The circumplanetary environments in our Solar System host a stunning array of moon and ring systems. Study of these environs has yielded valuable insights into planetary system formation and ...evolution, and there is every reason to believe that we will have much to learn from the moons and rings that are likely to exist in exoplanetary systems as well. This has motivated a small but growing number of researchers to investigate questions related to the formation, stability, long-term viability, composition, and observability of these exomoons and exorings. Still, due to a range of significant observational challenges, we remain at a relatively early stage of this work. As a result, we continue to face a number of difficult, unanswered questions, but this also means there are myriad opportunities for fundamental contributions to the field. In this review we will examine a variety of important issues for the astronomical community to consider, with an aim of providing a comprehensive understanding of ongoing efforts to identify and characterize exomoons and exorings, while also increasing interest and engagement. We begin with an overview of what we expect from systems hosting moons and/or rings in terms of their architectures, habitability, and observational signatures. We then highlight the contributions from a variety of works that have been aimed at detecting and characterizing them. We conclude by examining the outlook for finding these objects and discussing a number of ongoing challenges that we will want to overcome in the years ahead.
ABSTRACT
Diffuse gamma-ray emission from interstellar clouds results largely from cosmic ray (CR) proton collisions with ambient gas, regardless of the gas state, temperature, or dust properties of ...the cloud. The interstellar medium is predominantly transparent to both CRs and gamma-rays, so GeV emission is a unique probe of the total gas column density. The gamma-ray emissivity of a cloud of known column density is then a measure of the impinging CR population and may be used to map the k-scale CR distribution in the Galaxy. To this end, we test a number of commonly used column density tracers to evaluate their effectiveness in modeling the GeV emission from the relatively quiescent, nearby
ρ
Ophiuchi molecular cloud. We confirm that both H
i
and an appropriate
tracer are required to reproduce the total gas column densities probed by diffuse gamma-ray emisison. We find that the optical depth at 353 GHz (
) from
Planck
best reproduces the gamma-ray data overall, based on the test statistic across the entire region of interest, but near-infrared stellar extinction also performs very well, with smaller spatial residuals in the densest parts of the cloud.
Recently Kipping (2021) identified the so-called "exomoon corridor", a potentially powerful new tool for identifying possible exomoon hosts, enabled by the observation that fully half of all planets ...hosting an exomoon will exhibit transit timing variation (TTV) periodicities of 2-4 epochs. One key outstanding problem in the search for exomoons, however, is the question of how well the methods we have developed under the single moon assumption extend to systems with multiple moons. In this work we use \(N\)-body simulations to examine the exomoon corridor effect in the more general case of \(N \geq 1\) moons, generating realistic TTVs produced by satellite systems more akin to those seen in the outer Solar System. We find that indeed the relationship does hold for systems with up to 5 moons in both resonant and non-resonant chain configurations. Our results suggest an observational bias against finding systems with large numbers of massive moons; as the number of moons increases, total satellite mass ratios are generally required to be significantly lower in order to maintain stability, or architectures must be more finely tuned to survive. Moons produced in impact or capture scenarios may therefore dominate early detections. Finally, we examine the distribution of TTV periods measured for a large number of Kepler Objects of Interest (KOIs) and find the same characteristic exomoon corridor distribution in several cases. This could be dynamical evidence for an abundance of moons in the field, though we caution against strong inferences based on this result.
The search for exomoons in time-domain photometric data has to-date generally consisted of fitting transit models that are comprised of a planet hosting a single moon. This simple model has its ...advantages, but it may not be particularly representative, as most of the major moons in our Solar System are found in multi-moon satellite systems. It is critical that we investigate, then, the impact of applying a single-moon model to systems containing multiple moons, as there is the possibility that utilizing an inaccurate or incomplete model could lead to erroneous conclusions about the system. To that end, in this work we produce a variety of realistic multi-moon light curves, perform standard single-moon model selection, and analyze the impacts that this model choice may have on the search for exomoons. We find that the number of moons in a system fit with a single-moon model generally has little impact on whether we find evidence for a moon in that system, and other system attributes are individually not especially predictive. However, the model parameter solutions for the moon frequently do not match any real moon in the system, instead painting a picture of a ``phantom'' moon. We find no evidence that multi-moon systems yield corresponding multi-modal posteriors. We also find a systematic tendency to overestimate planetary impact parameter and eccentricity, to derive unphysical moon densities, and to infer potentially unphysical limb darkening coefficients. These results will be important to keep in mind in future exomoon search programs.
Targeted observations of possible exomoon host systems will remain difficult to obtain and time-consuming to analyze in the foreseeable future. As such, time-domain surveys such as Kepler, K2 and ...TESS will continue to play a critical role as the first step in identifying candidate exomoon systems, which may then be followed-up with premier ground- or space-based telescopes. In this work, we train an ensemble of convolutional neural networks (CNNs) to identify candidate exomoon signals in single-transit events observed by Kepler. Our training set consists of \({\sim}\)27,000 examples of synthetic, planet-only and planet+moon single transits, injected into Kepler light curves. We achieve up to 88\% classification accuracy with individual CNN architectures and 97\% precision in identifying the moons in the validation set when the CNN ensemble is in total agreement. We then apply the CNN ensemble to light curves from 1880 Kepler Objects of Interest with periods \(>10\) days (\(\sim\)57,000 individual transits), and further test the accuracy of the CNN classifier by injecting planet transits into each light curve, thus quantifying the extent to which residual stellar activity may result in false positive classifications. We find a small fraction of these transits contain moon-like signals, though we caution against strong inferences of the exomoon occurrence rate from this result. We conclude by discussing some ongoing challenges to utilizing neural networks for the exomoon search.
