The Next Generation Transit Survey (NGTS)
Peter J. Wheatley, Don L. Pollacco, Didier Queloz, Heike Rauer, Christopher A. Watson, Richard G. West, Bruno Chazelas, Tom M. Louden, Simon Walker, Nigel Bannister, Joao Bento, Matthew Burleigh, Juan Cabrera, Philipp Eigmueller, Anders Erikson, Ludovic Genolet, Michael Goad, Andrew Grange, Andres Jordan, Katherine Lawrie, James McCormac, Marion Neveu
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The Next Generation Transit Survey (NGTS)
Peter J. Wheatley , a , Don L. Pollacco , Didier Queloz , Heike Rauer , , Christopher A. Watson ,Richard G. West , Bruno Chazelas , Tom M. Louden , Simon Walker , Nigel Bannister ,Joao Bento , Matthew Burleigh , Juan Cabrera , Philipp Eigmüller , Anders Erikson , Lu-dovic Genolet , Michael Goad , Andrew Grange , Andrés Jordán , Katherine Lawrie , JamesMcCormac , and Marion Neveu Department of Physics, University of Warwick, Coventry CV4 7AL, UK Observatoire Astronomique de l’Université de Genève, 1290 Sauverny, Switzerland Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt, 12489 Berlin, Germany Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, 10623 Berlin, Germany Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Santiago, Chile Isaac Newton Group of Telescopes, 38700 Santa Cruz de la Palma, Canary Islands, Spain
Abstract.
The Next Generation Transit Survey (NGTS) is a new ground-based sky sur-vey designed to find transiting Neptunes and super-Earths. By covering at least sixteentimes the sky area of
Kepler , we will find small planets around stars that are su ffi cientlybright for radial velocity confirmation, mass determination and atmospheric characteri-sation. The NGTS instrument will consist of an array of twelve independently pointed20 cm telescopes fitted with red-sensitive CCD cameras. It will be constructed at theESO Paranal Observatory, thereby benefiting from the very best photometric conditionsas well as follow up synergy with the VLT and E-ELT. Our design has been verifiedthrough the operation of two prototype instruments, demonstrating white noise charac-teristics to sub-mmag photometric precision. Detailed simulations show that about thirtybright super-Earths and up to two hundred Neptunes could be discovered. Our scienceoperations are due to begin in 2014. Ground-based transit surveys such as WASP [1] and HAT [2] have discovered an extraordinary va-riety of mainly Jupiter and Saturn-sized exoplanets that are challenging our understanding of giantplanet formation and migration. Discoveries have included planets that are variously highly-inflated,extremely close-in, or in retrograde orbits. Meanwhile, space-based transit surveys such as CoRoT[3] and
Kepler [4] have extended our sensitivity to smaller planets, finding the first rocky exoplanetsas well as a startling variety of multiple systems. However, due to the relatively narrow fields of viewof space-based surveys, most small planet candidates orbit stars that are too faint for radial-velocityconfirmation. As a consequence, their masses are often unknown or poorly known, placing only weak a e-mail: [email protected] a r X i v : . [ a s t r o - ph . E P ] F e b PJ Web of Conferences constraints on their bulk composition. Most of these planets are also too faint for atmospheric char-acterisation, even with future instrumentation such as the E-ELT, JWST, EChO or FINESSE. There isthus a strong scientific imperative to find small exoplanets around brighter stars.Building on experience gained in the WASP project, we have designed the Next Generation TransitSurvey (NGTS) with the primary aim of discovering transiting Neptunes and Super-Earths aroundbright stars from the ground. Our objective is to find a statistically significant sample of such systemsthat are bright enough for radial-velocity confirmation and hence mass and density determination. Byconstraining the bulk compositions of our sample we will test population synthesis models of planetformation and evolution [e.g. 5]. Our brightest Neptunes and Super-Earths will also be suitable foratmospheric characterisation by transmission spectroscopy and secondary eclipse observations.
