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‘Warm Jupiters’ and Nearby Worlds

Where exactly do ‘hot Jupiters’ come from? I usually see explanations involving planetary migration for Jupiter-class objects with tight orbital periods of 10 days or less, the thinking being that such planets are too close to their host stars to have accumulated a Jovian-style gaseous envelope there. Migration explains their placement, with gas giants forming much further out in their planetary systems and then migrating disruptively inward to become hot Jupiters.

Does the scenario work? Consider the hot Jupiter WASP-47b, which has two low-mass planets nearby in its system. WASP-47b is a problem because a migrating gas giant should have produced profound gravitational issues for small worlds in the inner system, likely ejecting them entirely. A new paper from Chelsea Huang and Yanqin Wu (University of Toronto), working with Amaury Triaud (University of Cambridge), tries to explain the dilemma posed by WASP-47b.

The answer turns out to be that, according to Kepler data used by the researchers, systems in which true hot Jupiters have nearby companions are extremely rare. A sample of 45 hot Jupiters (28 of them confirmed) found none with small companions in nearby orbits either closer to the star or more distant. This tends to confirm that these planets migrated to their current orbits, with expected results for the inner system. WASP-47b remains a prominent and problematic outlier.

But here we have to be careful because Huang and company make a crucial distinction between ‘hot Jupiters’ (orbital periods of ten days or less) and ‘warm Jupiters,’ whose orbital periods range from ten days to 200. The paper describes the latter category this way:

…we refer specifically to those giant planets orbiting between 10 days and 200 days in period. Unlike the hot Jupiters (inward of 10 days), they are too far out to have experienced little if any tidal circularization and therefore may be difficult to migrate inward by mechanisms that invoke high-eccentricity excitation. On the other hand, they live inward of the sharp rise of giant planets outside ∼ 1AU – in fact, the period range of warm Jupiters corresponds to the so-called ’period-valley’, the observed dip in occupation in-between the hot Jupiters and cold Jupiters…

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Image: An artist’s portrayal of a Warm Jupiter gas-giant planet in orbit around its parent star, along with smaller companion planets. Credit: Detlev Van Ravenswaay/Science Photo Library.

Warm Jupiters present an entirely different picture than their hot, inner system cousins. In fact, among the researchers’ warm Jupiter sample (27 planets, 12 confirmed), 11 are found to have nearby worlds ranging in size from Earth to Neptune. Most of these companions are inner planets, which is interesting in itself, because outer planets would be less likely to make an observable transit. Hence the data point to outer planets being as common as inner ones. Formation in place seems likely here, a clear distinction between the warm and hot Jupiters:

Motivated by this discovery, and by recent theoretical progress in understanding gas accretion, we propose that a significant fraction of warm Jupiters are formed in situ. The prevalence of multiple low-mass planets in close proximity to one another and to the star can, in a fraction of the cases, permit some of the planets to accrete enough envelope and to trigger run-away growth. This process can operate in the warm Jupiter locale, but appears to become increasingly difficult towards the hot Jupiter region, explaining the rarity of systems like WASP-47b.

Huang speculates that the number of warm Jupiters with small neighboring worlds may encompass half of all such planets, with formation in situ becoming increasingly difficult for closer-in worlds. In this analysis, then, WASP-47b simply becomes the ‘hottest representative of the warm Jupiter population.’ We wind up with hot Jupiters being the result of violent dynamical processes that effectively eliminate (by ejection) nearby inner planets, while those warm Jupiters that form in place are much more benign neighbors and, we can add, interesting places to look for possible moons with habitable conditions on the surface.

Where next with this research? The paper suggests close monitoring of confirmed warm Jupiter systems in hopes of discovering smaller companion worlds. The masses of such planets, inner or outer, could be an interesting clue to the critical mass above which runaway gas accretion occurs. We also need more information about the warm Jupiter population to find out whether there is a second formation process that distinguishes two classes of such worlds.

The paper is Huang, Wu and Triaud, “Warm Jupiters are less lonely than hot Jupiters: close neighbours,” Astrophysical Journal Vol. 825, No. 2 (2016). Abstract / preprint.

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Comments on this entry are closed.

  • Harry R Ray July 22, 2016, 9:32

    Technically, then; Wasp 47b is NOT a hot Jupiter at all, but, instead, an Extremely Warm Jupiter!!! OUCH!!!!! It can ABSOLUTELY-POSITIVELY NOT have formed by core accretion. That leaves disk instability as a NATURAL solution. CRANK UP THE SUPERCOMPUTERS! We need a scenario that explains the whole process from start to finish and leaves us with a STABLE SYSTEM! If, after exhausting ALL OTHER POSSIBILITIES, and NOT finding a solution, we may have to consider what Villoreal et al have been considering recently, completely natural IMPOSSABILITIES! If there are aliens out there who are ADVANCED ENOUGH to reveal their presence by making a star COMPLETELY DISSAPEAR(OH BY THE WAY: Any UPDATE on their ONE VIABLE CANDIDATE, anybody)they would CERTAINLY BE ADVANCED ENOUGH to CONCTRUCT an otherwise IMPOSSIBLE solar system!

  • Michael July 22, 2016, 10:03

    I think once all the numbers are in they will find that the distribution of planets will be random, large and small, as the gas and dust cloud is not a true homogenous medium but tends to be clumpy. However the detection of large planets like Jup/Sat within say an AU of the HZ would indicate there been no planets in the HZ as they have a very disruptive effect.

  • Marshall Eubanks July 22, 2016, 13:33

    Was the discrepancy in the rate of hot Jupiters for near stars, and for the Kepler survey, ever resolved?

    http://sites.psu.edu/astrowright/2012/07/05/how-many-hot-jupiters/

    • Paul Gilster July 22, 2016, 13:42

      Not to my knowledge. Jason Wright’s post, which you reference, says this (for those who haven’t been following the issue:

      “The actual frequencies of hot Jupiters around normal stars is surprisingly hard to figure out. Kepler reports a very low rate: around 0.5% of stars have hot Jupiters (many of these may be false positives, so the true Kepler rate may be only 0.3%), but the Keck planet search reported a higher number that is consistent with the other radial velocity surveys: more like 1.2%. What’s going on?

      “To check, I recently ran the numbers again for the entire Lick and Keck radial velocity surveys. I scanned our archives for every star for which we had made at least 4 measurements of its velocity. Since our precision is below 5 m/s, even with the old Lick data, we can easily detect any hot Jupiter (which typically generates a signal of hundreds of m/s, with only a couple of measurements). So we should have a complete search here.

      “We find numbers consistent with our old reported value: 1.2% of stars have detected hot Jupiters. But this is much higher than the Kepler value, and much higher than the values from transit searches by the microlensing groups, which also agree with Kepler. What’s going on?”

      Wright’s post is good reading and gives a full overview.

  • tesh July 26, 2016, 3:31

    Going by our system, it seems that the inner planets (Mars inwards) have relatively fewer moons than do the outer planets (Jupiter outwards) – where even dwarf planets have multiple moons.

    How will this impact on the notion of finding moons around warm Jupiters? Would warm Jupiters, if they had formed beyond 3-4 AU with a large collection of moons keep them if they migrated inward? If the warm Jupiters formed where they did then would they be as fecund in moon formation as their colder brethren?