Exomoons are predicted to produce transit timing variations (TTVs) upon their host planet. Unfortunately, so are many other astrophysical phenomena - most notably other planets in the system. In this ...work, an argument of reductio ad absurdum is invoked, by deriving the transit timing effects that are impossible for a single exomoon to produce. Our work derives three key analytic tests. First, one may exploit the fact that a TTV signal from an exomoon should be accompanied by transit duration variations (TDVs), and that one can derive a TDV floor as a minimum expected level of variability. Cases for which the TDV upper limit is below this floor can thus be killed as exomoon candidates. Second, formulae are provided for estimating whether moons are expected to be 'killable' when no TDVs presently exist, thus enabling the community to estimate whether it's even worth deriving TDVs in the first place. Third, a TTV ceiling is derived, above which exomoons should never be able to produce TTV amplitudes. These tools are applied to a catalog of TTVs and TDVs for two and half thousand Kepler Objects Interest, revealing over two hundred cases that cannot be due to a moon. These tests are also applied to the exomoon candidate Kepler-1625b i, which comfortably passes the criteria. These simple analytic results should provide a means of rapidly rejecting putative exomoons and streamlining the search for satellites.
Exomoons are the natural satellites of planets orbiting stars outside our solar system, of which there are currently no confirmed examples. We present new observations of a candidate exomoon ...associated with Kepler-1625b using the Hubble Space Telescope to validate or refute the moon's presence. We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon. Self-consistent photodynamical modeling suggests that the planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. Since our inference is dominated by a single but highly precise Hubble epoch, we advocate for future monitoring of the system to check model predictions and confirm repetition of the moon-like signal.
Recently, Heller & Hippke argued that the exomoon candidates Kepler-1625 b-i and Kepler-1708 b-i were allegedly 'refuted'. In this Matters Arising, we address these claims. For Kepler-1625 b, we show ...that their Hubble light curve is identical to that previously published by the same lead author, in which the moon-like dip was recovered. Indeed, our fits of their data again recover the moon-like dip with improved residuals than that obtained by Heller & Hippke. Their fits therefore appear to have somehow missed this deeper likelihood maximum, as well producing apparently unconverged posteriors. Consequently, their best-fitting moon is the same radius as the planet, Kepler-1625 b; a radically different signal from that which was originally claimed. The authors then inject this solution into the Kepler data and remark, as a point of concern, how retrievals obtain much higher significances than originally reported. However, this issue stems from the injection of a fundamentally different signal. We demonstrate that their Hubble light curve exhibits ~20% higher noise and discards 11% of the useful data, which compromises its ability to recover the subtle signal of Kepler-1625 b-i. For Kepler-1708 b-i it was claimed that the exomoon model's Bayes factor is highly sensitive to detrending choices, yielding reduced evidence with a biweight filter versus the original claim. We use their own i) detrended light curve and ii) biweight filter code to investigate these claims. For both, we recover the original moon signal, to even higher confidence than before. The discrepancy is explained by comparing to their quoted fit metrics, where we again demonstrate that the Heller & Hippke regression definitively missed the deeper likelihood maximum corresponding to Kepler-1708 b-i. We conclude that both candidates remain viable but certainly demand further observations.
The transit method is presently the most successful planet discovery and characterization tool at our disposal. Other advanced civilizations would surely be aware of this technique and appreciate ...that their home planet's existence and habitability is essentially broadcast to all stars lying along their ecliptic plane. We suggest that advanced civilizations could cloak their presence, or deliberately broadcast it, through controlled laser emission. Such emission could distort the apparent shape of their transit light curves with relatively little energy, due to the collimated beam and relatively infrequent nature of transits. We estimate that humanity could cloak the Earth from Kepler-like broadband surveys using an optical monochromatic laser array emitting a peak power of about 30 MW for roughly 10 hours per year. A chromatic cloak, effective at all wavelengths, is more challenging requiring a large array of tunable lasers with a total power of approximately 250 MW. Alternatively, a civilization could cloak only the atmospheric signatures associated with biological activity on their world, such as oxygen, which is achievable with a peak laser power of just around 160 kW per transit. Finally, we suggest that the time of transit for optical SETI is analogous to the water-hole in radio SETI, providing a clear window in which observers may expect to communicate. Accordingly, we propose that a civilization may deliberately broadcast their technological capabilities by distorting their transit to an artificial shape, which serves as both a SETI beacon and a medium for data transmission. Such signatures could be readily searched in the archival data of transit surveys.
Exomoons represent a crucial missing puzzle piece in our efforts to understand extrasolar planetary systems. To address this deficiency, we here describe an exomoon survey of 70 cool, giant ...transiting exoplanet candidates found by Kepler. We identify only one which exhibits a moon-like signal that passes a battery of vetting tests: Kepler-1708 b. We show that Kepler-1708 b is a statistically validated Jupiter-sized planet orbiting a Sun-like quiescent star at ~1.6AU. The signal of the exomoon candidate, Kepler-1708 b-i, is a 4.8-sigma effect and is persistent across different instrumental detrending methods, with a 1% false-positive probability via injection-recovery. Kepler-1708 b-i is ~2.6 Earth radii and is located in an approximately coplanar orbit at ~12 planetary radii from its ~1.6AU Jupiter-sized host. Future observations will be necessary to validate or reject the candidate.
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