CHEOPS - The VLT Planet Finder

CHEOPS Logo

Markus Feldt1, Raffaele Gratton2, Stefan Hippler1, Hans Martin Schmid3, Massimo Turatto2, Rens Waters4

1Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

2Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy

3Astronomisches Institut der Eidgenössischen Technischen Hochschule, ETH Zentrum, CH-8092 Zürich, Switzerland

4Sterrenkundig Instituut ”Anton Pannekoek”, Universiteit van Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam,
the Netherlands

An eigthteen months study shows that direct observations of extra-solar planets are coming closer

The success story of the VLT/VLTI on Cerro Paranal in Chile encouraged ESO to set out on even more ambitious missions with a set of 2nd-generation instruments. One of this instruments is to become the VLT “Planet Finder”, which aims at the direct detection of extra-solar planets. With this instrument, the key questions of the search for other planetary systems, evidence for the existence of exo-planets in habitable zones, and the fundamental understanding of how planets form can for the first time be answered by a direct observation.

After a call for preliminary proposals issued on November 15th 2001 by ESO, the CHEOPS consortium formed and responded with a preliminary proposal by February 2002. In May 2003, a contract was signed with ESO, launching an 18 month phase-A study of what is now the CHEOPS project.

In the view of the consortium, CHEOPS (CHaracterizing Exo-planets by Opto-infrared Polarimetry and Spectroscopy) is an integrated project that contains the development of the instrument, and the preparation and execution of a dedicated science programme, aimed at the direct detection of at least three planets in each of three distinct age groups (few 107 yr, few 108 yr, and > 109 yr). While this programme is to be executed in the guaranteed time awarded to the consortium, the CHEOPS group will support a wider, comprehensive ESO survey of about 300 targets, which will finally yield the detection of a statistically relevant sample of planets between a 0.02 MJupiter and about 10 MJupiter.

The instrument optimized to achieve these goals will of course produce an enormous amount of data for faint objects and structures close to bright stars. This is particularly true for the fields of brown dwarfs, jets, and accretion and debris disks - the latter one being a prerequisite for the Darwin mission.

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Figure 1: Modular concept of CHEOPS at the VLT Nasmyth focus.

The instrument concept at the moment is based upon the principles of maximum stability, simplicity, modularity and upgradeability. Fig. 1 shows the three basic modules at the Nasmyth focus of the VLT: The common fore optics that contains an extreme AO system and keeps the instrumental polarization below 1%, and the two differential imagers, ZIMPOL and the IFS. The AO is extreme only in terms of actuator number and speed (~ 1600 actuators operating at 2 kHz). This otherwise standard-design system will deliver Strehl ratios between 0.4 and 0.85, depending on wavelength, conditions, and observing mode. ZIMPOL is an imaging polarimeter working between 0.65 µm and 0.95 µm. Due to its innovative lock-in technique, it can achieve polarimetric precisions better than 10-5 on a localized signal measured differentially against a smooth background. The IFS is a low-resolution integral field spectrograph working from 0.95 µm to 1.7 µm. Longer wavelengths have been excluded since they do not promise to increase the chances of detection. This simplifies the optical and mechanical design considerably. All three modules are located on a common bench, fixed to the Nasmyth platform to ensure maximum mechanical stability. Detailed optical and mechanical designs of all three modules, plus the instrument electronics and software demonstrate that the instrument is feasible with current day technology and can easily be integrated into the existing VLT environment.

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Figure 2: Planet signal and residual noise curves. Left: IFS, the orange curve (triangles) gives the planetary signal, the green curve (circles) gives 5 times the predicted residual noise. The planet can be detected from ~ 0.''6. Right: ZIMPOL observing Alpha-Centauri A. The thick curve gives the planetary signal, the dashed curve gives the predicted residual noise level. The two other curves give the residual noise for an enhanced polarimetric precision (to 10-6, dotted), and for the photon noise limit (thin solid). The planet size corresponds to that of Jupiter, assumed polarization is P(80o) = 0.5.

Our comprehensive analysis report shows that critical issues and components can be controlled. It arrives at detailed simulations of planet detections as e.g. shown in Fig. 2. This figure shows that CHEOPS will be able to detect intermediate age planets around solar-type stars at orbits with radii greater than 24 AU out to 40 pc. It also shows that ZIMPOL observing Jupiter in orbit around Alpha-Centauri can reach very high signal-to-noise ratios and could in fact detect planets of a few Earth masses around that star. There is also no doubt that the planet around Epsilon-Eridani will be detected by both channels - IFS and ZIMPOL - at high SNR.

The project plan presented in this report currently foresees a start of phase B of the CHEOPS project in May 2005, and commissioning of the instrument on Paranal in late 2009. This period is followed by a four-year core survey by the CHEOPS consortium lasting until 2013. The consortium comprises the Max Planck Institute for Astronomy in Heidelberg as the P.I. institute, Padova Observatory (INAF-OPD), the Institute for Astronomy at ETH Zurich, and the University of Amsterdam (UvA). The instrument will be actually constructed in three places: The ETH Zürich, Padova Observatory, and the Max-Planck-Institute for Astronomy (MPIA) in Heidelberg. Integration and Preliminary Acceptance Europe is foreseen to take place in MPIA’s integration hall. The total hardware costs are estimated to be 5 Million Euro, the total manpower at about 1,300 person months.