses lightcurves, and full phase curves of numerous exoplanets with an unprecedented signal-to-noise ratio. In this paper, we review some of today’s main scientific questions about gas giant exoplanets atmospheres, for which a future mission such as EChO will bring a decisive contribution

Empirically Constraining the Spectra of a Stars Heterogeneities From Its Rotation Lightcurve

Transmission spectroscopy is currently the most powerful technique to study a wide range of planetary atmospheres, leveraging the filtering of a stars light by a planets atmosphere rather than its own emission. However, both a planet and its star contribute to the information encoded in a transmission spectrum and a particular challenge relate to disentangling their contributions. As measurements improve, the lack of fidelity of stellar spectra models present a bottleneck for accurate disentanglement. Considering JWST and future high-precision spectroscopy missions, we investigate the ability to derive empirical constraints on the emission spectra of stellar surface heterogeneities (i.e., spots and faculae) using the same facility as used to acquire the transmission spectra intended to characterize a given atmosphere. Using TRAPPIST-1 as a test case, we demonstrate that it is possible to constrain the photospheric spectrum to 0.2% and the spectra of stellar heterogeneities to within 1-5%, which will be valuable benchmarks to inform the new generation of theoretical stellar models. Long baseline of observations (90% of the stellar rotation period) are necessary to ensure the photon-limited (i.e., instrument-limited) exploration of exoplanetary atmospheres via transmission spectroscopy.Comment: 10 pages, 3 figure

Unveiling the atmospheres of giant exoplanets with an EChO-class mission

More than a thousand exoplanets have been discovered over the last decade. Perhaps more excitingly, probing their atmospheres has become possible. With current data we have glimpsed the diversity of exoplanet atmospheres that will be revealed over the coming decade. However, numerous questions concerning their chemical composition, thermal structure, and atmospheric dynamics remain to be answered. More observations of higher quality are needed. In the next years, the selection of a space-based mission dedicated to the spectroscopic characterization of exoplanets would revolutionize our understanding of the physics of planetary atmospheres. Such a mission was proposed to the ESA cosmic vision program in 2014. Our paper is therefore based on the planned capabilities of the Exoplanet Characterization Observatory (EChO), but it should equally apply to any future mission with similar characteristics. With its large spectral coverage ($4-16\, \rm{\mu m}$), high spectral resolution ($\Delta\lambda/\lambda>300$ below $5\,\rm{\mu m}$ and $\Delta\lambda/\lambda>30$ above $5\,\rm{\mu m}$) and $1.5\rm{m}$ mirror, a future mission such as EChO will provide spectrally resolved transit lightcurves, secondary eclipses lightcurves, and full phase curves of numerous exoplanets with an unprecedented signal-to-noise ratio. In this paper, we review some of today's main scientific questions about gas giant exoplanets atmospheres, for which a future mission such as EChO will bring a decisive contribution.Comment: Accepted to the Experimental Journal of Astronom