Galaxy, Universe

Uranus And Neptune: Two Different Worlds

In the outer Solar System, strange stuff occurred when it was aborning first. The ice-giants, Uranus and Neptune, are the two outermost major planets of the family of our Sun and are very much alike in size, bulk, composition, and great distance from our Star. Both distant worlds are clearly different from the quartet of small rocky inner planets — Mercury, Venus, Earth, and Mars — as well as Jupiter and Saturn, the duo of gas-giant planets.

Ice giants are planets of components that are greater than hydrogen and helium, such as oxygen, iron, nitrogen, and sulfur. Though the two planets are supposed to be nearly identical twins, they are not. A team of planetary scientists from the University of Zurich in Bern, Switzerland, told the press in February 2020 they think they’ve figured out why.

“There are … startling discrepancies between the two planets that need clarification,” Dr. Christian Reinhardt commented in a PlanetS Press Release from February 2020. Dr. Reinhardt studied Uranus and Neptune with Dr. Alice Chau, Dr. Joachim Stadel and Dr. Ravit Helled, all members of PlanetS working at the Institute for Computational Science, University of Zurich.

In the same PlanetS Press Release, Dr. Stadel stated that one of the startling distinctions between the two planets is that “Uranus and its main satellites are inclined around 97 degrees towards the solar axis, so the earth is essentially moving retrograde towards the Sun.”

Additionally, the distant duo’s satellite systems are different. The main satellites of Uranus are on standard orbits and inclined to the surface, indicating that they were created from a disk, identical to Earth’s Moon. In contrast, Triton — the largest moon in Neptune — is very inclined, and therefore considered an object captured. Triton also exhibits significant parallels to the distant ice-dwarf world, Pluto, which indicates that the two may have been formed in the same region — the Kuiper belt that lies outside the orbit of Neptune, which is the frigid, dimly illuminated habitat to countless comet nuclei, tiny minor planets, and other frozen objects. Planetary scientists predict that the orbit of Triton will decline in the future to the point where it will crash into its parent-planet adopted.

In addition to other differences, Uranus and Neptune may also differ in respect to heat fluxes and internal structure.

Ice-Giants

The word “storms” applies to reactive chemical materials that have freezing points above around 100 K in astrophysics and planetary science. Such contaminants comprise carbon, nitrogen, and methane, respectively, with freezing points of 273 K, 195 K, and 91 K. Scientists first began to know back in the 1990s that Uranus and Neptune are a distinct type of giant planet, quite separate from the other two large denizens of the family of our Earth, Jupiter, and Saturn. The constituent compounds of the ice giant duo were solids when, during their ancient formation, they were primarily incorporated into the two planets-either directly in the form of ices, or enclosed in water ice. Little of the water in Uranus and Neptune currently remains in the form of ices. Instead, at temperatures and pressures inside them, water mostly exists as a supercritical fluid.

The ice giant duo ‘s overall composition is only about 20 percent mass of hydrogen and helium. This is significantly different from the composition of the two gas-giants of our Solar System. Jupiter and Saturn both have mass hydrogen and helium in excess of 90 percent.

It is relatively straightforward to model the formation history of the terrestrial and gas-giant planets which inhabit our solar system. The quartet of earth planets is generally thought to have been born as a result of collisions and planetesimal mergers inside the protoplanetary accretion disk. The accretion disk around our newborn Sun consisted of gas and dust, and the extremely fine motes of dust had a natural “stickiness.”

The tiny dust particles collided and merged into bodies that gradually grew in size — from the size of the pebble to the size of the boulder, to the size of the moon, and finally to the size of the planet. The primordial Solar System ‘s rocky and metallic planetesimals acted as the “seeds” from which the earth planets formed. Asteroids are the remaining remnants of a once-abundant rocky and metallic planetesimal species that gradually formed Mercury, Venus, Moon, and Mars.

In comparison, our own Solar System ‘s two gas-giant planets, as well as the extrasolar gas-giants that orbit stars outside our Sun, are thought to have developed after strong cores were created that weighed-in at around 10 times the Earth’s mass. Thus, as a result of the same process that produced the terrestrial planets, the gas-giant core, like Jupiter and Saturn, formed while accreting heavy gaseous envelopes from the ambient solar nebula over the passage of several to several million years. 

However, there are alternative core formation models that have been proposed more recently based on pebble accretion. Alternatively, any of the massive exoplanets could have formed as a consequence of instabilities in the gravitational accretion disk.

The birth of Uranus and Neptune is much more complicated — and problematic — through a similar process of core accretion. The velocity of escape from the center of our own Solar System for the small primordial protoplanets (still-forming baby planets) located about 20 astronomical units (AU) would have been comparable to their relative velocities. These objects that entered Jupiter or Saturn’s orbits may well have been sent on hyperbolic trajectories that propelled them out of the family of our Universe and into the frigid vacuum of interstellar space entirely.

Alternatively, such bodies would have probably been accreted into Jupiter or Saturn — or hurled into distant cometary orbits beyond Neptune, being snared by the duo of gas giants. One AU is equal to the Earth-Sun average distance which is about 93,000,000 miles.

Since 2004 many alien ice giant candidates have been observed orbiting distant stars despite the problematic modeling of their formation. This suggests they may be common denizens of our galaxy along the Milky Way.

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’s often assumed that both planets formed similarly,” Dr. Alice Chau noted in the PlanetS Press Release of February 2020. This will presumably describe their identical shapes, mean orbital distances from our Planet, and masses of their parentage.

But how can their differences be explained?

Our primordial solar system was a “cosmic shooting gallery,” where frequent occurrences were impacts from crashing objects — and the same is true of alien planetary systems beyond our Sun. For this cause, a disastrous giant effect as the root of the unexplained variations between Uranus and Neptune was previously suggested. Earlier work, however, only studied the impacts on Uranus or was limited due to strong simplifications regarding the impact calculations.

For the first time, the team of planetary scientists at the University of Zurich used high-resolution computer simulations to study a range of different collisions on both Uranus and Neptune. Starting with very similar pre-impact ice giants they demonstrated, the differences can be explained by the impact of a body with 1-3 times the mass of Earth on both Uranus and Neptune.

A grazing collision in the case of Uranus would tilt the planet but would not affect its interior. A head-on collision in Neptune ‘s past would, in dramatic contrast, affect its interior, but not form a disc. This is consistent with the absence of regular orbits of large moons, as seen at Neptune. Such a devastating collision, which churned the traumatized planet’s deep interior, is also indicated by Neptune ‘s greater measured heat flux.

Future missions to Uranus and Neptune by NASA and the European Space Agency (ESA) can create new and important constraints on these scenarios, improve our understanding of the formation of the Solar System and also provide astronomers with a better understanding of exoplanets in this particular mass range.

“We clearly show that the dichotomy observed in the properties of these fascinating outer planets can result in an initially similar formation pathway to Uranus and Neptune,” Dr. Ravit Helled commented to the press in February 2020.

This research was published under the title “Bifurcation in the history of Uranus and Neptune: the role of giant planets” in the issue of the Royal Astronomical Society’s (MNRAS) Monthly Notices 22 November 2019.

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