The main scientific goals of PLATO concern the detection and characterisation of extrasolar planets for their radius, mass, and age. PLATO’s ‘Rosetta stone targets’ will be the small planets, in particular those which orbit in the habitable zone of solar-like stars. They need to be characterised with high accuracy (3% in radius, 10% in mass and age). The total sample shall, however, also include a large number of characterised planets of all sizes at all orbital distances to provide substantial constraints to planet formation and evolution theories and place our solar system into a general context of planet evolution. To achieve these mission goals, the following parameters of science mission performance are important:

• The total number of bright stars which will be observed. This sample splits into three sub-samples:
  • P1 sample, which includes the ‘Rosetta stone targets’ of highly accurately characterised long-period small planets in the habitable zone of solar-like stars. The required high accuracies for the P1 sample are:
    • Radius: < 3% for an Earth-sized planet orbiting a G0V star with V£10
    • Mass: < 10% for suitably bright RV follow-up targets (assumed as V£11)
    • Age: 10% for a G0V star with V£10
  • P2 sample, which provides planets orbiting very bright stars (V< 8.2), whose atmospheres could be characterised by ground-based or space-borne facilities. It will allow for dedicated asteroseismologic studies of a wide sample of well characterised stars, needed to better calibrate stellar models.
  • P5-bright sample, with lower SNR observed in long pointings. This sample provides planet detections similar to what we know from NASA’s Kepler mission sample, but for stars brighter than magnitude 11, therefore allowing mass measurements from ground. Most of them will have lower accuracy in planet parameters than the P1 sample, but provide significant input into statistical analysis of exoplanet properties.

• The statistical P5 sample will be, as an added benefit, complemented by the faint (V≤13) targets in the long and short pointings. These targets provide additional light curves for exoplanet search. Planetary masses will be delivered with TTVs and RV for larger planets.

Our benchmark test for accuracies is therefore an Earth-sized planet around a Sun (V=10) orbiting with 1 year orbital period in the P1 sample. Obviously, accuracies will be better for planets with smaller orbital periods (more transits observed), brighter host stars (better SNR) and larger planets (larger transit signal). Here, we focus on the benchmark test to demonstrate the performance in the “most difficult case” scenario.

Furthermore, the final number of stars observed and resulting planet yield will depend on the observing scenario chosen within the 4 years science observation baseline. Our baseline observing scenario is a division into two long pointings of 2 years each. This scenario maximises PLATO’s unique science return, that is, small planets in the habitable zone of solar like stars. The PLATO mission is flexible enough to, e.g., stay one year longer on the first long pointing field if decided. In this case, the 4^thyear will be dedicated to a step-and-stare phase, maximising the statistics of well characterised short-period planets with PLATO.

The final baseline observing scenario concerning the share between long- and short-pointings will be decided two years before launch, but can also be adapted during mission operation to optimise the science return. In case an additional extended mission phase of 2-4 years can be granted, it is obvious that the planet yield will be extended enormously and it will be possible to maximise both, long and short periods, at the same time. However, the baseline of 4 years is sufficient to make PLATO a unique mission providing scientific results that no other exoplanet mission will be able to achieve before.

 

 

References

PLATO – Revealing habitable worlds around solar-like stars
Definition Study Report, ESA-SCI(2017)1, April 2017