Ice-Giant Planets: A Hard Rain’s A-Gonna Fall

In our Solar System’s distant, dark, and mysterious kingdom of ice, there are two bewitching ice giant planets circling our Sun in secretive splendor. Uranus and Neptune are the outermost planets of our Star’s family, and both of these beautiful big worlds have managed to keep their secrets very well, successfully hiding them from the prying eyes of astronomers who are trying to uncover their many mysteries. It has long been thought that the hidden interiors of these two planets are similar, and that both contain solid cores buried beneath a surrounding dense blanket of slush composed of different ices. However, this ice-giant duo may hold in their hard hearts a sparkling secret. In August 2017, astronomers announced that in an experiment they had designed to copy the bizarre conditions buried deep within the duo of icy giant planets, they were able to observe a strange “diamond rain” for the first time, seeing how it is thought to form in the high-pressure conditions that exist beneath the heavy gaseous blankets of Uranus and Neptune.

Extremely high pressure tightly and mercilessly squeezes the hydrogen and carbon existing within the interiors of these two distant worlds. This powerful squeeze could create solid diamonds that drift slowly, further and further down, into the hearts of these two strange and secretive worlds.

The existence of this sparkling rain has long been hypothesized to form more than 5,000 miles below the surface of Uranus and Neptune. This diamond downpour is thought to form as a result of the combinations of only hydrogen and carbon that occur quite frequently. In the case of the hard hearts of these two giant planets, “ice” refers to hydrogen molecules linked to lighter elements, such as carbon, oxygen and/or nitrogen.

Ice Giants

The outer region of our Solar System is the home of a majestic quartet of giant, gaseous planets. The two largest planets, Jupiter and Saturn, are classified as gas-giants, and they are different from the somewhat smaller–but nevertheless gigantic–duo of ice-giants. The banded behemoth, Jupiter, is our Solar System’s largest planet, and it is larger than all of the eight major planets of our Solar System combined. Saturn is smaller than Jupiter, even though it is the second-largest planet of our Sun’s family. Both Jupiter and Saturn have orbits that carry them closer to the Sun than the duo of ice-giants.

Uranus and Neptune are smaller than Jupiter and Saturn, and both possess larger solid cores than the two gas-giants. This distant duo of ice-giants also have thinner gaseous envelopes than the pair of gas-giants, although their blankets of gas are still both thick and heavy. The four majestic inhabitants of the outer limits of our Sun’s family are also circled by most of the moons in our Solar System. In stark contrast, the much smaller, rocky quartet of planets that inhabit our Star’s inner kingdom are almost entirely barren of moons. Little Mercury, the smallest major planet, as well as the closest to our Sun, is moonless. Likewise, the inhospitable Earth-size ball of hell, that is the planet Venus, is also moonless. Mars has two misshapen little moons, Phobos and Deimos, that are fascinating potato-shaped small objects, that are likely refugees from the Main Asteroid Belt, between Mars and Jupiter. Many astronomers think that the two Martian moons were gravitationally snared by the Red Planet when our 4.56 billion year old Solar System was young. Out of all four planetary denizens of the warm and well-lit inner region of our Solar System, only Earth possesses a large Moon.

It is almost certain that Uranus and Neptune were not born where they are now, 19 and 30 astronomical units (AU) from our Sun, respectively. One AU is the mean distance between Earth and Sun, which is about 93,000,000 miles. The accretionary processes that created both ice-giants in our young Solar System were comparatively slow farther from our Sun, where Uranus and Neptune currently dwell. The primordial protoplanetary accretion disk made of gas, dust, and ice was much too thin in this outer region to permit gaseous planets of this immense size to be born as rapidly as in the warmer, denser regions of the disk closer to our Sun.

Indeed, astronomers have had a hard time finding an explanation for how Uranus and Neptune could have grown to become majestic giants if they had been born where they are situated today. The protoplanetary accretion disk should have dissipated before worlds of this size could form in this outer region of our Solar System. For this reason, many astronomers propose that the cores of Uranus and Neptune were born closer to our Star, but later migrated to their current, much more remote, locations.

