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Tom Mazanec
2019-Mar-18, 01:46 PM
The articles describing this exomoon candidate say it has to be confirmed.
How would this be done? When can we expect confirmation?

StupendousMan
2019-Mar-19, 01:03 AM
Confirming the moon will require additional photometric measurements with very very high precision. Since Kepler is no longer working, the only telescope, I guess, which could definitely achieve the required precision is HST; after all, HST and Kepler provided the data from which the discovery was made. See

http://advances.sciencemag.org/content/4/10/eaav1784

It is possible that some ground-based telescopes could do the job, too, but it would be very difficult.

I'm not sure what priority the HST time-allocation committee would give to follow-up observations. If they judge it to be high, then confirmation _could_ be done in 2 or 3 years. If not, it might be a decade or more before some other instrument does the job.

Roger E. Moore
2019-Mar-19, 01:58 AM
This was the most recent paper on that giant exomoon, or whatever. It's a difficult road to confirm it no matter what.

https://arxiv.org/abs/1902.06018

An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625

René Heller, Kai Rodenbeck, Giovanni Bruno (Submitted on 16 Feb 2019)

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-1625b's 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.

Roger E. Moore
2019-Apr-03, 01:35 PM
Another paper pointing out that Kepler-1625's system needs a lot more observation before anything about a giant moon can be confirmed.

https://arxiv.org/abs/1901.06366

Transits of Inclined Exomoons - Hide and Seek and an Application to Kepler-1625

David V. Martin, Daniel C. Fabrycky, Benjamin T. Montet (Submitted on 18 Jan 2019 (v1), last revised 2 Apr 2019 (this version, v2))

A Neptune-sized exomoon candidate was recently announced by Teachey & Kipping, orbiting a 287 day gas giant in the Kepler-1625 system. However, the system is poorly characterized and needs more observations to be confirmed, with the next potential transit in 2019 May. In this Letter, we aid observational follow up by analyzing the transit signature of exomoons. We derive a simple analytic equation for the transit probability and use it to demonstrate how exomoons may frequently avoid transit if their orbit is larger than the stellar radius and sufficiently misaligned. The nominal orbit for the moon in Kepler-1625 has both of these characteristics, and we calculate that it may only transit roughly 40% of the time. This means that approximately six non-transits would be required to rule out the moon's existence at 95% confidence. When an exomoon's impact parameter is displaced off the star, the planet's impact parameter is displaced the other way, so larger planet transit durations are typically positively correlated with missed exomoon transits. On the other hand, strong correlations do not exist between missed exomoon transits and transit timing variations of the planet. We also show that nodal precession does not change an exomoon's transit probability and that it can break a prograde-retrograde degeneracy.

Roger E. Moore
2019-Apr-29, 03:29 PM
Another re-examination of whether Kepler 1625b has an exomoon.

https://arxiv.org/abs/1904.11896

Loose Ends for the Exomoon Candidate Host Kepler-1625b

Alex Teachey, David Kipping, Christopher J. Burke, Ruth Angus, Andrew W. Howard (Submitted on 26 Apr 2019)

The claim of an exomoon candidate in the Kepler-1625b system has generated substantial discussion regarding possible alternative explanations for the purported signal. In this work we examine in detail these possibilities. First, the effect of more flexible trend models is explored and we show that sufficiently flexible models are capable of attenuating the signal, although this is an expected byproduct of invoking such models. We also explore trend models using X and Y centroid positions and show that there is no data-driven impetus to adopt such models over temporal ones. We quantify the probability that the 500 ppm moon-like dip could be caused by a Neptune-sized transiting planet to be < 0.75%. We show that neither autocorrelation, Gaussian processes nor a Lomb-Scargle periodogram are able to recover a stellar rotation period, demonstrating that K1625 is a quiet star with periodic behavior < 200 ppm. Through injection and recovery tests, we find that the star does not exhibit a tendency to introduce false-positive dip-like features above that of pure Gaussian noise. Finally, we address a recent re-analysis by Kreidberg et al (2019) and show that the difference in conclusions is not from differing systematics models but rather the reduction itself. We show that their reduction exhibits i) slightly higher intra-orbit and post-fit residual scatter, ii) ≃ 900 ppm larger flux offset at the visit change, iii) ≃ 2 times larger Y-centroid variations, and iv) ≃ 3.5 times stronger flux-centroid correlation coefficient than the original analysis. These points could be explained by larger systematics in their reduction, potentially impacting their conclusions.