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Executive Summary Ref: PLATO-T-ASTR-TN-43 Issue: Issue 1 rev 0 Date: 20 Oct 2009 Page: 1 Title Assessment Study of the PLAnetary Transits and Oscillations of stars (PLATO) Mission Executive Summary Name & Function Date Signature Prepared by PLATO STUDY TEAM Oct 5th 2009 Verified by Approved by SEBASTIEN BOULADE Authorized by Oct 5th 2009 Study manager Doc type # WP Keywords Summary © EADS Astrium Ref: PLATO-T-ASTR-TN-43 Issue: Issue 1 rev 0 Date: 20 Oct 2009 Page: 2 1 PLATO MISSION OBJECTIVES PLATO mission objectives The PLAnetary Transits and Oscillations of stars (PLATO) mission aims at detecting and characterizing exoplanets by means of their transit signature in front of a very large sample of bright stars, and measuring the seismic oscillations of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems. The discovery of exoplanets from ground observations makes use of several methods: transits (as proposed for PLATO), radial velocity measurements of the parental stars line-of-sight movements (the first successful method) and gravitational lensing. The success of ground observations has been highly confirmed by the CNES/ESA/European/Brazilian spacecraft CoRoT (Convection, Rotation and planetary Transits). Astroseismology is also performed on CoRoT, although carried out on different targets. The Kepler NASA discovery mission, launched on March 6th 2009, aims also at finding Earth-like planets with the methods of transit and assessing also some astroseismology. Kepler shall observe 100,000 targets of the main sequence of magnitude <14, in a FOV of 105 deg². Both CoRoT and Kepler missions feature limitations in terms of minimum planet size, maximum orbital period, number of detected exoplanets and capability of further characterization of exoplanets and their host stars. PLATO will offer order of magnitude improvement of the science with respect to CoRoT and Kepler missions, filling the need for a further generation mission, observing more stars with increased magnitude and observing significantly smaller exoplanets, with significantly longer orbital periods. PLATO Science objectives The objective of the PLATO mission is to detect planetary transits in front of stars that can be characterized in terms of fundamental physical parameters. The stars characterization is obtained both from PLATO data themselves via asteroseismology (stellar masses and ages are measured), from the ESA GAIA mission (stellar radius at first order, augmented by PLATO data) and from the ground using e.g. high resolution spectroscopy. When discovered and confirmed, planets characteristics will be inferred from the gathered information on the planet/star radius and mass ratios, coupled to the measurement of the star’s radius and mass. Asteroseismology is therefore a key complement to transits method in the process of planets detection and characterization. In addition to the seismic analysis of planet host stars, which represents the highest priority goal of the mission, asteroseismology of the many other stars present in the field of view will be used to study stellar evolution. The planets to be detected and characterised are of the same type as the Terrestrial planets in the Solar System (Venus, Earth, Mars), orbiting within the inner part of their systems, where they could in principle be the hosts for life (the “Habitable Zone”). Only approximately 1% of the solar-like stars with planets should show Earth-size transits (due to the geometry of the problem). This poor transit probability, associated to the fact that not all stars feature planetary systems, makes the problem a highly statistical one. A large number of stars shall be surveyed in order to get sufficiently information on planets and stars characteristics. The two major requirements of PLATO are: • Observe continuously (minimizing interruptions) 20,000 bright cool dwarf stars during 2 years with 27 ppm/hr photometric accuracy. • Observe continuously (minimizing interruptions) 250,000 faint cool dwarf stars during 2 years with 80 ppm/hr photometric accuracy PLATO Executive Summary -2- © EADS Astrium Ref: PLATO-T-ASTR-TN-43 Issue: Issue 1 rev 0 Date: 20 Oct 2009 Page: 3 Measured transit depth Planet movement on its orbit Light signal received by the observer transit No transit Figure 1-1: The transit method for detection of planets (Left) and astroseismology using amplitude spectra of the signal (Sun oscillations - Right) The method of transits is based on the characterization of the continuous signal received from the star, where an occultation by the planet can be analysed. The geometry of the observation requires a large number of stars to be surveyed to detect planets. The spectrum of stars will be measured on PLATO in the 0.2 µHz-10 mHz range with high accuracy of frequency separations measurements 2 FROM SCIENCE TO SYSTEM REQUIREMENTS Measurement based on aperture photometry To achieve its scientific objectives, PLATO is relying on the aperture photometry technique: a star is constantly monitored by the telescope and the light collected