Telescopes and Instrumentation

TRAPPIST: TRAnsiting Planets and PlanetesImals Small Telescope

Emmanuël Jehin1 Michaël Gillon1 Didier Queloz2 E. Jehin/ESO E. Pierre Magain1 Jean Manfroid1 Virginie Chantry1 Monica Lendl2 Damien Hutsemékers1 Stephane Udry2

1 Institut d’Astrophysique de l’Université de Liège, Belgium 2 Observatoire de l’Université de Genève, Switzerland

TRAPPIST is a 60-cm robotic telescope that was installed in April 2010 at the ESO La Silla Observatory. The project is led by the Astrophysics and Image Pro- cessing group (AIP) at the Department of Figure 1. The TRAPPIST telescope in its 5-metre time is obviously needed to monitor the Astrophysics, Geophysics and Ocean- enclosure at the La Silla Observatory, Chile. activity of several comets with a fre­ ography (AGO) of the University of Liège, quency of a few times per week. Some in close collaboration with the Geneva TRAPPIST is an original project using a comets are known, but others appear Observatory, and has been funded by single telescope that has been built and serendipitously. For the latter, telescope the Belgian Fund for Scientific Research optimised to allow the study of those two availability is crucial if we want to react (F.R.S.-FNRS) and the Swiss National aspects of the growing field of astrobiol­ rapidly to observe those targets at the Science Foundation (SNF). It is devoted ogy. It provides high quality photometric appropriate moment and for several hours to the detection and characterisation of data of exoplanet transits and allows or nights in a row; this strategy can pro­ exoplanets and to the study of comets the gaseous emissions of bright comets vide unique datasets impossible to obtain and other small bodies in the Solar Sys- to be monitored regularly. The project otherwise. tem. We describe here the goals of the is centred on three main goals: (1) the project and the hardware and present detection of the transits of new exoplan­ some results obtained during the first six ets; (2) the characterisation of known Telescope and instrumentation months of operation. transiting planets, in particular the pre­ cise determination of their size; and (3) For low cost operations and high flexibil­ the survey of the chemical composition of ity, TRAPPIST (see Figure 1) had to be The science case bright comets and the evolution of their a robotic observatory. The observation activity during their orbit. programme, including the calibration The hundreds of exoplanets known today plan, is prepared in advance and submit­ allow us to place our own Solar System ted daily to a specific software installed in the broad context of our own Galaxy. A dedicated robotic telescope on the computer controlling the obser­ In particular, the subset of known exo­ vatory. This computer controls all the planets that transit their parent stars are The basic project concept is a robotic tel­ technical aspects of the obser­vations: key objects for our understanding of escope fully dedicated to high precision dome control, pointing, focusing, image the formation, evolution and properties of exoplanet and comet time-series pho­ acquisition, astrometry and software planetary systems. The objects of the tometry, providing the large amount of guiding, calibrations, data storage... It is Solar System are, and will remain, exqui­ observing time requested for those in sleep mode during daytime and wakes site guides for helping us understand research projects. Exoplanet transits typi­ up one hour before sunset, opening the the mechanisms of planetary formation cally last several hours, up to a full night. dome and starting to cool the CCD. This and evolution. Comets, in particular, are There are now many known transiting process is made possible thanks to a most probably remnants of the initial planets, and many more candidates found ­collection of computer programs working population of planetesimals of the outer by transit surveys which need to be together and interacting with the tele­ part of the protoplanetary disc. Therefore ­confirmed and characterised. Moreover scope, dome, CCD camera, filter wheels the study of their physical and chemical these targets need to be observed at and meteorological station. Such a properties allows the conditions that very specific times, during eclipses, put­ ­complete and rapid integration, using prevailed during the formation of the four ting even more constraints on telescope mostly off-the-shelf solutions, would have giant planets to be probed. availability. Similarly a lot of observing been impossible a few years ago and

