Lonely Planets of the Cosmos

Hot Jupiters are special beasts in the exoplanetary menagerie. These giant worlds orbit their parent stars incredibly tightly, sometimes zipping around in barely a day or two, and so close that they can disturb the stellar atmosphere itself - as well as throwing themselves at the mercy of gravitational tides and scorching radiation.

They were also the very first type of exoplanets to be detected around normal, hydrogen-burning, stars like our Sun in 1995. This was both a great triumph of the ingenuity and perseverance of a few astronomers, and a great surprise. Up to this point it was almost an unwritten expectation that other planetary systems would in some way be like ours. Smaller rocky worlds would orbit closer to stars and gas and ice giants would orbit at a distance, mimicking the solar system, but also matching our relatively simple picture of how planets should form.

Hot Jupiters threw all of this for a loop. There was no way that they could have formed in-situ, suggesting immediately that some mechanism had moved them, or migrated them, into their toasty environments from an origin much further out. There are a number of possibilities for how this can happen. A massive proto-planet, more than 10 or 15 times the mass of the Earth, can set up density waves, or wakes, in the vast disks of gas and dust surrounding a proto-star. These spiral patterns of matter in turn exert a gravitational force on the planet, sometimes driving it inwards by bleeding off angular momentum. A young giant can quite rapidly burrow its way in close to a parent star this way. Another good possibility is that proto-planets form in configurations that are inherently unstable. Their gravitational pulls on each other eventually perturbing orbits to a point where planets collide, dive into the parent star, are ejected altogether to interstellar space, or are simply pushed into elliptical paths with small periastrons (closest approaches to the star) where gravitational tides collapse the orbits into the tight, nearly circular shapes that hot Jupiters inhabit.

What has remained somewhat unclear however is what any of these scenarios mean for other, smaller planets in these systems. Some computer simulations of planet formation have indicated that terrestrial-sized planets could perhaps survive the inwards migration of giant worlds in roughly 30% of cases, but other scenarios predict that to make a hot Jupiter a system must sacrifice many of its worlds.

Now a new study by Steffen et al. in the Proceedings of the National Academies uses data from the Kepler planet-finding mission to look for the signatures of smaller worlds around stars already proven to harbor giant hot Jupiters. In 63 systems the authors searched for signals of other transiting planets, or planets that might be perturbing the hot Jupiters - tweaking their orbital timing by their gravitational pulls.

The conclusion? Hot Jupiters with orbital periods of less than 3 days show no significant evidence for other planets (as small as 2/3rds to 5 times the mass of Earth) near to them. By contrast, 'warm' Jupiters on somewhat larger, though still small, orbits, and 'hot Neptunes' (lower mass giants) do show statistical evidence for neighboring worlds. This apparent isolation of hot Jupiters (although we don't yet know for sure about even smaller mass planetary neighbors) indicates a very distinct pathway to their formation - planet on planet interactions that drive these worlds into elliptical orbits that then erode down to small sizes by tides. In this scenario very few, if any, hot Jupiter systems will ever harbor Earth-sized planets anywhere in the habitable zone. The situation for hot Neptunes, or hot Earths may however be different.

So hot Jupiters seem likely to spend their hellish lifetimes isolated from planetary sisters, the price paid for a very specific type of wild youth.