Exoplanet science is now in its full expansion, particularly after the CoRoT and Kepler space missions that led us to the discovery of thousands of extra-solar planets. The last decade has taught us that UV observations play a major role in advancing our understanding of planets and of their host stars, but the necessary UV observations can be carried out only by HST, and this is going to be the case for many years to come. It is therefore crucial to build a treasury data archive of UV exoplanet observations formed by a dozen "golden systems" for which observations will be available from the UV to the infrared. Only in this way we will be able to fully exploit JWST observations for exoplanet science, one of the key JWST science case

Alibert, Y.

Ayres, T. R.

Ballester, G. E.

Barnes, J.

Bisikalo, D. V.

Bourrier, V.

Cameron

Collier, A.

Czesla, S.

Desert, J. -M.

Ehrenreich, D.

Etangs, A. Lecavelier des

Fossati, L.

France, K.

Guedel, M.

Guenther, E.

Haswell, C. A.

Helling, Ch.

Heng, K.

Homstrom, M.

Kaltenegger, L.

Kislyakova, K. G.

Koskinen, T.

Lammer, H.

Lanza, A. F.

Linsky, J. L.

Mordasini, C.

Pagano, I.

Pollacco, D.

Rauer, H.

Reiners, A.

Salz, M.

Schneider, P. C.

Shematovich, V. I.

Staab, D.

Vidotto, A. A.

Wheatley, P. J.

Wood, B. E.

Yelle, R. V.

English

arXiv

Fossati, Luca

Bourrier, Vincent

Ehrenreich, David

Haswell, Carole A.

Kislyakova, Kristina G.

Lammer, Helmut

Lecavelier des Etangs, Alain

Alibert, Yann

Ayres, Thomas R.

Ballester, Gilda E.

Barnes, John

Bisikalo, Dmitry V.

Cameron, Andrew Collier

Czesla, Stefan

Desert, Jean-Michel

France, Kevin

Güdel, Manuel

Guenther, Eike

Helling, Christiane

Heng, Kevin

Homström, Mats

Kaltenegger, Lisa

Koskinen, Tommi

Lanza, Antonino F.

Linsky, Jeffrey L.

Mordasini, Chris

Pagana, Isabella

Pollacco, Don

Rauer, Heike

Reiners, Ansgar

Salz, Michael

Schneider, Peter C.

Shematovich, Valery I.

Staab, Daniel

Vidotto, Aline A.

Wheatley, Peter J.

Wood, Brian E.

Yelle, Roger V.

University of St. Andrews - Pure

Characterising exoplanets and their environment with UV transmission spectroscopy

Kislyakova, K.G.

Lecavelier des Etangs, A.

Cameron, A. Collier

Desert, J.-M.

Gudel, M.