2 The Messenger 145 – September 2011 WASP—43b TRAPPIST I+z

1

0.99

Flux 0.98

0.97 σ = 0.0003

0.96 –0.04 –0.02 0 0.02 0.04 dT (days)

aluminium components, it weighs only Figure 3. TRAPPIST I + z transit photometry of the 65 kg and was allied to a compact Ger­ planet WASP-43b, period-folded and binned per two minute intervals, with the best-fit transit model man equatorial mount, the New Technol­ superimposed. The residuals of the fit, shifted along ogy Mount NTM-500, from the same the y-axis for the sake of clarity, are shown below company. This robust mount uses direct and their standard deviation is 300 parts per million drive technology to avoid the well-known (ppm). This light curve results from the global analy­ sis of 20 transits observed by TRAPPIST for this exo­ periodic errors found on the usual equa­ planet. Figure 2. Close-up of the 60-cm TRAPPIST torial mounts for small telescopes and telescope. therefore permits accurate pointing and tracking. The accuracy of the tracking (1.3 arcseconds per pixel) and a 10% allowed us to set up the experiment in allows an exposure time of four minutes accuracy are B-band 16.2, V-band less than two years. maximum, which is usually enough for 16.4, Rc-band 16.4, Ic-band 15.5 and our bright targets. Each frame is cali­ I + z-band 15.6; and in 200 seconds, The observatory is controlled through a brated in right ascension and declination B-band 19.7, V-band 19.4, Rc-band 19.2 VPN (Virtual Private Network) connection and software guiding runs continuously to and ­Ic-band 18.1. between La Silla and Liège University. keep the target centred on the same few The telescope and each individual sub­ pixels for the whole exposure sequence. The camera is fitted with a double filter system can be used from anywhere in wheel specifically designed for the pro­ the world, provided an internet connec­ The CCD camera was built by Finger ject and allowing a total of 12 different tion is available. In case of a low-level Lakes Instrumentation, with thermo-­ 5 × 5 cm filters and one clear position. mechanical failure, we can count on the electric cooling and a CCD of the latest One filter wheel is loaded with six broad­ help of the Swiss technician on site or generation. This is a thinned broad- band filters (Johnson-Cousins BVRcIc, the La Silla staff. band backside-illuminated Fairchild chip Sloan z’, and a special I + z filter for exo­ with 2048 × 2048 15-µm pixels providing planet transits) and the other filter wheel Hundreds of images, amounting to a field of view of 22 by 22 arcminutes is loaded with six narrowband filters 2–15 GB, are produced every night. and a plate scale of 0.6 arcseconds per for the comet programme. The comet Reduction pipelines run on a dedicated pixel. The sensitivity is excellent over ­filters were designed by NASA for the computer installed in the control room. all the spectral range, with a peak of 98% international Hale–Bopp campaign For the exoplanet programme, only tables at 750 nm, declining to around 80% ­(Farnham et al., 2000). Four filters iso­ and plots with the final results are trans­ at 550 nm and 60% at 300 nm. It is opti­ lating the main molecular emission lines ferred to Liège, while for the comet pro­ mised for low fringe level in the far red present in cometary spectra (OH [310 nm], gramme, it is often necessary to transfer and achieves a sensitivity of 40% at CN [385 nm], C3 [405 nm], C2 + NH2 dozens of frames in order to perform 950 nm. The gain is set to 1.1 e-/ADU. [515 nm]) are permanently mounted, more interactive tasks on the images. There are three different readout modes: while the two other filters of the set + + Every third month, a backup disk is sent a low noise readout mode (readout noise (CO [427 nm] and H2O [705 nm]) are to Belgium and transferred to the archive [RON] 9.7 e- in 8s), a fast mode (RON also available. In addition two narrow­ machine. 14 e- in 4s) and a very fast readout of 2s band filters, isolating “continuum win­ using two quadrants. The cooling is dows” (BC [445 nm] and GC [525 nm]) for The telescope is a 60-cm f/8 Ritchey– –55 deg below ambient, usual operation the estimation of the solar spectrum Chrétien design built by the German being at –35 °C with a dark count of reflected by the dust of the comet, are ASTELCO company (see Figure 2). Owing 0.11 e-/s/pixel. Typical magnitudes mounted. to its open design with carbon fibre and reached in 20s with a 2 × 2 binning

The Messenger 145 – September 2011 3 Telescopes and Instrumentation Jehin E. et al., TRAPPIST : TRAnsiting Planets and PlanetesImals Small Telescope

