Enter the ‘Synestia’

What happens when giant objects collide? We know the result will be catastrophic, as when we consider the possibility that the Moon was formed by a collision between the Earth and a Mars-sized object in the early days of the Solar System. But Sarah Stewart (UC-Davis) and Simon Lock (a graduate student at Harvard University) have produced a different possible outcome. Perhaps an impact between two infant planets would produce a single, disk-shaped object like a squashed doughnut, made up of vaporized rock and having no solid surface.

Call it a ‘synestia,’ a coinage invoking the Greek goddess Hestia (goddess of the hearth, family, and domestic life, although the authors evidently drew on Hestia’s mythological connections to architecture). Stewart and Lock got interested in the possibility of such structures by asking about the effects of angular momentum, which would be conserved in any collision. Thus two giant bodies smashing into each other should result in the angular momentum of each being added together. Given enough energy (and there should be plenty), the hypothesized structure should form, an indented disk much larger than either planet.

Image: The structure of a planet, a planet with a disk and a synestia, all of the same mass. Credit: Simon Lock and Sarah Stewart.

Moreover, this process should be widespread (if generally short-lived) in young, evolving planetary systems. As planet formation ends, planetary collisions should produce rapidly rotating, partially vaporized rocky objects. The researchers developed a computer code called HERCULES that allows them to calculate the physical structures of bodies in varying temperatures and rotational states. There are combinations of rotational rate and thermal energy that make it impossible for a planet to rotate like a solid body. Instead, we get an inner region with its own rotation connecting to a disk-like outer region moving at orbital velocities.

The paper on this work notes that “…the structure of post-impact bodies influences the physical processes that control accretion, core formation and internal evolution. Synestias also lead to new mechanisms for satellite formation.” Moreover, Stewart and Lock believe that rocky planets are vaporized multiple times during their formation. Thus synestias should be a common outcome in young systems. From the paper:

…there is a corotation limit for the structure of terrestrial bodies that depends on mass, compositional layering, thermal state, and AM [angular momentum]. We have named super-CoRoL [corotation limit] structures synestias. Synestias typically consist of an inner corotating region connected to an outer disk-like region. By analyzing the results of N-body simulations of planet formation, we found that high-entropy, highly vaporized post-impact states are common during terrestrial planet accretion. Given the estimated range of planetary AM during the giant impact stage, we find that many post-impact structures are likely to be synestias.

Remember that a sufficiently large impact will have produced molten or gaseous material in vast quantities, expanding in volume and responding to all that angular momentum. One outcome, given the size of the impacting objects and the energy involved, could be a disk of material surrounding the impacted planet. But the researchers believe a synestia is likely at some point, perhaps lasting as little as a hundred years. For the same amount of mass, a synestia would be much larger than a solid planet with a disk of material around it.

Could we ever hope to observe such an object given how short its lifetime is expected to be? Perhaps, and not just by a stroke of good luck. For Stewart and Lock argue that a synestia formed from larger objects like gas giants or even stars could potentially last much longer. That would make a synestia a possible observable in young extrasolar systems. With that in mind, it will be interesting to see whether the HERCULES code produced in this work will find its way into new studies of planet formation and evolution.

The paper is Lock & Stewart, “The structure of terrestrial bodies: Impact heating, corotation limits, and synestias,” Journal of Geophysical Research: Planets 122 (2017). Abstract / preprint.