Chaotic quadruple secular evolution and the production of misaligned exomoons and Warm Jupiters in stellar multiples

Chaotic quadruple secular evolution and the production of misaligned exomoons and Warm Jupiters in stellar multiples 

Authors:

Grishin et al

Abstract:
We study the chaotic and secular evolution of hierarchical quadruple systems in the 3+1 configuration, focusing on the evolution of mutual inclination of the inner binaries as the system undergoes coupled Lidov-Kozai (LK) oscillations. We include short-range forces (SRF; such as those due to tidal and rotational distortions) that control the eccentricity excitation of the inner binary. The evolution of mutual inclination is described, a priori, by two dimensionless parameters, $\pazocal{R}_0$, the ratio between the inner and outer LK time-scales and ϵSRF, the ratio between the SRF precession and the inner LK precession rates. We find that the chaotic zones for the mutual inclination depend mainly on $\pazocal{R}_0$, while ϵSRF controls mainly the range of eccentricity excitation. The mutual inclination evolves chaotically for $1\lesssim \pazocal{R}_0\lesssim 10$, leading to large misalignments. For $0.4 \lesssim \pazocal{R}_0 \lesssim 0.8$, the system could be weakly excited and produce bimodal distribution of mutual inclination angles. Our results can be applied to exomoons-planets in stellar binaries and Warm/Hot Jupiters in stellar triples. Such systems could develop large mutual inclination angles if the inner binary is tight enough, and also high eccentricities, depending of the strength of the short-range forces. Future detections of tilted Warm/Hot Jupiters and exomoons could put our mechanism under observational tests.

The nature of the giant exomoon candidate Kepler-1625 b-i

The nature of the giant exomoon candidate Kepler-1625 b-i 

Authors:

Heller et al

Abstract:
The recent announcement of a Neptune-sized exomoon candidate around the transiting Jupiter-sized object Kepler-1625 b could indicate the presence of a hitherto unknown kind of gas giant moons, if confirmed. Three transits have been observed, allowing radius estimates of both objects. Here we investigate possible mass regimes of the transiting system that could produce the observed signatures and study them in the context of moon formation in the solar system, i.e. via impacts, capture, or in-situ accretion. The radius of Kepler-1625 b suggests it could be anything from a gas giant planet somewhat more massive than Saturn (0.4 M_Jup) to a brown dwarf (BD) (up to 75 M_Jup) or even a very-low-mass star (VLMS) (112 M_Jup ~ 0.11 M_sun). The proposed companion would certainly have a planetary mass. Possible extreme scenarios range from a highly inflated Earth-mass gas satellite to an atmosphere-free water-rock companion of about 180 M_Ear. Furthermore, the planet-moon dynamics during the transits suggest a total system mass of 17.6_{-12.6}^{+19.2} M_Jup. A Neptune-mass exomoon around a giant planet or low-mass BD would not be compatible with the common mass scaling relation of the solar system moons about gas giants. The case of a mini-Neptune around a high-mass BD or a VLMS, however, would be located in a similar region of the satellite-to-host mass ratio diagram as Proxima b, the TRAPPIST-1 system, and LHS 1140 b. The capture of a Neptune-mass object around a 10 M_Jup planet during a close binary encounter is possible in principle. The ejected object, however, would have had to be a super-Earth object, raising further questions of how such a system could have formed. In summary, this exomoon candidate is barely compatible with established moon formation theories. If it can be validated as orbiting a super-Jovian planet, then it would pose an exquisite riddle for formation theorists to solve.

Search for possible exomoons with FAST telescope

Search for possible exomoons with FAST telescope 

Author: 

Lukic

Abstract:
Our knowledge of the Solar System, encourages us to beleive that we might expect exomoons to be present around some of the known exoplanets. With present hardware and existing optical astronomy methods we shall not be able to find exomoons at least 10 years from now and even then, it will be a hard task to detect them. Using data from the Exoplanet Orbit Database (EOD) we find stars with Jovian exoplanets within 50 light years. Most of them will be fully accessible by the new radio telescope, The Five-hundred-meter Aperture Spherical radio Telescope (FAST) under construction, now in the test phase. We suggest radio astronomy based methods to search for possible exomoons around two exoplanets.