Installation, first light and start of allowing us to constrain its bulk compo­ Characterisation of known transiting operations sition. Furthermore, the special geometry planets of the orbit makes the study of impor- Once a transiting planet is detected, it is The telescope was installed in April 2010 tant properties of the planet (e.g., atmos­ of course desirable to characterise it in the T70 Swiss telescope building pheric composition, orbital obliquity, etc) thoroughly with high precision follow-up belonging to Geneva University (Figure 1). possible without the challenge of having measurements. Assuming a sufficient This facility had not been used since to spatially resolve it from its host star. precision, a transit light curve allows a the 1990s and was completely refurbished Transiting planets are thus key objects for number of parameters to be thoroughly in early 2010. The old 5-metre dome our understanding of the vast planetary constrained: (i) the planet-to-star radius (­AshDome) was equipped with new azi­ population hosted by the Galaxy. ratio; (ii) the orbital inclination; (iii) the muth motors and computer control. A ­stellar limb-darkening coefficients; and Boltwood II meteorological station with a Discovery of more transiting planets is (iv) the stellar density (assuming the cloud sensor and an independent rain important to assess the diversity of plan­ orbital period is known). This last quantity sensor was installed on the roof to record etary systems, to constrain their forma­ can be used with other measured stellar the weather conditions in real time. In tion and the dependence of planetary quantities to deduce, via stellar model­ case of bad conditions (clouds, strong properties on external conditions (orbit, ling, the mass of the star, which leads wind, risk of condensation, rain or snow), host star, other planets, etc.). TRAPPIST finally to the stellar and planet radii (Gillon the dome is automatically closed and is participating in this effort through sev­ et al., 2007; 2009). So far, we have the observations interrupted to guarantee eral different projects. ­gathered many high precision light curves the integrity of the telescope and equip­ for two dozen transiting planets. These ment. An uninterruptable power supply Detection of new transiting planets data will not only allow us to improve our (UPS) keeps the observatory running for On account of its extended temporal knowledge of these planets (size, struc­ 45 minutes during an electrical power availability and high photometric preci­ ture), but also to search for transit timing cut and an emergency shutdown is trig­ sion, TRAPPIST has very quickly become variations that could reveal the presence gered at the end of this period. Several an important element for the transit of other planets in the system. Since webcams inside and outside the building ­surveys WASP2 and CoRoT3. It is used to TRAPPIST is dedicated to this research help us to check what is going on in the confirm the candidate transits detected project, it can monitor dozens of tran- observatory if needed. After two months by these surveys and to observe them sits of the same planet, leading to an of commissioning on site, TRAPPIST with better time resolution and precision exquisite global precision, as shown in “first light” took place remotely on 8 June to discriminate eclipsing binaries from Figure 3. 2010, together with a press conference planetary transits. TRAPPIST observa­ at Liège University1. Technical tests, fine tions have so far rejected more than Transit search around ultra-cool dwarf tuning of the software as well as the first 30 WASP candidates as being eclipsing stars (UCDs) scientific observations were performed binaries. It has confirmed, and thus co- We have selected a sample of ten rela­ in remote control mode until November discovered, ten new transiting planets tively bright late-M stars and brown 2010. The fully robotic operation then (e.g., Triaud et al., 2011; Csizmadia et al., dwarfs. For each of them, we have started started smoothly in December with sev­ 2011; Gillon et al., 2011). an intense monitoring campaign (several eral months of superb weather until the full nights) to search for the transits of start of the winter. The search for transits of the planets the ultra-short period (less than one day) detected by the radial velocity (RV) tech­ terrestrial planets that are expected by The two scientific aspects of this dedi­ nique is another important science driver some planetary formation theories. The cated telescope and the first results are for TRAPPIST. RV surveys monitor stars photometric variability of these UCDs described below. significantly brighter than the transit sur­ brings a lot of information on their atmos­ veys. The few RV planets that were pheric and magnetic properties, and revealed afterwards to be transiting, have the by product of this TRAPPIST project Survey of transiting exoplanets brought improved knowledge of exo­ will thus be a significant contribution planet properties because a thorough to the understanding of these fascinating The transit method used by TRAPPIST is characterisation is possible (e.g., Deming UCDs that dominate the Galactic stellar an indirect technique, based on the & Seager, 2009). These planets thus population. measurement of the apparent brightness play a major role in exoplanetology. In this of a star. If a planet passes in front of context, TRAPPIST is used to search for the star, there is a slight observable the possible transits of the planets Survey of the chemical composition of decline in the apparent luminosity, as the detected by the HARPS (Mayor et al., comets planet eclipses a small fraction of the 2003) and CORALIE (Queloz et al., 2000) stellar disc. Recording this periodic event Doppler surveys. For the late M-dwarfs TRAPPIST is the only telescope in the allows the radius of the planet to be observed by HARPS, TRAPPIST is even southern hemisphere equipped with measured. Combined with the radial able to detect the transit of a massive the instrumentation to detect gaseous velocity method, the transit method pro­ rocky planet. comet emissions on a daily basis. As vides the mass and density of the planet, recently outlined during a NASA work­

