The Persistent Case for Exomoon Candidate Kepler-1708 b-i

We began discovering a variety of ‘scorching Jupiters’ within the early days of planet searching just because, though their existence was not extensively predicted, they had been the most definitely planetary sorts to set off our radial velocity detection strategies. These star-hugging worlds produced a Doppler sign that readily confirmed the consequences of planet on star, whereas smaller worlds, and planets farther out of their orbits, remained undetected.

David Kipping (Columbia College) makes use of scorching Jupiters as an analogy when describing his personal indefatigable work searching exomoons. We have already got one in all these – Kepler-1625 b-i – nevertheless it stays problematic and unconfirmed. If this turned out to be the primary in a string of exomoons, we’d effectively anticipate all of the early finds to be giant moons just because utilizing transit strategies, these can be the simplest to detect.

Kepler-1625 b-i is problematic as a result of the info may very well be displaying the consequences of different planets in its system. If actual, it might be a moon far bigger than any in our system, a Neptune-sized object orbiting a fuel large. In any case, its knowledge got here not from Kepler however from Hubble’s Large Discipline Digicam 3, and with however a single transit detection. In a brand new paper in Nature Astronomy , Kipping describes it as an ‘intriguing trace’ and is fast to level out how far we nonetheless are from confirming it.

This ‘trace’ is intriguing, nevertheless it’s additionally helpful in making the case that what exomoon hunters want is an extension of the search house. Whereas the proof for Kepler-1625 b-i turned up in transit timing variations that indicated some perturbing impact on the planet, how way more helpful would such variations be if detected in a survey of fuel giants specializing in long-period, cool worlds, of which there’s a small however rising catalog? These planets are tougher to search out transiting as a result of, being a lot farther from their star, they transfer throughout its floor as seen from Earth solely in cycles measured in years. Which means, after all, that as time goes by, we’ll discover increasingly more of them.

However we now have a pattern of 73 cool worlds that Kipping and colleagues analyze, bringing their exomoon detection toolkit to bear. The tactic of their choice properties in on worlds with a radius at least half that of Jupiter, and with both a interval of greater than 400 days or an equilibrium temperature lower than 300 Okay. A closing qualifier is the quantity of stellar radiation acquired from the star. Of the preliminary 73 worlds, three needed to be rejected as a result of the info on them proved insufficient for evaluation.

So we’ve 70 fuel giants and a deep dive into their properties, on the lookout for any traces of an exomoon. The crew sought planets in near-circular orbits, realizing that eccentric orbits would decrease the soundness wanted to provide a moon, additionally on the lookout for at the very least two transits (or ideally extra), the place transit timing variations may very well be detected. The mannequin of planet plus moon wanted to face out, with the authors insisting that it’s favored over a planet-only mannequin by an element of at least 10; Kipping describes this as “the canonical commonplace of sturdy proof in mannequin choice research.”

Out of the preliminary screening standards, 11 planets emerged and had been subjected to further assessments, refitting their mild curves with different fashions to look at the robustness of the detection. Solely three planets survived these further checks, and just one emerged with a possible exomoon candidate: KIC-8681125.01. KIC stands for Kepler Enter Catalog, a designation that adjustments when a planet is confirmed, as this one subsequently has been. Thus our new exomoon candidate: Kepler 1708-b-i.

Picture: The invention of a second exomoon candidate hints on the risk that exomoons could also be as widespread as exoplanets. Simply as our Photo voltaic System is filled with moons, we will anticipate others to be, and it appears cheap that we’d detect extraordinarily giant exomoons earlier than any others. Picture credit score: Helena Valenzuela Widerström.

