One of the surprises of the early planet-hunting era has been the discovery of ‘hot Jupiters,’ giant planets orbiting extremely close to their parent star. That these planets should be prolific in our catalog at present makes sense given the nature (and limitations) of radial velocity detection methods, but before we started finding them, there seemed little reason to believe gas giants would exist at orbits within 0.1 AU. Now we see them as evidence that protoplanets can migrate during the formation period, probably causing havoc as they pass through the inner system.
Are hot Jupiters the bane of terrestrial planets? You would think so, given the above scenario, with a gas giant clearing planet-forming materials out of the inner disk during its passage. But Martyn Fogg and Richard Nelson (University of London) think otherwise. Their new paper looks at models of terrestrial planet formation and finds that inner disks survive the passage of the inbound giant, resuming their planetary formation once the hot Jupiter has closed to its new, searingly close orbit.
A sample scenario goes like this, using a simulation based on a system that has evolved for a million years before the giant planet appears and begins its migration:
The results show that the passage of the giant does not sweep the inner system clear of planet-forming material. Instead, the giant planet shepherds the solids disk inward, compacting it and exciting the orbits of objects captured at mean-motion resonances. Much of this excited material eventually experiences a close encounter with the giant planet and is expelled into an exterior orbit, augmenting a new disk of solid material that progressively builds up in orbits external to the ﬁnal position of the hot-Jupiter. In this particular case, 86% of the solids disk survives, with 82% of it residing in the external scattered disk.
If this work is correct, the passage of a wandering gas giant has a different aspect than we have assumed. The inner disk is only slightly diluted by the event. Moreover, the materials left behind are prompted to new planetary formation with volatile-rich material moving inward. And note this comment on orbital eccentricity (internal references deleted for brevity):
The results of further simulation of accretion in this scattered disk show that the initially eccentric orbits of protoplanets are rapidly damped and circularized via dynamical friction exerted by smaller bodies and possibly via tidal drag exerted by the remaining gas. Planetary growth resumes and over the following ~ 10–100 Myr gives rise to a set of water-rich terrestrial planets in stable orbits external to the hot-Jupiter.
Thus another paradigm shift: This paper suggests that all those hot Jupiter systems we’ve been more or less writing off for Earth-like planets are back in the game. These systems account for roughly one-quarter of all exoplanets thus far found, so the finding isn’t insignificant. Can Kepler, Darwin or perhaps a mission like ESA’s proposed PLATO track down a terrestrial world in such a system? The technology should be up to the task, but the first step is the realization, more than a little surprising, that we may need to add hot Jupiter systems to the target list.
The paper is Fogg and Nelson, “Can Terrestrial Planets Form in Hot-Jupiter Systems?” to appear in Extreme Solar Systems, ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan (abstract). Personally, I’m finding the interest in extreme systems fascinating, because part of the study is figuring out which systems are actually extreme, and which scenarios, however unlike our own, may be common.