4 The Messenger 145 – September 2011 OH CN C3

C2 GC H2O

Figure 4. Comet 103P/Hartley 2 imaged with calibration, image analysis can reveal follow-up of split comets and of special ­TRAPPIST through the different cometary filters on coma features (jets, fans, tails), that could outburst events is possible very shortly 5 November 2010: OH, CN, C3, C2, green continuum + lead to the detection of active regions after an alert is given and can thus pro­ (GC) and H2O . Note the different shapes and inten­ sities of the cometary coma in each filter. and determination of the rotation period vide important information on the nature of the nucleus. Such regular measure­ of comets. Light curves from these data ments are rare because of the lack of tel­ are useful to assess the gas and dust shop4, the huge amount of data col­ escope time on larger telescopes, yet are activity of a given comet in order, for lected by ­TRAPPIST will bring crucial very valuable as they show how the gas instance, to prepare more detailed obser­ new information on comets and will production rate of each species evolves vations with larger telescopes, espe- ­rapidly increase statistics, allowing com­ with respect to the distance to the Sun. cially the southern ESO telescopes. Hun­ ets to be classified on the basis of their These observations will allow the compo­ dreds of photometric and astrometric chemical composition. Linking those sition of the comets and the chemical measurements of all the moving targets in chemical classes to dynamical types (for class to which they belong (rich or poor in our frames are reported each month to instance short period comets of the carbon chain elements for instance) to the IAU Minor Planet Center. Two new Jupiter family and new long period com­ be determined, possibly revealing the ori­ asteroids were found during a laboratory ets from the Oort Cloud) is a funda­ gin of those classes. Indeed with about session with students of Liège University. mental step in understanding the forma­ five to ten bright comets observed each The observatory code attributed by the tion of comets and the Solar System. year, this programme will provide a good IAU is I40. statistical sample after a few years. For relatively bright comets (V ≤ 12 mag), Our first target was periodic comet 103P/ about twice a week, we measure gase­ Broadband photometry is also performed Hartley 2, which made a close approach ous production rates and the spatial dis­ once a week for fainter comets, usually to Earth in October 2010 and was tribution of several molecular species, far from the Sun, in order to measure the observed in great detail during the NASA including OH, CN, C2, and C3 (see Fig­ dust production rate from the R-band, EPOXI spacecraft flyby on 4 November. ure 4 for an example). In addition to pro­ to catch outbursts and find interesting We monitored this small (2 km) but very viding the production rates of the differ­ targets for the main programme. Owing active comet roughly every other night for ent species through a proper photometric to the way the telescope is operated, the four months and collected ~ 4000 frames

The Messenger 145 – September 2011 5 Telescopes and Instrumentation Jehin E. et al., TRAPPIST : TRAnsiting Planets and PlanetesImals Small Telescope

­Asteroid Belt and not the result of key asset for confirming and characteris­ cometary activity (Jehin et al., 2010). ing these planets.