On The Feasibility of Exomoon Detection Via Exoplanet Phase Curve Spectral Contrast

On The Feasibility of Exomoon Detection Via Exoplanet Phase Curve Spectral Contrast

Authors:

Forgan et al

Abstract:
An exoplanet-exomoon system presents a superposition of phase curves to observers - the dominant component varies according to the planetary period, and the lesser varies according to both the planetary and the lunar period. If the spectra of the two bodies differs significantly, then it is likely there are wavelength regimes where the contrast between the moon and planet is significantly larger. In principle, this effect could be used to isolate periodic oscillations in the combined phase curve. Being able to detect the exomoon component would allow a characterisation of the exomoon radius, and potentially some crude atmospheric data. We run a parameter survey of combined exoplanet-exomoon phase curves, which show that for most sets of planet-moon parameters, the lunar component of the phase curve is undetectable to current state-of-the-art transit observations. Even with future transit survey missions, measuring the exomoon signal will most likely require photometric precision of 10 parts per million or better. The only exception to this is if the moon is strongly tidally heated or in some way self-luminous. In this case, measurements of the phase curve at wavelengths greater than a few microns can be dominated by the lunar contribution. Instruments like the James Webb Space Telescope and its successors are needed to make this method feasible.

Kepler-1625b I: A Half Neptune Sized Exomoon Orbiting a 10 Jupiter Mass Exoplanet?

HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I 

Authors:

Teachey et al

Abstract: 

Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability. In this work, we stack the phase-folded transits of 284 viable moon hosting Kepler planetary candidates, in order to search for satellites. These planets range from Earth-to-Jupiter sized and from ~0.1 to 1.0 AU in separation - so-called "warm" planets. Our data processing includes two-pass harmonic detrending, transit timing variations, model selection and careful data quality vetting to produce a grand light curve with a r.m.s. of 5.1 ppm. We find that the occurrence rate of Galilean-analog moon systems can be constrained to be η less than 0.38 to 95% confidence for the 284 KOIs considered, with a 68.3% confidence interval of η=0.16+0.13−0.10. A single-moon model of variable size and separation locates a slight preference for a population of Super-Ios, ~0.5 R_Earth moons orbiting at 5-10 planetary radii. However, we stress that the low Bayes factor of just 2 in this region means it should be treated as no more than a hint at this time. Splitting our data into various physically-motivated subsets reveals no strong signal. The dearth of Galilean-analogs around warm planets places the first strong constraint on exomoon formation models to date. Finally, we report evidence for an exomoon candidate Kepler-1625b I, which we briefly describe ahead of scheduled observations of the target with the Hubble Space Telescope.

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Authors:

Yang et al

Abstract:

Many exoplanets have been found in orbits close to their host stars and thus they are subject to the effects of photo-evaporation. Previous studies have shown that a large portion of exoplanets detected by the Kepler mission have been significantly eroded by photo-evaporation. In this paper, we numerically study the effects of photo-evaporation on the orbital evolution of a hypothesized moon system around a planet. We find that photo-evaporation is crucial to the stability of the moon system. Photo-evaporation can erode the atmosphere of the planet thus leading to significant mass loss. As the planet loses mass, its Hill radius shrinks and its moons increase their orbital semi-major axes and eccentricities. When some moons approach their critical semi-major axes, global instability of the moon system would be triggered, which usually ends up with two, one or even zero surviving moons. Some lost moons could escape from the moon system to become a new planet orbiting the star or run away further to become a free-floating object in the Galaxy. Given the destructive role of photo-evaporation, we speculate that exo-moons are less common for close-in planets (<0.1 AU), especially those around M-type stars, because they are more X-ray luminous and thus enhancing photo-evaporation. The lessons we learn in this study may be helpful for the target selection of on-going/future exomoon searching programs.

The Fate of Exomoons in White Dwarf Planetary Systems

The Fate of Exomoons in White Dwarf Planetary Systems

Authors:

Payne et al

Abstract:

Roughly 1000 white dwarfs are known to be polluted with planetary material, and the progenitors of this material are typically assumed to be asteroids. The dynamical architectures which perturb asteroids into white dwarfs are still unknown, but may be crucially dependent on moons liberated from parent planets during post-main-sequence gravitational scattering. Here, we trace the fate of these exomoons, and show that they more easily achieve deep radial incursions towards the white dwarf than do scattered planets. Consequently, moons are likely to play a significant role in white dwarf pollution, and in some cases may be the progenitors of the pollution itself.

