The combination of high-precision photometry, large number of planets, and well-characterised host stars that will be provided by PLATO will significantly increase our chances of detecting planetary rings, moons, Trojan planets, and exo-comets.
Aside from the detection of exoplanets, searching for modulations in transit light curves also allows for the detection of planetary rings (Barnes & Fortney 2004; Ohta et al. 2009) and large moons (Sartoretti & Schneider 1999). One of the main drivers for the search of moons is that they might share the orbits of Jupiter-sized planets in the habitable zone (Heller & Barnes 2012), and thus be interesting targets for atmospheric characterisation. There are well-developed projects searching for moons around transiting extrasolar planets in the Keplermission (Kipping et al. 2012, 2013a, 2013b, 2014, 2015; Simon et al. 2012, 2015; Hippke 2015), but so far they remain elusive.
Moons produce two types of observable effects: photometric transits superimposed on planetary transits, and perturbations in the timing and length of the host planet’s transits. Unfortunately, for typical solar system satellites such as Ganymede around Jupiter, the amplitude of the timing perturbations is extremely small, on the order of several seconds. This is well below current detection limits. Furthermore, the photometric transit of a moon, when superimposed onto planetary transits, can very well be confused with the patterns produced by spot crossings (Silva-Valio & Lanza 2011; Sanchís-Ojeda et al. 2012) or instrumental systematics. On the other hand, planets that orbit closer than 0.1 au to their host star are not thought to be able to host stable moons (Namouni 2010). This means that we can focus on finding moons around planets with large orbital separations, which reduces the number of possible transit events for a given duration of observations. The scarcity of the observations, and the fact that the orbital phase of the moon is sampled at the orbital period of the planet (below the Nyquist frequency of the moon’s orbit), make the characterisation of these systems extremely challenging.
An efficient method to find evidence for satellites around a transiting planet is in the search for an extra signal due to moons in the phase-folded transit light curve of the planet.
After about a dozen transits, a moon on a circular orbit around its planet will create a symmetric extra dip in the wings of the planetary transit light curvebecause the moon’s positions relative to the planet will behave more and more like a ring the more transits are being folded. In fact, the moon’s orbit is statistically sampled after many transits, which is why this transit effect has been termed the Orbital Sampling Effect, or OSE for short (Heller 2014). The OSE could be detectable with Kepler for moons as small as Ganymede around photometrically quiet M dwarf stars (Heller 2014).
We have carried out additional simulations for PLATO assuming the most favourable scenario of a bright (V=8) white-noise dominated G dwarf star that is transited by a Jupiter-sized planet with a 0.7RE-sized moon. We found that PLATO would detect the OSE of such a moon with < 10% error in the retrieved moon radius and orbital semi-major axis after about 8 transits or a little less than a year of continuous observations, assuming the planet is in a ~45d orbit around the star.
Taking into account red noise effects and the fact that the star might be slightly dimmer than 8th magnitude, we estimate that PLATO could detect large moons (if they exist) after about 2 years of continuous observations. An extension to 3 years would certainly increase the odds significantly.
Trojan orbits close to the Lagrange points L4 and L5, and in 1:1 mean-motion resonance with planets, are thought to be very stable configurations for planets up to and including super-Earth sized bodies. In our solar system there are multiple examples of bodies in such orbits, albeit with sizes comparable with asteroids, so planetary objects in Trojan orbits would be a new class of system. To date, none have been detected (see Cabrera 2008; Ford & Holman 2007, and references therein), but PLATO will have the precision to detect Trojan-planets as small as the Earth.
Finally, exo-cometary tails lead to transit light curves that can be as deep as those produced by Earth-sized planets, albeit with a different shape (Lecavelier des Etangs 1999), and exo-cometary tails detected around nearby stars might be observable with future direct imaging experiments (Jura 2005). Giant planets can also develop comet-like tails (Schneider et al. 1998), and indications for such tails have already been found in Keplerdata (Budaj et al. 2013). Exo-comets are of interest, as they might provide a route for redistribution of organic material, both within and between planetary systems.
The tantalising discoveries of the peculiar light curves of KIC 8462852 (Boyajian et al. 2016) and EPIC 204278916 (Scaringi et al. 2016) by Keplerand K2 have intrigued researchers about possible families of comets (Bodman & Quillen 2016) or dust disks (Thompson et al. 2016) which could explain the evolution of the stellar brightness observed (Montet & Simon 2016; Hippke et al. 2016). Such rare systems might be found around brighter, closer-by stars by PLATO, continuing further their characterisation with ground based instruments.