Among other related projects we joined Further information and the latest news an international collaboration whose about TRAPPIST can be found on our goal is to catch rare stellar occultations web page6. by large trans-Neptunian objects (TNOs). This technique provides the most accu­ rate measurements of the diameter of Acknowledgements these very remote and poorly constrained We would like to thank the following: Grégory icy bodies (provided at least two chords ­Lambert of the Geneva Observatory for the continu­ are observed). About one to two events ous technical support on site and Vincent Megevand per month are expected for a dozen big when he was in charge; Michel Crausaz, Nigel TNOs. On 6 November 2010, a unique ­Evershed, Jean-Francois Veraguth, Francesco Pepe, Charles Maire, and Michel Fleury from Geneva observation was performed. A faint star Observatory for the refurbishment phase of the T70 was occulted by the dwarf planet Eris building; Andrew Wright and Alexis Thomas from for 29 seconds. Eris is the most distant ESO and Pierre Demain from Liège University for the Figure 5. The TRAPPIST image of the the activated object known in the Solar System by set-up of VPN at each site; Karina Celedon from asteroid (596) Scheila taken on 18 December 2010. ESO for the very efficient work and great help in the far (three times the distance of Pluto) and delivery of the many telescope parts to Chile and supposedly the biggest TNO — it was the La Silla Observatory; David Schleicher from even named the tenth planet for a few ­Lowell Observatory for having recovered and lent through ten different filters. Our contribu­ months in 2006. This was the third posi­ one ­complete set of NASA cometary filters and Alain ­Gilliotte from ESO for the optical characterisation tion to the worldwide campaign on this tive occultation by a TNO ever recorded of those ­filters; Sandrine Sohy and Robert Sip from comet was recently published in Meech and it allowed a very accurate radius Liège University for setting up all the computers et al. (2011). The quality of the data for Eris (to a few kilometres) to be derived, and backup procedures and Sandrine for being the allowed us to observe periodic variations providing a huge improvement in the webmaster. in the gaseous flux of the different spe­ determination of its size (previously known We would finally like to pay special thanks to the cies from which we could determine the to within about 400 km). The surprise whole staff of La Silla, and especially Gerardo Ihle and rotation of the nucleus and show that was to discover that Eris is a twin of Pluto Bernardo Ahumada, for their constant help and sup­ the rotation was slowing down by about and that it is not much bigger — remem­ port, most particularly during the installation phase. one hour in 100 days (Jehin et al., 2010). ber that Pluto was demoted as a planet in M. Gillon and E. Jehin are FNRS Research Associ­ This behaviour had never been so clearly 2006 because Eris was found to be big­ ates, J. Manfroid is an FNRS Research Director and observed before. The long-term monitor­ ger — both then received the new status D. Hutsemékers is an FRNS Senior Research ing of the production rates of the different of dwarf planets! A paper describing ­Associate. species is nearly completed and will be these results has been accepted for pub­ combined with high-resolution spectro­ lication in Nature (Sicardy et al., 2011). References scopic data in the visible and infrared that we obtained at the ESO Very Large Tele­ Csizmadia, Sz. et al. 2011, A&A, 531, 41 Deming, D. & Seager, S. 2009, Nature, 462, 301 scope (VLT) to provide a clear picture of Perspectives Farnham, T. L. et al. 2000, Icarus, 147, 180 the chemical composition of this unusu­ Gillon, M. et al. 2011, A&A (accepted) ally active comet from the Jupiter family. After only six months of robotic opera­ Gillon, M. et al. 2007, A&A, 466, 743 tions, TRAPPIST is already recognised Gillon, M. et al. 2009, A&A, 496, 259 Hsieh, H. & Jewitt D. 2006, Science 312, 561 On account of the fast reaction time in the exoplanet and comet communities Jehin, E. et al. 2010, CBET #2589 (a few hours), TRAPPIST is an invaluable as a unique tool on account of, among Jehin, E. et al. 2010, CBET, #2632 instrument for catching rare and short- other things, the large amount of tele­ Mayor, M. et al. 2003, The Messenger, 114, 20 term events. As an example, the night scope time available under photometric Meech, K. et al. 2011, ApJL, 734, L1 Queloz, D. et al. 2000, A&A, 354, 99 after the announcement that asteroid conditions for performing time-consuming Sicardy, B. et al. 2011, Nature, accepted (596) Scheila was behaving like a comet research. In particular, TRAPPIST has Triaud, A. et al. 2011, A&A, 513, A24 and could be a new Main Belt comet very quickly become a key element in the (only five of them are known — Hsieh & follow-up effort supporting WASP. In Links Jewitt, 2006), we began a programme future, TRAPPIST will play a similar role to monitor the expanding coma and the for the successor of WASP, the Next 1 ESO PR on TRAPPIST: http://www.eso.org/public/ brightness of the nucleus every night Generation Transit Survey (NGTS)5, a pro­ news/eso1023/ 2 ­during a period of three weeks (see Fig­ ject led by Geneva Observatory and sev­ Superwasp: http://www.superwasp.org 3 CoRoT: http://smsc.cnes.fr/COROT/index.htm ure 5 for one of the images). From imag­ eral UK universities, that will be installed 4 Comet Taxonomy, NASA workshop held 12–16 ing with TRAPPIST and spectroscopy at ESO Paranal Observatory in 2012. March 2011, Annapolis, USA with the ESO VLT we concluded that this NGTS will focus on detecting smaller 5 Next Generation Transit Survey: http://www. behaviour was the result of a collision planets than WASP, and the high photo­ ngtransits.org/ 6 TRAPPIST web page: http://www.ati.ulg.ac.be/ with a smaller asteroid in the Main metric precision of TRAPPIST will be a TRAPPIST/Trappist_main/Home.html

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