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…

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.