We all know all too little about this candidate apart from the persistence of the proof for it. Certainly, in a Cool Worlds video describing the trouble, Kipping will get throughout simply how firmly his crew tried to quash the exomoon speculation, motivated not solely by the required rigors of investigation but in addition by frustration born of years of unsuccessful looking out. But the proof wouldn’t go away. Let me quote from the paper on this:

The Bayes issue of the planet–moon mannequin in opposition to the planet solely is 11.9, formally passing our threshold of 10 (sturdy proof on the Kass and Raftery scale). Inspection of the maximum-likelihood moon match, proven in Fig. 2, reveals that the sign is pushed by an sudden lower in brightness on the shoulder of previous the primary planetary transit, in addition to a corresponding improve in brightness previous the egress of that very same occasion. The time interval between these two anomalies is roughly equal to the length of the planetary transit, which is in line with that anticipated for an exomoon . The second transit exhibits extra marginal proof for the same impact. The planet–moon mannequin is ready to effectively clarify these options, indicative of an exomoon on a reasonably compact orbit, to clarify the shut proximity of the anomalies to the principle transit…

That ‘pre-transit shoulder’ shouldn’t occur, and it turns up in all of the fashions used. It appears remarkably just like the signature of an exomoon. Right here’s the determine talked about above:

Picture: That is Determine 2 from the paper. Caption: Transit mild curves of Kepler-1708 b. The left/proper column exhibits the primary/second transit epoch, with the maximum-likelihood planet–moon mannequin overlaid in stable pink. The gray line above exhibits the contribution of the moon in isolation. Decrease panels present the residuals between the planet–moon mannequin and the info, in addition to the planet-only mannequin. BJD, barycentric Julian date; UTC, coordinated common time. Credit score: Kipping et al.

I received’t undergo the entire battery of assessments the crew used to hammer away on the exomoon speculation – all the small print can be found within the paper – however as you’d think about, starspots had been thought of and dominated out, the moon mannequin match the info higher than all options, the transit sign to noise ratio was sturdy, and the pre-transit ‘shoulder’ is compelling. Quite a few completely different algorithms for mild curve evaluation had been utilized to the modeling of this dataset. Certainly, the paper’s dialogue of strategies is itself an training in lightcurve evaluation.

Describing Kepler 1708-b-i as “a candidate we can not kill,” Kipping and crew current a moon candidate that’s 2.6 instances bigger than Earth and 12 planetary radii from its host planet, which occurs to be concerning the distance of Europa from Jupiter. The exomoon’s mass is unknown, however constrained to be lower than 37 Earth plenty. The F-class host star, round which the planet orbits each 737 days, is a few 5500 mild years from Earth.

How such a moon may type raises a number of questions:

There are a number of broad situations for moon formation: planet–planet collisions, formation of moons inside gaseous circumplanetary disks (for instance the Galilean moons) or direct seize—both by tidal dissipation or pulldown throughout the development of the planet. For a gaseous planet, the primary situation is unlikely to provide a particles disk large sufficient to type a moon this huge. The moon can also be on the excessive finish of the mass vary produced by primordial disks within the conventional core-collapse image of giant-planet formation, however is less complicated within the case the place planets type by disk instability. Such fashions additionally naturally produce moons on low-inclination orbits. Direct seize by tidal dissipation can also be attainable, though the parameter vary for seize with out merger is restricted. Pulldown seize can produce giant moons inside ~10 Jupiter radii, with a variety of inclinations relying on the timescale for planetary development.

In our personal system, after all, we see no moons at Venus or Mercury, and it’s price asking whether or not moons are uncommon for planets near their host stars. Be that as it could, the supposition right here is that if Neptune-sized moons do exist, they’ll represent the majority of our early catalog of exomoons, simply as scorching Jupiters dominated our early exoplanet finds. Certainly, it’s laborious to see how something smaller may very well be present in Kepler knowledge.

This exomoon candidate is smaller than the earlier candidate – Kepler-1625 b-i – and on a tighter orbit. Whereas each these discoveries retain their candidate standing, they trace on the risk that enormous moons like these might start turning up in JWST or PLATO knowledge. The authors name for follow-up transit photometry for each Kepler-1708 b-i and Kepler-1625 b-i, including “we will discover no grounds to reject Kepler-1708 b-i as an exomoon candidate at the moment, however urge each warning and additional observations.”

The paper is Kipping et al., “An exomoon survey of 70 cool large exoplanets and the brand new candidate Kepler-1708 b-i,” Nature Astronomy 13 January 2022 (full textual content).


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