Weird things happened in the outer Solar System when it was first a’borning. The ice-giants, Uranus and Neptune, are the two outermost major planets of our Sun’s family, and in size, bulk, composition, and great distance from our Star, they are very much alike. Both distant worlds are clearly different from the quartet of small rocky inner planets–Mercury, Venus, Earth, and Mars–as well as from the duo of gas-giant planets, Jupiter and Saturn. Ice giants are planets that contain elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. Although the two planets should be almost identical twins, they are not. In February 2020, a team of planetary scientists from the University of Zurich in Bern, Switzerland, told the press that they believe they have discovered why.
“There are… striking differences between the two planets that require explanation,” commented Dr. Christian Reinhardt in a February 2020 PlanetS Press Release. Dr. Reinhardt studied Uranus and Neptune with Dr. Alice Chau, Dr. Joachim Stadel and Dr. Ravit Helled, who are all PlanetS members working at the University of Zurich, Institute for Computational Science.
Dr. Stadel commented in the same PlanetS Press Release that one of the striking differences between the two planets is that “Uranus and its major satellites are tilted about 97 degrees into the solar plane and the planet effectively rotates retrograde with respect to the Sun”.
In addition, the satellite systems of the distant duo are different. Uranus’s major satellites are on regular orbits and tilted with the planet, which suggests that they formed from a disk, similar to Earth’s Moon. In contrast, Triton–Neptune’s largest moon–is very inclined, and is therefore considered to be a captured object. Triton also displays important similarities to the distant ice-dwarf planet, Pluto, which suggests that the two may have been born in the same region–the Kuiper belt that is situated beyond Neptune’s orbit, and is the frigid, dimly lit home of myriad comet nuclei, small minor planets, and other frozen bodies. Planetary scientists predict that in the future Triton’s orbit will decay to the point that it will crash down into its adopted parent-planet.
In addition to other differences, Uranus and Neptune may also differ in respect to heat fluxes and internal structure.
Ice-Giants
In astrophysics and planetary science the term “ices” refers to volatile chemical compounds that possess freezing points above around 100 K. These compounds include water, ammonia, and methane, with freezing points of 273 K, 195 K, and 91 K, respectively. Back in the 1990s, scientists first came to the realization that Uranus and Neptune are a distinct class of giant planet, very different from the two other giant denizens of our Sun’s family, Jupiter and Saturn. The constituent compounds of the duo of ice giants were solids when they were primarily incorporated into the two planets during their ancient formation–either directly in the form of ices or encased in water ice. Currently, very little of the water in Uranus and Neptune remains in the form of ices. Instead, water mostly exists as supercritical fluid at the temperatures and pressures within them.
The overall composition of the duo of ice giants is only about 20% hydrogen and helium in mass. This differs significantly from the composition of our Solar System’s two gas-giants. Jupiter and Saturn are both more than 90% hydrogen and helium in mass.
Modelling the formation history of the terrestrial and gas-giant planets inhabiting our Solar System is relatively straightforward. The quartet of terrestrial planets are generally thought to have been born as the result of collisions and mergers of planetesimals within the protoplanetary accretion disk. The accretion disk surrounding our newborn Sun was composed of gas and dust, and the extremely fine dust motes possessed a natural “stickiness”. The tiny particles of dust collided into one another and merged to form bodies that gradually grew in size–from pebble size, to boulder size, to moon size, and ultimately to planet size. The rocky and metallic planetesimals of the primordial Solar System served as the “seeds” from which the terrestrial planets grew. Asteroids are the lingering relics of this once-abundant population of rocky and metallic planetsimals that ultimately became Mercury, Venus, Earth, and Mars.
In contrast, the two gas-giant planets of our own Solar System, as well as the extrasolar gas-giants that circle stars beyond our Sun, are believed to have evolved after the formation of solid cores that weighed-in at about 10 times the mass of Earth. Therefore, the cores of gas-giants, like Jupiter and Saturn, formed as a result of the same process that produced the terrestrial planets–while accreting heavy gaseous envelopes from the ambient solar nebula over the passage of a few to several million years. However, there are alternative models of core formation based on pebble accretion that have been proposed more recently. Alternatively, some of the giant exoplanets may have emerged as the result of gravitational accretion disk instabilities.
The birth of Uranus and Neptune via a similar process of core accretion is much more complicated–and problematic. The escape velocity for the small primordial protoplanets (still-forming baby planets) situated about 20 astronomical units (AU) from the center of our own Solar System would have been comparable to their relative velocities. Such bodies crossing the orbits of Jupiter or Saturn could well have been sent on hyperbolic trajectories that shot them howling out of our Sun’s family altogether, and into the frigid darkness of interstellar space. Alternatively, such bodies, being snared by the duo of gas giants, would likely have been accreted into Jupiter or Saturn–or hurled into distant cometary orbits beyond Neptune. One AU is equal to the average distance between Earth and Sun, which is about 93,000,000 miles.
Since 2004, despite the problematic modelling of their formation, many alien ice giant candidates have been observed orbiting distant stars. This suggests that they may be common denizens of our Milky Way Galaxy.
Taking into account the orbital challenges of protoplanets situated 20 AU or more from the center of our Solar System, it is likely that Uranus and Neptune were born between the orbits of Jupiter and Saturn, before being gravitationally scattered into the more distant, dark, and frigid domains of our Sun’s family.
Two Different Worlds
“It is often assumed that both planets formed in a similar way,” Dr. Alice Chau noted in the February 2020 PlanetS Press Release. This woould likely explain their similar compositions, mean orbital distances from our Sun, and their kindred masses.
But how can their differences be explained?
Our primordial Solar System was a “cosmic shooting gallery”, where impacts from crashing objects were frequent occurrences–and the same is true for alien planetary systems beyond our Sun. For this reason, a catastrophic giant impact was previously proposed as the source of the mysterious differences between Uranus and Neptune. However, earlier work only studied the impacts on Uranus or was limited because of strong simplifications in respect to the impact calculations.
For the first time, the team of planetary scientists at the University of Zurich studied a range of different collisions on both Uranus and Neptune using high resolution computer simulations. Starting with very similar pre-impact ice giants they demonstrated than an impact of a body with 1-3 times the mass of Earth on both Uranus and Neptune can explain the differences.
In the case of Uranus, a grazing collision would tilt the planet but not affect its interior. In dramatic contrast, a head-on collision in Neptune’s past, would affect its interior, but not form a disk. This is consistent with the absence of large moons on regular orbits as seen at Neptune. Such a catastrophic crash, that churned the deep interior of the traumatized planet, is also suggested by the larger observed heat flux of Neptune..
Future NASA and European Space Agency (ESA) missions to Uranus and Neptune can provide new and important constraints on these scenarios, improve our understanding of Solar System formation, and also provide astronomers with a better understanding of exoplanets in this particular mass range.
“We clearly show that an initially similar formation pathway to Uranus and Neptune can result in the dichotomy observed in the properties of these fascinating outer planets,” Dr. Ravit Helled commented to the press in February 2020.
This research was published in the November 22, 2019 issue of the Monthly Notices of the Royal Astronomical Society (MNRAS) under the title “Bifurcation in the history of Uranus and Neptune: the role of giant planets.”