from this star on the detector is measured by addition of the signal of the pixels in, and at the vicinity, of the star image. The window, called “aperture” is the set of pixels where the signal is collected. The aperture shall best fit the star image on the detector so that to minimize the noise coming from the background or other stars, but shall collect as much as possible signal from the target star, to improve signal to noise ratio. The collected signal for a star is therefore measured along time in a “light curve”. Un-interrupted measurement of this light curve allows detection of a transit in the time domain, or astroseismology science when passing in the frequency domain. There is no absolute photometry here, nor imaging quality and the PLATO main requirements are therefore linked to this measurement principle based on relative photometry only: the telescope shall continuously monitor a high number of stars to obtain un-interrupted light curves, and the star position on the detector in the aperture shall be constant so that the only variations of the light curves do correspond to the science signal. Performance requirements (number of stars and accuracy) Seven different samples have been defined as targets for observation by PLATO (Figure 2-1 below). These samples are characterized by the number of stars to be observed (cool dwarfs stars mainly, except for sample #3b where the whole main sequence is considered) and the accuracy of the observation, expressed in photonic noise level. The main requirement for non-photonic noise level is that it shall be below 1/3rd of the photonic noise one. Beyond noises induced by the detection chain, the main source of this non-photonic noise is the displacement of the star image inside the aperture defined by the pixels where the signal is summed. Whenever a displacement occurs, the collected signal changes because of several effects such as Pixel Response Non-Uniformity (PRNU), truncature of the image by the aperture, confusion of the signal by a nearby star entering the aperture, or pixels weights factors de-optimisation. The sum of all these effects defines a requirement on the maximum movement of the star image on the detector that can be tolerated PLATO Executive Summary -3- © EADS Astrium Ref: PLATO-T-ASTR-TN-43 Issue: Issue 1 rev 0 Date: 20 Oct 2009 Page: 4 (this movement comes mainly from the spacecraft attitude control system accuracy, differential velocity aberration and thermo-elastic deformations). Based on the heritage gained on CoRoT, this requirement was assumed to be 0.2 arcsec rms (corresponding to 10 % of stars that would not meet the 1/3rd requirement due to this jitter effect in the magnitude 10-11 range domain). When analysing the relation between the required total collective area of the telescope to meet the photonic noise level, and the required Field of View (FoV) to obtain the proper number of stars with this collective area (by eliminating in the equations the magnitude parameter), it can be demonstrated that the sample #1 is actually the driving requirement (others are met when sample #1 is), and that a whole set of potential designs for PLATO can be positioned on the curve drawing FoV as a function of the collective area. This curve (Figure 2-2) was used extensively during the assessment study to compare potential PLATO designs (high FoV, low collective area or low FoV, higher collective area, FoV splitting, overlapping or duplication, …) and their respective performance with respect to the requirements. This curve was also used to demonstrate analytically that the sample #2b requirement (corresponding to a re- visit of sample#2 stars were a planet has been detected, but with a higher accuracy) would lead to a design featuring less performance for sample #1. For this last reason, the sample #2b objective was not retained for the proposed PLATO reference design, although it could be implemented in further phases. Sample #1 Sample #2 Sample #2b Sample #3a Sample #3b Sample #4 Sample #5 Number of stars 20,000 80,000 ~ 80 1,000 300 3,000 250,000 Magnitude range NS NS NS Mv ≤ 8 Mv ≤ 8 Mv ≤ 8 (G) 11-14 (G) 27 ppm/h Photonic noise level 27 ppm/h 80 ppm/h 27 ppm/h (G) 27 ppm/h 27 ppm/h 80 ppm/h (G) 2 colours Nb of observed star fields 2 2 NS 2 2 1 2 Figure 2-1: Synthesis of MRD V2.1 requirements on performance (number of stars, accuracy) (NS = Not Specified, G = Goal requirement). Sample #2b was not retained as a requirement for this assessment phase since it would lead to a design featuring less performance on sample #1. 2000 1800 Eddington Kepler 1600 1400 towards larger FOV, 1200 smaller aperture Zone where the design is oversized with respect 1000 to PLATO requirements FoV (deg²) 800 600 towards smaller FOV, larger aperture 400 Curve of minimum design meeting PLATO Requirements Not Meeting 200 PLATO Requirements 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 1,5 Collecting area (m²) Figure 2-2: Sample #1 requirement curve This curve can be calculated to account for systematic noises (saturation, jitter, …).
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