These requirements provide the specification of the mission capabilities, and are derived from the high-level science requirements.
R-SCI-035 The capability shall be provided to carry out two phases during nominal science operations:
- A Long-Duration Observation Phase of at least two years (LOP)
- A Step-and-stare Observation Phase (SOP)
The Long-Duration Observation Phase is required for the detection of planets with periods long enough to orbit in the habitable zone of solar-type stars. The Step-and-stare Observation Phase is intended to include (i) repointings of the spacecraft to observe more transits of long-period Earth-like planets detected in the Long-Duration Observation Phase; (ii) observations of large numbers of targets in different areas of the sky to satisfy the Level-0 requirements R-SCI-L0-40, -45 and -65.
R-SCI-037 The in-orbit nominal science operations duration shall be four years.
R-SCI-045 During a Long-Duration Observation Phase it shall be possible to observe one field for up to 4 years.
The minimum duration of the LOP observations is 2 years, so that at least 2 consecutive transits for Sun-Earth analogues can be detected. For the P1 sample (< 34 ppm in 1 hour), the SNR of a single transit for an Earth-Sun analogue is 9 and 13 when integrating 2 transits. It is generally considered that a SNR > 10 guarantees the detectability of 100% of the transit signals (see Fressin et al. 2013, based on studies by Jenkins et al. 1996, 2010). A third transit event is beneficial to confirm the consistency of previous observations.
The requirements are defined in this way to allow for flexibility in the definition of the observation strategy, such that it can be adapted to the scientific priorities in exoplanet science in the next decade.
For the seismic analysis of the target stars, the total monitoring time must be sufficient to yield a relative precision of 10-4for the measurement of individual mode frequencies, which is needed to extract valuable stellar modelling information from the oscillation frequencies. For solar-type stars, this comes down to an absolute precision of 0.2 to 0.1 μHz, which translates into a minimum monitoring time of 5 months for a reasonable SNR of 10 in the power spectrum needed to achieve the envisioned mass, radius, and age precision of the core sample stars.
R-SCI-060 During a Step-and-stare Observation Phase, it shall be possible to observe between 6 and 3 fields for between 2 and 5 months, when feasible due to the position of the Sun.
Experience with CoRoT and Kepler has shown that two months is the shortest time base required to perform useful asteroseismology modelling of various types of stars.
Stellar samples specifications
In order to achieve the science requirements, four stellar samples have been defined for the PLATO observations. The definition of each star sample is displayed in the Table below.
Sample 1 (P1) is the backbone of the PLATO mission and must be considered as the highest priority objective. It consists of dwarfs and subgiants with spectral types from F5 to K7 and magnitudes down to V=11, observed with a maximum random noise (or random Noise-to-signal ratio, NSR) of 34 ppm in one hour. Ground-based radial velocity follow-up will be most effective for these stars.
Sample 2 (P2) consist of stars brighter than V= 8.2, with the same spectral types and maximum random noise as sample 1, observed over long time periods. Observing very bright stars will provide new significant facts about the physics of a large number of different stellar classes. Furthermore, the detection of a number of short period planets around such bright stars will also be used as input for observatories aimed at characterising exo-planetary atmospheres. In this regards, the E-ELT first light is targeted for 2024. In addition, JWST would be in its extended mission operations, which could continue until ~2028 (the JWST mission lifetime after launch will be between 5.5 years and 10 years).
The P4 sample is composed of nearby M dwarfs. These stars are cool enough that their habitable zone is relatively close-in, therefore planets in the habitable zone have orbital periods of just a few weeks.
Finally, the fifth sample (P5) is derived from the requirement to observe an even higher number of stars and obtain statistical information on planet occurrence and system properties.
PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017