Our scientific goals require the detection of transits with depths at the 1 mmag level. While this levelof accuracy is routinely achieved from the ground in narrow-field observations of individual objects,it is unprecedented for a ground-based wide-field survey. To reach this level of accuracy we havedrawn on experience from the WASP project in order to minimise known sources of red noise relatedto imprecise pointing, focus and flat fielding.The NGTS facility will be an array of twelve 20cm f / ffl e in order to minimise sensitivity to scattered moonlight. The CCD cameras are also ofcustom design, by Andor Technology in the UK, employing e2v CCDs with back-illuminated deep-depletion silicon and anti-fringing technology that is optimised for the far red of the optical spectrum.This improves our sensitivity to smaller stars (K and M types) and hence smaller planets.The telescopes and cameras are mounted on independent equatorial fork mounts by OMI in theUSA, and will be located in a single enclosure with a slide o ff -roof by GR PRO in the UK. Theenclosure is designed to shelter the telescope units from the prevailing Northerly wind, while allowingeach unit to point independently without obscuring the view of any other. The full telescope array hasan instantaneous field of view of 96 square degrees, and we intend to observe around four fields eachyear. The instrument design is described in more detail in [6].The deployment phase of NGTS is fully funded by our consortium institutes (DLR, Geneva, Le-icester, QUB, Warwick) and most of the equipment has been procured. A complete telescope unit hasbeen assembled and tested in Geneva, shown in the left hand panel of Fig. 1. On-site construction isdue to begin in summer 2013, with science operations from 2014. Our data will be made publiclyavailable after a proprietary period through the normal ESO archive. In order to develop and verify our instrument design we have deployed two prototype instruments.The first was operated on La Palma in 2010, and it verified the photometric performance of the back-illuminated deep-depletion e2v CCDs, and demonstrated the value of precise autoguiding. The second ot Planets and Cool Stars
Figure 1. Left: the first of twelve NGTS telescope units. This unit was operated at Geneva during summer 2012.
Right: the noise characteristics of the NGTS telescope unit measured in Geneva. These noise measurementshave been made using an ensemble of bright stars from across the whole field of view. The results show whitenoise behaviour to well below 1 mmag precision. prototype was the complete NGTS telescope unit shown in Fig. 1. Tests with this unit have verified theoptical performance of the telescope, as well as the pointing precision of the mount. The right handpanel of Fig. 1 shows the results of end-to-end tests of our photometry from Geneva on the
Kepler field. The median fractional RMS variability of an ensemble of bright stars from across the field ofview is plotted as a function of binned exposure time. The measured noise is consistent with theexpected scintillation level for the Geneva site [7]. We initially found a noise floor of 3-mmag that weascribe to variable water vapour extinction at the Geneva site, but since this was strongly correlatedbetween all stars it was readily detrended using the standard SysRem algorithm [8]. Fig. 1 then showspurely white-noise down to sub-mmag photometry.
We have performed detailed simulations of the NGTS planet catch for a four year baseline surveythat covers sixteen times the area of the
Kepler field. A population of host stars was drawn fromthe Besancon model of the Galaxy [9], and assigned a population of planets based on the planetsize distributions and occurrence rates from
Kepler [10]. This population was then sampled withthe known characteristics of the NGTS prototype instruments, accounting for realistic source andbackground spectra, weather statistics for Paranal and scintillation. The resulting planet candidatepopulation was then filtered with the sensitivity limits of the HARPS and ESPRESSO radial-velocityspectrographs, and the final predicted confirmable population is shown in Fig. 2 assuming a totalof 10 h exposure time per candidate. We find that HARPS / HARPS-N observations are capable ofconfirming 37 Neptunes from NGTS compared with only 7 from
Kepler . Allowing the HARPS-Nexposure times to increase to 20 h per candidate retrieves 21 Neptunes from
Kepler , but still only 1super-Earth. In contrast, follow up of NGTS candidates with ESPRESSO on the VLT is sensitive to231 Neptunes and 39 super-Earths (with only 10 h total exposure time per candidate).
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Figure 2.
Our simulated population ofNGTS planets that can be confirmed in10 h with HARPS or ESPRESSO(blue). These are compared with theknown transiting planets withradial-velocity confirmation (green)and the
Kepler candidates that areconfirmable with HARPS-N (red). Thissimulation shows a total of 39confirmable super-Earths from NGTSand 231 Neptunes.
The smallest NGTS confirmed planets will be prime targets for the ESA S Mission CHEOPS,which will provide precise radii and hence densities of our super-Earths. As well as testing modelsof bulk composition of super-Earths, this will allow us to prioritise objects by scale height for atmo-spheric follow up with VLT and
HST , and eventually E-ELT and
JWST (as well as dedicated missionssuch as EChO or FINESSE).
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