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsCharacterising exoplanets and their environment withUV transmission spectroscopyOtherHow to cite:Fossati, L.; Bourrier, V.; Ehrenreich, D.; Haswell, C. A.; Kislyakova, K.G.; Lammer, H.; Lecavelier des Etangs,A.; Alibert, Y.; Ayres, T. R.; Ballester, G. E.; Barnes, J.; Bisikalo, D. V.; Cameron, A. Collier; Czesla, S.; Desert,J.-M.; France, K.; Gudel, M.; Guenther, E.; Helling, Ch.; Heng, K.; Homstrom, M.; Kaltenegger, L.; Koskinen, T.;Lanza, A. F.; Linsky, J. L.; Mordasini, C.; Pagano, I.; Pollacco, D.; Rauer, H.; Reiners, A.; Salz, M.; Schneider, P. C.;Shematovich, V. I.; Staab, D.; Vidotto, A. A.; Wheatley, P. J.; Wood, B. E. and Yelle, R. V. (2015). Characterisingexoplanets and their environment with UV transmission spectroscopy. Space Telescope Science Institute.For guidance on citations see FAQs.c© [not recorded]Version: Accepted ManuscriptLink(s) to article on publisher’s website:http://arxiv.org/pdf/1503.01278v1.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukarXiv:1503.01278v1  [astro-ph.SR]  4 Mar 2015Characterising exoplanets and their environmentwith UV transmission spectroscopyL. Fossati1, V. Bourrier2, D. Ehrenreich2, C.A. Haswell3, K.G. Kislyakova4, H. Lammer4, A. Lecavelier desEtangs5, Y. Alibert6, T.R. Ayres7, G. E. Ballester8, J. Barnes3, D. V. Bisikalo9, A. Collier, Cameron10, S.Czesla11, J.-M. De´sert7, K. France7, M. Gu¨del12, E. Guenther13, Ch. Helling10, K. Heng6, M. Homstro¨m14,L. Kaltenegger15, T. Koskinen8, A. F. Lanza16, J. L. Linsky17, C. Mordasini6, I. Pagano16, D. Pollacco18 ,H. Rauer19, A. Reiners20, M. Salz11, P.C. Schneider21, V. I. Shematovich9, D. Staab3, A. A. Vidotto2, P. J.Wheatley18, B. E. Wood22, R.V. Yelle81. Argelander Institut fu¨r Astronomie, Universita¨t Bonn, Germany, lfossati@astro.uni-bonn.de; 2.Observatoire astronomique de l’Universite´de Gene`ve, Switzerland; 3.Open University, UK; 4. Space Research Institute, Graz, Austria; 5. Institut d’astrophysique de Paris, France;6. University of Bern, Switzerland; 7. CASA, University of Colorado, USA; 8. LPL, University of Arizona, USA; 9. INASAN, Moscow, Russia;10. University of St. Andrews, UK; 11. Universita¨t Hamburg, Germany; 12. University of Vienna, Austria; 13. TLS Tautenburg, Germany;14. Swedish Institute of Space Physics, Sweden; 15.MPIA, Heidelberg, Germany; 16. INAF-Catania, Italy; 17. JILA, University of Colorado,USA; 18. University of Warwick, UK; 19. TU Berlin, Germany; 20.Universita¨t Go¨ttingen, Germany; 21. ESA, The Netherlands; 22. US NRL,Washington, D.C., USA;IntroductionExtra-solar planets (exoplanets) have provided many fascinating surprises and puzzles since their discovery.Given the youth of exoplanet science, the field is strongly driven by observations: the Hubble Space Telescope(HST) is providing fundamental observations for the advance of exoplanet science, particularly in terms ofatmospheric characterisation, crucial for our further understanding of planet formation and evolution.The very first detection of an exoplanet atmosphere was via STIS transmission spectroscopy of the close-ingiant planet (hot-Jupiter) HD209458 b that led to the detection of sodium in the planet atmosphere [1]. Follow-ing this seminal detection, Vidal-Madjar et al. (2003; [2]) used STIS far-ultraviolet (far-UV) Lyα transmissionspectroscopy of the same system to reveal that the planet possesses an extended hydrogen atmosphere thatis escaping hydrodynamically [3]. This result led to the full realisation that atmospheric evaporation is a keyfactor shaping planet evolution (e.g., [4]).Evaporation has also an impact on our understanding of planet formation and population. Atmosphericescape must be taken into account in order to use Gyr old planets to understand planet formation (e.g., theefficiency of accretion of primordial H/He atmospheres). High mass-loss rates would imply the existence of aconsistent population of almost atmosphereless low mass close-in planets (e.g., CoRoT-7 b and Kepler-10 b),where some of these might be the remnant core of evaporated Neptune-mass planets [5,6,7]. High mass-lossrates would also imply that close-in planets may be engulfed by the host star [8] or become undetectable,leaving behind a gap in the observed planet population distribution [9,10,11].The mechanism of evaporation, the upper atmosphere (exosphere) of exoplanets, and its interactions withthe host star can be thoroughly studied only at UV wavelengths and HST is the only facility capable to collectthe necessary observations. HST has now a limited lifetime and the next major UV space telescope, and stillnot as powerful as HST, might be able to fly at best only in 10 years time, or more (e.g., WSO-UV1, ARAGO2).It is only in the last few years that a consistent number of planets orbiting stars bright enough for UVtransit observations with HST (V < 10mag) has been discovered by ground-based surveys (e.g., SuperWASP,HATNet, KELT), and many more are expected to be found in the next couple of years from the ground andby the CHEOPS and TESS space missions. It is therefore now time to use the exceptional capabilities of HSTto create a treasury data archive of UV