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Register a new userCatching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I
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The (yet-to-be confirmed) discovery of a Neptune-sized moon around the ~3.2 Jupiter-mass planet in Kepler 1625 puts interesting constraints on the formation of the system. In particular, the relatively wide orbit of the moon around the planet, at ~40 planetary radii, is hard to reconcile with planet formation theories. We demonstrate that the observed characteristics of the system can be explained from the tidal capture of a secondary planet in the young system. After a quick phase of tidal circularization, the lunar orbit, initially much tighter than 40 planetary radii, subsequently gradually widened due to tidal synchronization of the spin of the planet with the orbit, resulting in a synchronous planet-moon system. Interestingly, in our scenario the captured object was originally a Neptune-like planet, turned into a moon by its capture.
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Transit photometry of the exoplanet candidate Kepler-1625b has recently been interpreted to show hints of a moon. We aim to clarify whether the exomoon-like signal is really caused by a large object in orbit around Kepler-1625b. We explore several detrending procedures, i.e. polynomials and the Cosine Filtering with Autocorrelation Minimization (CoFiAM). We then supply a light curve simulator with the co-planar orbital dynamics of the system and fit the resulting planet-moon transit light curves to the Kepler data. We employ the Bayesian Information Criterion (BIC) to assess whether a single planet or a planet-moon system is a more likely interpretation of the light curve variations. We carry out a blind hare-and-hounds exercise using many noise realizations by injecting simulated transits into different out-of-transit parts of the original Kepler-1625 data: 100 sequences with 3 synthetic transits of a Kepler-1625b-like planet and 100 sequences with 3 synthetic transits of this planet with a Neptune-sized moon. The statistical significance and characteristics of the exomoon-like signal strongly depend on the detrending method, and the data chosen for detrending, and on the treatment of gaps in the light curve. Our injection-retrieval experiment shows evidence for moons in about 10% of those light curves that do not contain an injected moon. Strikingly, many of these false-positive moons resemble the exomoon candidate. We recover up to about half of the injected moons, depending on the detrending method, with radii and orbital distances broadly corresponding to the injected values. A $Delta$BIC of -4.9 for the CoFiAM-based detrending indicates an exomoon around Kepler-1625b. This solution, however, is only one out of many and we find very different solutions depending on the details of the detrending method. It is worrying that the detrending is key to the interpretation of the data.
We have investigated i) the formation of gravitationally bounded pairs of gas-giant planets (which we call binary planets) from capturing each other through planet-planet dynamical tide during their close encounters and ii) the following long-term orbital evolution due to planet-planet and planet-star {it quasi-static} tides. For the initial evolution in phase i), we carried out N-body simulations of the systems consisting of three jupiter-mass planets taking into account the dynamical tide. The formation rate of the binary planets is as much as 10% of the systems that undergo orbital crossing and this fraction is almost independent of the initial stellarcentric semi-major axes of the planets, while ejection and merging rates sensitively depend on the semi-major axes. As a result of circularization by the planet-planet dynamical tide, typical binary separations are a few times the sum of the physical radii of the planets. After the orbital circularization, the evolution of the binary system is governed by long-term quasi-static tide. We analytically calculated the quasi-static tidal evolution in later phase ii). The binary planets first enter the spin-orbit synchronous state by the planet-planet tide. The planet-star tide removes angular momentum of the binary motion, eventually resulting in a collision between the planets. However, we found that the binary planets survive the tidal decay for main-sequence life time of solar-type stars (~10Gyrs), if the binary planets are beyond ~0.3 AU from the central stars. These results suggest that the binary planets can be detected by transit observations at >0.3AU.
Kepler-22b is the first transiting planet to have been detected in the habitable-zone of its host star. At 2.4 Earth radii, Kepler-22b is too large to be considered an Earth-analog, but should the planet host a moon large enough to maintain an atmosphere, then the Kepler-22 system may yet possess a telluric world. Aside from being within the habitable-zone, the target is attractive due to the availability of previously measured precise radial velocities and low intrinsic photometric noise, which has also enabled asteroseismology studies of the star. For these reasons, Kepler-22b was selected as a target-of-opportunity by the Hunt for Exomoons with Kepler (HEK) project. In this work, we conduct a photodynamical search for an exomoon around Kepler-22b leveraging the transits, radial velocities and asteroseismology plus several new tools developed by the HEK project to improve exomoon searches. We find no evidence for an exomoon around the planet and exclude moons of mass >0.5 Earth masses to 95% confidence. By signal injection and blind retrieval, we demonstrate that an Earth-like moon is easily detected for this planet even when the time-correlated noise of the data set is taken into account. We provide updated parameters for the planet Kepler-22b including a revised mass of <53 Earth masses to 95% confidence and an eccentricity of 0.13(-0.13)(+0.36) by exploiting Single-body Asterodensity Profiling (SAP). Finally, we show that Kepler-22b has a >95% probability of being within the empirical habitable-zone but a <5% probability of being within the conservative habitable-zone.
Kepler and Hubble photometry of a total of four transits by the Jupiter-sized Kepler-1625b have recently been interpreted to show evidence of a Neptune-sized exomoon. The profound implications of this first possible exomoon detection and the physical oddity of the proposed moon, that is, its giant radius prompt us to re-examine the data and the Bayesian Information Criterion (BIC) used for detection. We combine the Kepler data with the previously published Hubble light curve. In an alternative approach, we perform a synchronous polynomial detrending and fitting of the Kepler data combined with our own extraction of the Hubble photometry. We generate five million MCMC realizations of the data with both a planet-only model and a planet-moon model and compute the BIC difference (DeltaBIC) between the most likely models, respectively. DeltaBIC values of -44.5 (using previously published Hubble data) and -31.0 (using our own detrending) yield strongly support the exomoon interpretation. Most of our orbital realizations, however, are very different from the best-fit solutions, suggesting that the likelihood function that best describes the data is non-Gaussian. We measure a 73.7min early arrival of Kepler-1625b for its Hubble transit at the 3 sigma level, possibly caused by a 1 day data gap near the first Kepler transit, stellar activity, or unknown systematics. The radial velocity amplitude of a possible unseen hot Jupiter causing Kepler-1625bs transit timing variation could be some 100m/s. Although we find a similar solution to the planet-moon model as previously proposed, careful consideration of its statistical evidence leads us to believe that this is not a secure exomoon detection. Unknown systematic errors in the Kepler/Hubble data make the DeltaBIC an unreliable metric for an exomoon search around Kepler-1625b, allowing for alternative interpretations of the signal.
We test the high-eccentricity tidal migration scenario for Kepler-419b, a member of the eccentric warm Jupiter class of planets whose origin is debated. Kepler-419 hosts two known planets (b,c). However, in its current configuration, planet c cannot excite the eccentricity of planet b enough to undergo high-eccentricity tidal migration. We investigate whether the presence of an undiscovered fourth body could explain the orbit of Kepler-419b. We explore the parameter space of this potential third giant planet using a suite of N-body simulations with a range of initial conditions. From the results of these simulations, coupled with observational constraints, we can rule out this mechanism for much of the parameter space of initial object d conditions. However, for a small range of parameters (masses between 0.5 and 7 $m_{rm{Jup}}$, semi-major axes between 4 and 7.5 AU, eccentricities between 0.18 and 0.35, and mutual inclinations near 0$^{circ}$) an undiscovered object d could periodically excite the eccentricity of Kepler-419b without destabilizing the system over 1 Gyr while producing currently undetectable radial velocity and transit timing variation signals.
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