Our primordial Solar System was not peaceful. Instead, it was a “cosmic shooting gallery” where objects both large and small, rocky and icy, continually blasted into each other, wreaking havoc. However, frequently these ancient bodies merged together when they collided, rather than breaking each other up into fragments. When these mergers occurred, increasingly larger and larger bodies formed–growing from pebble-size, to boulder-size, to mountain-size, to moon-size, to planet-size. Sometimes planet-size worlds crashed into one another. Deadly gravitational dances between migrating planets tossed some planets into other regions of our Solar System, or even out of our Solar System altogether. For this reason, it is entirely possible that the cores of both ice-giants originated where Jupiter and Saturn are today, and then migrated further and further outward until they finally went into more distant orbits because of gravitational waltzes with other bodies.

Uranus is the seventh planet from our Star, and it has the third-largest planetary radius and fourth-largest planetary mass in our Sun’s family. It is similar in composition to Neptune, and both giant planets have different chemical compositions from that of Jupiter and Saturn.

However, Uranus’s atmosphere does share some similarities with the atmospheres of Jupiter and Saturn, but it contains more “ices” such as ammonia, methane, and water, along with scanty amounts of other hydrocarbons. It is also the coldest major planet in our Solar System, with a minimum temperature of -371 degrees Fahrenheit. Uranus also sports a complicated layered cloud structure with water believed to compose the lowest clouds and methane the uppermost layer of clouds. The secretive interior of Uranus is thought to be primarily made up of ices and rock.

Like the other three giant planets of our Solar System’s outer regions, Uranus sports a system of rings, a magnetosphere, and many, many moons. The Uranium system possesses a unique configuration among its sibling planets. This is because its axis of rotation is tilted sideways, nearly into the plane of its orbit around our Sun. For this reason, its north and south poles are situated where most other planets have their equators.

Back in 1986, images obtained from Voyager 2 showed Uranus to be a featureless planet in visible light, without the intriguing bands of colorful clouds or the whirling storms associated with the three other giant planets. However, observations from Earth later revealed seasonal changes and increased weather activity as Uranus approached its equinox in 2007. Wind speeds on this greenish-blue world can reach up to 560 miles per hour.

The other ice-giant, Neptune, is the eighth and most distant known major planet from our Star. It is also the smallest of the quartet of giant gaseous planets inhabiting our Solar system’s outer limits, which makes it the fourth largest planet in our Sun’s family. All four of the solid worlds within our Star’s warm and well-lit inner regions are much smaller than the four giant worlds. Neptune is also the densest of the gaseous giants, and it is 17 times the mass of Earth–making it slightly more massive than its near-twin Uranus, which is about 15 times the mass of Earth, and slightly larger than Neptune.

Neptune circles our Sun once every 164.8 years at its average distance of 30.1 AU. Like Jupiter and Saturn, Neptune’s atmosphere is made up mostly of hydrogen and helium, along with trace quantities of hydrocarbons and possibly nitrogen. However, it contains a higher percentage of “ices”. Neptune’s hidden interior, like that of Uranus, is composed primarily of ices and rock. Small quantities of methane in the outermost regions of Neptune account for the planet’s beautiful deep blue hue.

The planetary families of distant stars beyond our Sun may also contain the same glittering secret as Uranus and Neptune. For example, the exoplanet 55 Cancri e, discovered in orbit around a nearby star inhabiting our own Milky Way Galaxy is very, very dark. This is because it is richly endowed with the element carbon. This faraway Super-Earth is in orbit around a Sun-like star, dubbed 55 Cancri, which is located about 40 light-years from our own planet in the constellation Cancer. A Super-Earth is a type of exoplanet that is unlike any of the planets in our own Solar System. These alien planets are smaller than the quartet of giant gaseous planets in our Sun’s family, but more massive than Earth, and they can be composed of rock or gas or both!

