In a wonderfully punchy, and technically brilliant paper the scientists Konstantin Batygin and Mike Brown of Caltech have set the world abuzz with their claim of a giant planet lurking well beyond Pluto and its Kuiper-belt kin.
In brief, the oddly clustered orbital configurations of a bunch of distant Kuiper-belt bodies might be best explained by the gravitational perturbations of a planet of at least 10 Earth masses, orbiting with a semi-major axis of some 700 astronomical units (AU), and an ellipticity of 0.6 (which would bring it as close to the Sun as about 280 AU at perihelion, during a roughly 19,000 year orbit).
It's pretty heady stuff, and although proposals have been made before for such a planet, this new analysis is perhaps the most compelling yet.
But chucking a new giant planet in our solar system's fringes raises all sorts of questions, so let me try to address some of those here.
Would it be a 'super-Earth' or a 'mini-Neptune'?
I would have to go with 'mini-Neptune' (a term that may have first been coined in this 2009 paper). From what we're learning about planet formation and the population of exoplanets, a world of this mass is very likely to be blanketed by a deep atmosphere dominated by primordial hydrogen and helium. Since Neptune is about 17 times the mass of the Earth this newly proposed world is more like an object on the low-end of ice-giant worlds rather than on the high-end of rocky planets.
What would it imply for the status of our solar system?
Until now the solar system has seemed to be somewhat unusual in terms of its planetary contents and architecture compared to the wealth of exoplanetary systems being discovered. Specifically, a majority of planetary systems around cool stars seem to contain at least one world with a mass between that of the Earth and Neptune (perhaps as many as 60% of systems). Adding in a mini-Neptune around the Sun would suddenly place us in a far more ordinary category - which is good or bad depending on your preconceptions.
Could this planet be habitable?
On the face of it you'd think not. It's far enough from the Sun that any solar heating is pitifully small. But a thick hydrogen atmosphere, with high pressure at depth, is remarkably opaque to infrared radiation and serves as a great thermal blanket. So much so that the original heat of formation would not have yet leaked out from a 10 Earth mass world. It is therefore possible that in some deep layer there are conditions suitable for liquid water to exist - making this a world with a dark ocean whose surface might be comparable in pressure and temperature to Earth's abyssal depths. Whether that could actually support life is another question. Unless the rocky core was also in contact with the ocean there might not be enough chemical feedstock for any kind of biological system to function.
What is a planet like this doing all the way out there?
Although our theories of planet formation, in the solar system or elsewhere, are far from complete or confirmed, there are ideas that could support the existence of a world like this. Conventional astrophysical wisdom tells us that unless the early solar system had an exceptionally large and rich proto-stellar disk, planets this large shouldn't form this far from their stars - there is neither material or time enough. But some of the most promising models for the necessary dynamical jostling and orbital reconfiguration of the major planets some 4.5 billion years ago include a fifth giant world. To arrive at the arrangement we see today of inner worlds and Kuiper-belt material that planet would have served as a sacrificial mass - getting scattered outwards. It would have either been ejected to interstellar space - as a 'rogue' wanderer - or cast onto a long orbit with high ellipticity. The planet proposed by Batygin & Brown might just fit the bill.
So would it be the 9th planet or something else?
Take your pick. The whole process of classifying planets should (in principle) serve to enhance our scientific understanding, but at the same time history surely plays a role. If this object had formed among the other giant worlds then it probably should be considered the 9th major planet.