Detecting Exomoons Through Micro Lensing

Properties of microlensing events by wide separation planets with a moon

Authors:

Chung et al

Abstract:

We investigate the properties of microlensing events caused by planetary systems where planets with a moon are widely separated from their host stars. From this investigation, we find that the moon feature generally appears as an very short-duration perturbation on the smooth asymmetric light curve of the lensing event induced by the wide separation planet; thus it can be easily discriminated from the planet feature responsible for the overall asymmetric light curve. For typical Galactic lensing events with the Einstein radius of ∼2 AU, the asymmetry of the light curves due to bound planets can be noticed up to ∼20AU. We also find that the perturbations of the wide planetary systems become dominated by the moon as the projected star-planet separation increases, and eventually the light curves of events produced by the systems appear as the single lensing light curve of the planet itself with a very short-duration perturbation induced by the moon, which is a representative light curve of the event induced by a star and a planet, except on the Einstein timescale of the planet. We also study the effect of a finite source star on the moon feature in the wide planetary lensing events. From this study, we find that when the lunar caustic is sufficiently separated from the planetary caustic, the lower limit on the ratio of the lunar caustic size to the source radius causing a ≥5% lunar deviation depends mostly on the projected planet-moon separation regardless of the moon/star mass ratio, and it decreases as the planet-moon separation becomes smaller or larger than the planetary Einstein radius.

Possibility for albedo estimation of exomoons: Why should we care about M dwarfs?

Possibility for albedo estimation of exomoons: Why should we care about M dwarfs?

Authors:

Dobos et al

Abstract:

Occultation light curves of exomoons may give information on their albedo and hence indicate the presence of ice cover on the surface. Icy moons might have subsurface oceans thus these may potentially be habitable. The objective of our paper is to determine whether next generation telescopes will be capable of albedo estimations for icy exomoons using their occultation light curves. The success of the measurements depends on the depth of the moon's occultation in the light curve and on the sensitivity of the used instruments. We applied simple calculations for different stellar masses in the V and J photometric bands, and compared the flux drop caused by the moon's occultation and the estimated photon noise of next generation missions with 5 σ confidence. We found that albedo estimation by this method is not feasible for moons of solar-like stars, but small M dwarfs are better candidates for such measurements. Our calculations in the J photometric band show that E-ELT MICADO's photon noise is just about 4 ppm greater than the flux difference caused by a 2 Earth-radii icy satellite in a circular orbit at the snowline of an 0.1 stellar mass star. However, considering only photon noise underestimates the real expected noise, because other noise sources, such as CCD read-out and dark signal become significant in the near infrared measurements. Hence we conclude that occultation measurements with next generation missions are far too challenging, even in the case of large, icy moons at the snowline of small M dwarfs. We also discuss the role of the parameters that were neglected in the calculations, e.g. inclination, eccentricity, orbiting direction of the moon. We predict that the first albedo estimations of exomoons will probably be made for large icy moons around the snowline of M4 -- M9 type main sequence stars.

Modeling the Orbital Sampling Effect of Extrasolar Moons

Modeling the Orbital Sampling Effect of Extrasolar Moons

Authors:

Heller et al

Abstract:

The orbital sampling effect (OSE) appears in phase-folded transit light curves of extrasolar planets with moons. Analytical OSE models have hitherto neglected stellar limb darkening and non-zero transit impact parameters and assumed that the moon is on a circular, co-planar orbit around the planet. Here, we present an analytical OSE model for eccentric moon orbits, which we implement in a numerical simulator with stellar limb darkening that allows for arbitrary transit impact parameters. We also describe and publicly release a fully numerical OSE simulator (PyOSE) that can model arbitrary inclinations of the transiting moon orbit. Both our analytical solution for the OSE and PyOSE can be used to search for exomoons in long-term stellar light curves such as those by Kepler and the upcoming PLATO mission. Our updated OSE model offers an independent method for the verification of possible future exomoon claims via transit timing variations and transit duration variations. Photometrically quiet K and M dwarf stars are particularly promising targets for an exomoon discovery using the OSE.