This “oddball” planetary child of an alien sun is one of a quintet of planets orbiting its star, and it zips around 55 Cancri at an almost incredible speed–circling it in a mere 18 hours. This dramatically differs from one Earth-year which is 365 days long. 55 Cancri e is searing-hot–broiling at a temperature of 3,900 degrees Fahrenheit. In October 2012, a team of astronomers announced that at least one-third of this bizarre carbon-rich world could be composed of diamond.

The detection of this carbon-rich Super-Earth suggests that distant rocky planets orbiting stars beyond our Sun can no longer be assumed to possess atmospheres, interiors, chemical constituents, or biologies similar to those on our own Earth. Indeed, 55 Cancri e may have no water at all. In fact this weird “oddball” is composed mostly of carbon–in the form of diamond and graphite. Iron, silicon, and possibly silicates may also exist on this distant world. Some planetary scientists have suggested that at least a third of 55 Cancri es mass–which is equivalent to that of three Earths–could be made of diamond.

A Hard Rain’s A-Gonna Fall

A team of scientists simulated the environment thought to exist deep within the ice-giants, Uranus and Neptune, by creating shock waves in plastic with an intense optical laser at the Matter in Extreme Conditions (MEC) instrument at the SLAC National Accelerator Laboratory’s x-ray free-electron laser, the Linac Coherent Light Source (LCLS). SLAC is one of ten U.S. Department of Energy (DOE) Office of Science laboratories. It is located in Menlo Part, California, and is operated by Stanford University.

During the course of the experiment, the scientists were able to observe that nearly every carbon atom belonging to the original plastic was captured into tiny diamond structures up to a few nanometers wide. The study authors predict that, on Uranus and Neptune, the diamonds would become ever larger and larger–perhaps growing to become millions of carats in weight. The researchers also propose it is possible that, over thousands of years, the diamonds would slowly sink through the ice-giants’ layers of ice and assemble themselves into a thick layer surrounding the core.

“Previously, researchers could only assume that the diamonds had formed. When I saw the results of this latest experiment, it was one of the best moments of my scientific career,” commented Dr. Dominik Kraus in an August 22, 2017 Lawrence Livermore National Laboratory (LLNL) Press Release. Dr. Kraus, currently at Helmholtz Zentrum Dresden-Rossendorf (Germany) is lead author of the publication describing the experiment. The idea for the experiment was born within LLNL’s NIF & Photon Science Directorate, where Dr. Kraus was stationed as a University of California, Berkeley postdoc.

Improving scientific understanding of when and how combinations of carbon and hydrogen separate under these extreme conditions is also relevant to inertial confinement fusion (ICF) experiments.

“It is important to mitigate mass density fluctuations as a conequence of species separation because they could be seeds for hydrodynamic instabilities. In current ICF explosions the first shock drives plastic at higher pressures to significantly higher temperatures compared to this recent LCLS work, to conditions where we are confident that species separation does not occur,” explained Dr. Tilo Doeppner in the August 22, 2017 LLNL Press Release. Dr. Doepper, a coauthor on the paper, is LLNL’s experimental lead for ICF implosions using a CH (plastic) ablator.

Earlier static compression experiments also turned up tattle-tale hints of carbon forming graphite or diamond at lower pressures than those created in this experiment–but with other materials introduced that altered the reactions. The combination of high-energy optical lasers at MEC with LCLS’s brilliant X-ray pulses enabled the scientists for the first time to directly measure the species separation at ultra-fast time scales and free from the impact materials that contain the sample.

The results presented in this experiment are the first unamibuous detection of high-pressure diamond creation from mixtures. These findings agree with theoretical predictions about the conditions under which such precipitation can form, and will also provide scientists with better information that they can use to describe and classify other worlds. The scientists plan to use the same methods to detect other processes that occur deep within the secretive interiors of planets.

In addition to the scientific insights they provide for planetary scientists, nanodiamonds made on Earth could possibly be used for commercial purposes–which would include scientific equipment, medicine, and electronics. Currently, nanodiamonds are commercially manufactured from explosives: laser production may offer a more easily controlled and cleaner method.

This research was supported by the Department of Energy’s Office of Science and the National Nuclear Security Administration.