transit observations for planet atmospheric characterisation, allowingthe whole community to bring forward this fundamental branch of exoplanet research for the next decade, andmore.Current status of researchTo date, only three hot-Jupiters have been thoroughly observed at UV wavelengths. HD209458 b, with multipletransits observed at both far-UV and near-UV wavelengths, is a bloated (i.e., low density) hot-Jupiter orbiting1http://www.wso-uv.org/wso-uv2/index.php?lang=en2http://lesia.obspm.fr/UVMag/a Sun-like star. HD189733 b, with multiple transits observed only in the far-UV, is a hot-Jupiter orbiting a veryactive K-type star. WASP-12 b, with multiple transits observed only in the near-UV, is a bloated hot-Jupiterorbiting a late F-type star and one of the hottest planets known to date3.The common result obtained from the analysis of the UV data collected for these three planets is thepresence of an extended, evaporating upper atmosphere [2,14,15]. This so-called hydrodynamic “blow-off” setsin once the stellar X-ray and extreme-UV (EUV) incident flux is strong enough to greatly heat the planet upperatmosphere [16,17,18,19,20,21]. As a result, the atmosphere starts to dynamically expand pushing the upperatmosphere beyond the planet’s Roche lobe or protecting magnetosphere [22], leading to a consistent mass-loss[5]. Heavy atoms (e.g., C, O, Mg, and Fe; [23,15,24,25,26,27]) have also been detected in the exosphere of theseplanets, proving that heavy elements present in the lower atmosphere can be dragged up in the evaporatingexosphere.Despite these general similarities, the HST data led to the detection/prediction of several different unex-pected phenomena: the presence of a cometary tail of planet evaporated material in the case of HD209458 band HD189733 b [28], and, for WASP-12 b, the detection of an early ingress, likely caused by a dense bow-shockahead of the planet [15,29,30] or of a plasma torus originating from an active satellite [31], and of a circumstellarcloud of planet evaporated material [25,32]. In addition, scarce observational hints collected so far with HSTprovide tantalizing indications that surprisingly large signatures of atmospheric escape could be retrieved evenfrom moderately irradiated gas giants [12] and Neptune-mass planets [13].Atmospheric evaporation is the only major common denominator gathered so far from the observations, andmodels predict that many close-in planets should be affected by evaporation [33,34,35,36]. The reality is that itis completely obscure to us how the wealth of phenomena discovered so far depend upon the system parameters:stellar mass / temperature /metallicity / activity, planet mass / radius / lower atmospheric structure /magneticfield, orbit eccentricity, and semi-major axis. A large number of models has been made to make such predictions,but we completely lack the observational material necessary to constrain and improve them.The small number of observed systems does not allow us to understand the phenomenology of the planetupper atmosphere and plasma environment. This is fundamental to further improve our general understandingof planet atmosphere, formation, and evolution.Science goals of a UV observational parameter studyThe solution is to collect a treasury data archive of UV transit observations for a sample of systems selectedon the basis of their stellar, planet, and orbital characteristics in order to perform an observational parameterstudy. There are now a sufficient number and variety of planetary systems allowing us to perform such a study,and HST is the only telescope capable of collecting the necessary observations for many years to come. Thepossible legacy impact of such a data archive is shown for example by the continuous and extensive use of theSTIS observations of HD209458 b obtained in 2002 [2] that over the years have been analysed more than adozen times to extract information about the planet, its plasma environment, and host-star properties.Planet evolution via evaporation − Evaporation has a major impact on the evolution of a planet atmo-sphere. Since a phenomenon can usually be studied best where it is strongest, UV transit light curves of avariety of close-in planets with different characteristics would give us the best possible opportunity to studyplanet evaporation. This is key to understand for example the gap observed by Kepler for intermediate-massplanets with a short orbital separation [37,11,38]. It is crucial to now use HST to understand the diversityof nearby exoplanets that will be discovered with current and future space missions, such as K2, CHEOPS,TESS, and PLATO.Circumplanetary environment − Only with further UV transit observations of different close-in planets wecan constrain the various models (e.g., [39,29,30,31]) of the early-ingress phenomenon, discovered for WASP-12 b.The WASP-12 b observations led also to the detection of a thick circumstellar cloud formed of material lostby the planet. There are both observational [40] and theoretical [32] works suggesting that this is a commonfeature of many close-in planets. Because of the different effects these clouds have on different stellar emission3Single-transit