Orbital characteristics of planetesimals captured by circumplanetary gas disks

Orbital characteristics of planetesimals captured by circumplanetary gas disks

Authors:

Suetsugu et al

Abstract:

Sufficiently massive growing giant planets have circumplanetary disks, and the capture of solid bodies by the disks would likely influence the growth of the planets and formation of satellite systems around them. In addition to dust particles that are supplied to the disk with inflowing gas, recent studies suggest the importance of capture of planetesimals whose motion is decoupled from the gas, but orbital evolution of captured bodies in circumplanetary gas disks has not been studied in detail. In the present work, using three-body orbital integration and analytic calculations, we examine orbital characteristics and subsequent dynamical evolution of planetesimals captured by gas drag from circumplanetary gas disks. We find that the semi-major axes of the planet-centered orbits of planetesimals at the time of permanent capture are smaller than about one third of the planet's Hill radius in most cases. Typically, captured bodies rapidly spiral into the planet, and the rate of the orbital decay is faster for the retrograde orbits due to the strong headwind from the circumplanetary gas. When a planetesimal captured into a retrograde orbit suffers from sufficiently strong gas drag before spiraling into the planet, its orbit turns to the prograde direction at a radial location that can be explained using the Stokes number. We also find that those captured into certain types of orbits can survive for a long period of time even under gas drag both in the prograde and retrograde cases, which may be important for the origin of irregular satellites of giant planets.

Capture of planetesimals by waning circumplanetary gas disks

Capture of planetesimals by waning circumplanetary gas disks

Authors:

Suetsugu et al

Abstract:

When gas giant protoplanets grow sufficiently massive, circumplanetary disks would form. While solid bodies captured by the circumplanetary disks likely contribute to the growth of the planets and regular satellites around them, some of captured bodies would remain in planet-centered orbits after the dispersal of the disk. We examine capture and subsequent orbital evolution of planetesimals in waning circumplanetary gas disks using three-body orbital integration. We find that some of captured planetesimals can survive in the circumplanetary disk for a long period of time under such weak gas drag. Captured planetesimals have semi-major axes smaller than about one third of the planet's Hill radius. Distributions of their eccentricities and inclinations after disk dispersal depend on the strength of gas drag and the timescale of disk dispersal, and initially strong gas drag and quick disk dispersal facilitates capture and survival of planetesimals. However, in such a case, final orbital eccentricities and inclinations of captured bodies remain rather large. Although our results suggest that some of the present irregular satellites of gas giant planets with small semi-major axes would have been captured by gas drag, other mechanisms are required to fully explain their current orbital characteristics.

Detecting Exomoons Around Self-luminous Giant Exoplanets Through Polarization

Detecting Exomoons Around Self-luminous Giant Exoplanets Through Polarization

Authors:

Sengupta et al

Abstract:

Many of the directly imaged self-luminous gas giant exoplanets have been found to have cloudy atmospheres. Scattering of the emergent thermal radiation from these planets by the dust grains in their atmospheres should locally give rise to significant linear polarization of the emitted radiation. However, the observable disk averaged polarization should be zero if the planet is spherically symmetric. Rotation-induced oblateness may yield a net non-zero disk averaged polarization if the planets have sufficiently high spin rotation velocity. On the other hand, when a large natural satellite or exomoon transits a planet with cloudy atmosphere along the line of sight, the asymmetry induced during the transit should give rise to a net non-zero, time resolved linear polarization signal. The peak amplitude of such time dependent polarization may be detectable even for slowly rotating exoplanets. Therefore, we suggest that large exomoons around directly imaged self-luminous exoplanets may be detectable through time resolved imaging polarimetry. Adopting detailed atmospheric models for several values of effective temperature and surface gravity which are appropriate for self-luminous exoplanets, we present the polarization profiles of these objects in the infrared during transit phase and estimate the peak amplitude of polarization that occurs during the inner contacts of the transit ingress/egress phase. The peak polarization is predicted to range between 0.1 and 0.3 % in the infrared.

