Scientific American 201907

72 Scientific American, July 2019

diurnal cycle is linked to the giant impact by a fundamental law

of physics, the conservation of angular momentum. Going back

in time, the moon was closer to Earth, and to conserve angular

momentum, Earth spun faster—much faster: it would have had

a five-hour day. Other scientists had found that a grazing giant

impact by a Mars-size body could set the total angular momen-

tum of Earth and the moon. But if something else had set Earth’s

length of day, then the moon-forming event could have had

more (or less) angular momentum, opening the door for a much

wider array of possible impact scenarios. And a giant impact

with more angular momentum and more energy could, in very

rare cases, lead to an equitable mix of material from the two col-

liding bodies, potentially explaining Earth and the moon’s sta-

tus as isotopic twins.

Examining this problem in simulations of about 100 differ-

ent scenarios for a high-energy, high-angular-momentum, giant

moon-forming impact, we were confronted by seemingly non-

sensical results. Our plots of each postimpact scenario did not

show the tidy division between “planet” and “disk” that we had

expected. The postimpact planets were hot and huge, their rocky

mantles partially vaporized and puffed up to more than 100

times Earth’s present-day volume, so swollen that they became

connected to the encircling disk. The resulting objects did not

look like normal planets or disks anymore but instead something

in between. In a flash of insight, we realized that these giant

impacts were making something new. But we could not immedi-

ately understand what it was. We did not know what to call it at

the time, but we had seen our first synestia.

To understand what we were seeing, we went back to first

principles, reexamining fundamentals such as the working defi-

nition for a “planet.” A planet is defined in part by its spheroidal

shape, which comes from the body’s self-gravity being strong

enough to deform the rock as if it were a fluid. And planets

rotate around together, with only small variations arising from

any internal dynamics. We used a fluid dynamics code to calcu-

late what happens to an Earth-like planet as its rocky mantle is

slowly heated, watching as our models showed the planet swell-

ing up as its rocks began to vaporize. At the extreme tempera-

tures after a giant impact, the body resembles a gas giant—hot

enough that it lacks a true surface, just a thick rock vapor atmo-

sphere that becomes denser with depth. If such a world rotates

with a five-hour day, it maintains a roughly spheroidal shape

even as it balloons in size as the temperatures rise.

But if the planet is rotating even faster, as it heats up, some-

thing surprising happens. As the planet’s equator expands, it

reaches a point at which the equator rotates as fast as if it were in

orbit. We call this point the corotation limit. With just a little more

heat, material will then flow from the planet’s equator into orbit.

Suddenly, a fin of vapor protrudes from its equator, and the planet

becomes something else. Unlike a planet, it is no longer a simple

spheroid. Furthermore, unlike a planet, it no longer rotates cohe-

sively, instead featuring an inner corotating region and an outer

region that rotates more slowly. After some thought, we chose to

name this new celestial creature a synestia, after Hestia, the Greek

goddess of the hearth and home—because we believe Earth used

to be one of these fiery objects. (The “syn” emphasizes the synergy

that exists between all the interconnected material in the planet

and the disk.) A synestia is what a planet turns into when heat and

spin force it to exceed the limit of a spheroidal shape.

Soon we were manufacturing synestias by the hundreds in our

computer models, heating spinning planets beyond the corotation

limit. Synestias can have a wide range of shapes and sizes, depend-

ing on how mass, energy and angular momentum are distributed

throughout the body. The properties of a synestia depend on how

it was made. Gently heating a planet makes a synestia that looks

like a squat flying saucer, but giant impacts make

huge puffy synestias shaped more like doughnuts or

cream-filled pastries. Armed with a better under-

standing of how these objects arise and manifest,

we began digging through all our previous simula-

tions of giant impacts and finding synestias there,

too. It turned out that we had been making synestias

by accident for years. In fact, most scientists work-

ing on giant impacts unknowingly had synestias sit-

ting in their modeling data, just waiting to be recog-

nized as strange objects new to science.

The fact that no one had noticed them earlier had been a

matter of misplaced expectations. In the range of possible moon-

forming giant impacts, the energy and angular momentum of a

canonical Mars-size impact are too low to produce a synestia. By

focusing on the Mars-size impactor, the entire field—genera-

tions of scientists—had been misled into thinking that a planet

and disk were the standard outcome of giant impacts.

For us, the next obvious step was modeling how often synes-

tias should emerge from the complex process of planet formation.

We developed techniques to map out which impacts could trans-

form planets into synestias. By comparing these results with

models of growing planets, we have found that synestias are not

extremely rare oddballs but are actually a very common but tran-

sient feature of young planetary systems. Indeed, our simula-

tions suggest that most of the universe’s rocky planets may have

transformed into synestias one or more times during their for-

mation. We now believe that most giant impacts forming an

Earth-mass body will also make a synestia. In a flash, we had dis-

covered a missing piece in the cosmic history of planets.

BACK TO THE MOON

And yet the motivAting question remains: Could a synestia

explain our moon’s unique relation to Earth? A synestia is a very

different environment for lunar accretion than a traditional cir-

cumplanetary disk. We have found that forming the moon from

a synestia offers solutions to many of the issues that have

plagued the giant impact model for lunar origin.

A synestia’s surface temperature is set by the boiling point of

rock, which is about 2,300 kelvins (nearly 3,700 degrees Fahren-

heit) at its low-pressure outer edge. There, cooled by radiating

heat to space, rock vapor from the moon-forming synestia would

We are still learning from the

samples collected by the Apollo

missions, but they are a limited

resource with enormous gaps.