far-UV observations exist also for the 55Cnc [12] and Gliese 436 [13] systems, but the results require furtherconfirmation.lines, far-UV observations would allow us to directly detect the clouds and to characterise their physicalproperties.Host star characterisation and effects on exoplanets − The UV properties of late-type stars are stillpoorly constrained and UV planet transit observations would provide important information on stellar activity(i.e., UV intensity and variability) and its evolution, which has an important impact on planet evolution aswell [41].Stellar winds play a fundamental role in shaping the formation and evolution of planetary systems, startingwith the protoplanetary disk phase [42], but, because of their optically thin nature, they are exceptionallyhard to observe and study (e.g., [43]). Charge exchange between stellar wind protons and planetary exosphereneutral hydrogen atoms leads to planet evaporation [44]; constraining stellar winds is therefore crucial tounderstand planet evaporation as well. In addition, the far-UV radiation has an impact on the atmosphericphotochemistry of the planet [45] and allows one to empirically estimate the stellar EUV flux responsible forthe heating of the planet atmosphere [46].UV transit observations of close-in planets give us the unique opportunity to constrain the wind physicalproperties (temperature, density, and velocity), particularly around the crucial transition region where thewind is accelerated, which is otherwise possible only for the Sun (e.g., [28,47]).Planet atmospheric structure and composition − The analysis of transit light curves obtained at wave-lengths of specific UV spectral lines allows one to infer the temperature, pressure, and dynamical structureof an exoplanet upper atmosphere. As different wavelengths probe different planet atmospheric depths (e.g.,[48]), observations obtained in the UV (HST), optical (HST), and infrared (HST/Spitzer/JWST) give us theunique opportunity to consistently constrain models of a whole exoplanet atmosphere, currently possible justfor a few Solar System planets and HD209458 b. A set of UV transit observations together with future JWSTdata will provide the necessary set of observational constraints for the advance of planet atmospheric models.A large theoretical and observational effort is being put into trying to understand the chemical compositionof extrasolar planets and its importance in governing structures (e.g., planetary mass and radius). Variousatoms and molecules have been so far discovered through transmission/emission spectroscopy at differentwavelengths. UV transit observations allow one to constrain the abundance of atoms in the upper atmosphere,an information that would be extremely important to best model the chemistry of the lower atmosphere inorder to interpret JWST data in the most accurate way.Optical transit observations revealed that high altitude clouds are a common feature in the atmosphereof close-in planets (e.g., [49]). Their presence is recognised by a planet radius vs. wavelength relation thatfollows a Rayleigh/Mie scattering law. The STIS observations obtained to date concentrate in the optical andfor many planets only the addition of data-points in the near-UV would allow one to constrain the chemicalcomposition, particle size, and altitude of the clouds [50].Planet magnetic moment − The magnetic field of an exoplanet plays a significant role in the evolutionof a planetary atmosphere, but the magnetic properties of exoplanets are still unknown. The detection of anexoplanet magnetic field has been attempted several times, mostly with radio observations [51], but with littlesuccess. UV observations, combined with modelling, can instead provide estimates of the planet magneticmoment [29,31,47], which plays an important role in the escape processes, and hence on the whole atmosphericstructure [52,53].Planet habitability − In the early stages of the evolution of planetary systems, planets in the habitable zonewill experience thermospheric expansion. As a result, evaporation is a key process in the determination of theactual habitability of planets orbiting in the habitable zone [54,55]. It is only with further UV observationsthat we can constrain the numerous planet atmosphere evolution models that are currently used to study anddetermine habitability.SummaryExoplanet science is now in its full expansion, particularly after the CoRoT and Kepler space missions that ledus to the discovery of thousands of extra-solar planets. The last decade has taught us that UV observationsplay a major role in advancing our understanding of planets and of their host stars, but the necessary UVobservations can be carried out only by HST, and this is going to be the case for many years to come. Itis therefore crucial to build a treasury data archive of UV exoplanet observations formed by a dozen “goldensystems” for which observations will be available from the UV to the infrared. 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Characterising exoplanets and their environment with UV transmission spectroscopy

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