Predictable patterns in planetary transit timing variations and transit duration variations due to exomoons

Predictable patterns in planetary transit timing variations and transit duration variations due to exomoons

Authors:

Heller et al

Abstract:

We present new ways to identify single and multiple moons around extrasolar planets using planetary transit timing variations (TTVs) and transit duration variations (TDVs). For planets with one moon, measurements from successive transits exhibit a hitherto undescribed pattern in the TTV-TDV diagram, originating from the stroboscopic sampling of the planet's orbit around the planet-moon barycenter. This pattern is fully determined and analytically predictable after three consecutive transits. The more measurements become available, the more the TTV-TDV diagram approaches an ellipse. For planets with multiple moons in orbital mean motion resonance (MMR), like the Galilean moons, the pattern is much more complex and addressed numerically in this report. Exomoons in MMR can also form closed, predictable TTV-TDV figures if the drift of the moons' pericenters is sufficiently slow. We find that MMR exomoons produce loops in the TTV-TDV diagram and that the number of these loops is equal to the order of the MMR, or the largest integer in the MMR ratio. We use a Bayesian model and Monte Carlo simulations to test the discoverability of exomoons using TTV-TDV diagrams with current and near-future technology. In a blind test, two of us (BP, DA) successfully retrieved a large moon from simulated TTV-TDV by co-authors MH and RH, which resembled data from a known Kepler planet candidate. Single exomoons with a 10% moon-to-planet mass ratio, like to Pluto-Charon binary, can be detectable in the archival data of the Kepler primary mission. Multi-exomoon systems, however, require either larger telescopes or brighter target stars. Complementary detection methods invoking a moon's own photometric transit or its orbital sampling effect can be used for validation or falsification. A combination of TESS, CHEOPS, and PLATO data would offer a compelling opportunity for exomoon discoveries around bright stars.

Are Exomoons Impacting on White Dwarfs?

Liberating exomoons in white dwarf planetary systems

Authors:

Payne et al

Abstract:

Previous studies indicate that more than a quarter of all white dwarf (WD) atmospheres are polluted by remnant planetary material, with some WDs being observed to accrete the mass of Pluto in 106 yr. The short sinking time-scale for the pollutants indicates that the material must be frequently replenished. Moons may contribute decisively to this pollution process if they are liberated from their parent planets during the post-main-sequence evolution of the planetary systems. Here, we demonstrate that gravitational scattering events amongst planets in WD systems easily trigger moon ejection. Repeated close encounters within tenths of planetary Hill radii are highly destructive to even the most massive, close-in moons. Consequently, scattering increases both the frequency of perturbing agents in WD systems, as well as the available mass of polluting material in those systems, thereby enhancing opportunities for collision and fragmentation and providing more dynamical pathways for smaller bodies to reach the WD. Moreover, during intense scattering, planets themselves have pericentres with respect to the WD of only a fraction of an astronomical unit, causing extreme Hill-sphere contraction, and the liberation of moons into WD-grazing orbits. Many of our results are directly applicable to exomoons orbiting planets around main-sequence stars.

Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Exomoon Climate Models with the Carbonate-Silicate Cycle and Viscoelastic Tidal Heating

Authors:

Forgan et al

Abstract:

We presence of tidal heating, eclipses and planetary illumination in the exomoon energy budget combine to produce both circumstellar and circumplanetary habitable regions. Analytical calculations suggest that the circumplanetary habitable region is defined only by an inner edge (with its outer limits determined by orbital stability). Subsequent calculations using 1D latitudinal climate models indicated that the combined effect of eclipses and ice-albedo feedback can produce an outer edge to the circumplanetary habitable zone. But is this outer edge real, or an artefact of the climate model's relative simplicity?

We present an upgraded 1D climate model of Earth-like exomoon climates, containing the carbonate-silicate cycle and viscoelastic tidal heating. We conduct parameter surveys of both the circumstellar and circumplanetary habitable zones, and we find that the outer circumplanetary habitable edge remains provided the moon's orbit is not inclined relative to that of the planet. Adding the carbonate-silicate cycle pushes the circumplanetary habitable zone outward, by allowing increases in atmospheric partial pressure of carbon dioxide to boost the greenhouse effect. Viscoelastic tidal heating widens the habitable zone compared to standard, fixed-Q models. Weakening the tidal heating effect due to melting allows moons to be habitable at higher eccentricity, and pushes the inner circumstellar and circumplanetary habitable zone boundary inward.

Exomoons are Removed if Exoplanets Migrate in too far

Resonant Removal of Exomoons During Planetary Migration

Authors:

Spalding et al

Abstract:

Jupiter and Saturn play host to an impressive array of satellites, making it reasonable to suspect that similar systems of moons might exist around giant extrasolar planets. Furthermore, a significant population of such planets is known to reside at distances of several Astronomical Units (AU), leading to speculation that some moons thereof might support liquid water on their surfaces. However, giant planets are thought to undergo inward migration within their natal protoplanetary disks, suggesting that gas giants currently occupying their host star's habitable zone formed further out. Here we show that when a moon-hosting planet undergoes inward migration, dynamical interactions may naturally destroy the moon through capture into a so-called "evection resonance." Within this resonance, the lunar orbit's eccentricity grows until the moon eventually collides with the planet. Our work suggests that moons orbiting within about 10 planetary radii are susceptible to this mechanism, with the exact number dependent upon the planetary mass, oblateness and physical size. Whether moons survive or not is critically related to where the planet began its inward migration as well as the character of inter-lunar perturbations. For example, a Jupiter-like planet currently residing at 1AU could lose moons if it formed beyond 5AU. Cumulatively, we suggest that an observational census of exomoons could potentially inform us on the extent of inward planetary migration, for which no reliable observational proxy currently exists.

Spectroastrometric Detection of Exomoons

The Center of Light: Spectroastrometric Detection of Exomoons

Authors:

Agol et al

Abstract:

Direct imaging of extrasolar planets with future space-based coronagraphic telescopes may provide a means of detecting companion moons at wavelengths where the moon outshines the planet. We propose a detection strategy based on the positional variation of the center of light with wavelength, "spectroastrometry." This new application of this technique could be used to detect an exomoon, to determine the exomoon's orbit and the mass of the host exoplanet, and to disentangle of the spectra of the planet and moon. We consider two model systems, for which we discuss the requirements for detection of exomoons around nearby stars. We simulate the characterization of an Earth-Moon analog system with spectroastrometry, showing that the orbit, the planet mass, and the spectra of both bodies can be recovered. To enable the detection and characterization of exomoons we recommend that coronagraphic telescopes should extend in wavelength coverage to 3 micron, and should be designed with spectroastrometric requirements in mind.

Irregular Satellite Swarms Around Fomalhaut b

Collisional Cascade Caclulations for Irregular Satellite Swarms in Fomalhaut b

Authors:

Kenyon et al

Abstract:

We describe an extensive suite of numerical calculations for the collisional evolution of irregular satellite swarms around 1--300 M-earth planets orbiting at 120 AU in the Fomalhaut system. For 10--100 M-earth planets, swarms with initial masses of roughly 1% of the planet mass have cross-sectional areas comparable to the observed cross-sectional area of Fomalhaut b. Among 30--300 M-earth planets, our calculations yield optically thick swarms of satellites for ages of 1-10 Myr. Observations with HST and ground-based AO instruments can constrain the frequency of these systems around stars in the beta Pic moving group and possibly other nearby associations of young stars.

Can CHEOPS Detect Exomoons?

CHEOPS performance for exomoons: The detectability of exomoons by using optimal decision algorithm

Authors:

Simon et al

Abstract:

Many attempts have already been made for detecting exomoons around transiting exoplanets but the first confirmed discovery is still pending. The experience that have been gathered so far allow us to better optimize future space telescopes for this challenge, already during the development phase. In this paper we focus on the forthcoming CHaraterising ExOPlanet Satellite (CHEOPS),describing an optimized decision algorithm with step-by-step evaluation, and calculating the number of required transits for an exomoon detection for various planet-moon configurations that can be observable by CHEOPS. We explore the most efficient way for such an observation which minimizes the cost in observing time. Our study is based on PTV observations (photocentric transit timing variation, Szab\'o et al. 2006) in simulated CHEOPS data, but the recipe does not depend on the actual detection method, and it can be substituted with e.g. the photodynamical method for later applications. Using the current state-of-the-art level simulation of CHEOPS data we analyzed transit observation sets for different star-planet-moon configurations and performed a bootstrap analysis to determine their detection statistics. We have found that the detection limit is around an Earth-sized moon. In the case of favorable spatial configurations, systems with at least such a large moon and with at least Neptune-sized planet, 80\% detection chance requires at least 5-6 transit observations on average. There is also non-zero chance in the case of smaller moons, but the detection statistics deteriorates rapidly, while the necessary transit measurements increase fast.