Uranus and Neptune: Two different worlds

Strange things were happening in the outer solar system when it was first born. That ice giants, Uranus and Neptune are the two outermost major planets of our Sun’s family, and they are very similar in size, mass, composition and great distance from our star. Both distant worlds are distinctly different from the quartet of small rocky inner planets – Mercury, Venus, Earth and Mars – and 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 figured out why.

“There are… glaring differences between the two planets that require explanation,” commented Dr. Christian Reinhardt in a February 2020 PlanetS Preat release. dr Reinhardt studied Uranus and Neptune with Dr. Alice Chau, Dr. Joachim Stadel and Dr. Ravit Helled, who are all together PlanetS Members who work at the University of Zurich Institute for Computational Science.

dr Stadel expressed himself in the same way PlanetS press release that one of the striking differences between the two planets is that “Uranus and its main satellites are tilted about 97 degrees into the sun’s plane and the planet is effectively retrograde with respect to the sun”.

In addition, the remote duo’s satellite systems are different. The major satellites of Uranus are in regular orbits and are tilted with the planet, suggesting they were formed from a disk similar to Earth’s moon. In contrast, Triton – Neptune’s largest moon – is very tilted and is therefore considered a captured object. Triton also shares important similarities with the distant ice dwarf planet, Pluto, suggesting the two may have been born in the same region – the Kuiper belt Located beyond Neptune’s orbit, this is the cold, dimly lit home of countless cometary nuclei, small minor planets, and other frozen bodies. Planetary scientists predict that Triton’s orbit will disintegrate in the future to the point where it will crash onto its assumed parent planet.

In addition to other differences, Uranus and Neptune may also differ in terms of heat flows and internal structure.

ice giants

In astrophysics and planetary science, the term “ice” refers to volatile chemical compounds that have freezing points above about 100K. These compounds include water, ammonia and methane with freezing points of 273 K, 195 K and 91 K, respectively. In the 1990s, scientists first came to the conclusion that Uranus and Neptune are a separate class of giant planets that differ greatly from the two other giant inhabitants of our Sun’s family, Jupiter and Saturn. The constituent compounds of the duo of ice giants were solids when they were incorporated into the two planets, mainly during their initial formation – either directly in the form of ice or encased in water ice. There is currently very little water in Uranus and Neptune in the form of ice. Instead, water is usually present as a supercritical fluid at the temperatures and pressures it contains.

The overall composition of the duo ice giants consists of only about 20% hydrogen and helium. This differs significantly from the composition of the two gas giants in our solar system. Jupiter and Saturn are both made up of more than 90% hydrogen and helium.

Modeling the formation history of the terrestrial and gas giant planets that inhabit our solar system is relatively easy. It is generally believed that the quartet of terrestrial planets formed as a result of collisions and mergers of planetesimale within the protoplanetary accretion disk. That accretion disk surrounding our newborn sun was composed of gas and dust, and the extremely fine grains of dust had a natural “stickiness”. The tiny dust particles collided with each other and merged into bodies that gradually increased in size – from the size of a pebble to the size of a boulder to the size of the moon and finally the size of a planet. Rocky and metallic planetesimale of the original solar system served as the “seeds” from which the terrestrial planets grew. Asteroids are the remaining relics of this once rich rock and metal population planetsimals this eventually became Mercury, Venus, Earth and Mars.

In contrast, the two gas giant planets in our own Solar System, as well as the extrasolar gas giants orbiting stars beyond our Sun, are thought to have evolved after the formation of solid cores weighing about 10 times the mass of Earth. Hence, the cores of gas giants like Jupiter and Saturn formed as a result of the same process that gave birth to the terrestrial planets –while accumulating heavy gaseous envelopes from the surrounding solar nebula over the course 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 could have formed as a result of gravity accretion disc instabilities.

The birth of Uranus and Neptune through a similar process of core accretion is far more complicated — and problematic. The escape speed for the small primal creature protoplanets (still forming baby planets) in about 20 Astronomical Units (AU) from the center of our own solar system would have been comparable to their relative speeds. Such bodies crossing the orbits of Jupiter or Saturn may well have been sent on hyperbolic trajectories that howled them out of our Sun’s family and into the frigid darkness of interstellar space. Alternatively, such bodies captured by the gas giant duo would likely have been accumulated on Jupiter or Saturn — or ejected into distant cometary orbits beyond Neptune. one AU corresponds to the average distance between the earth and the sun, which is about 93,000,000 miles.

Since 2004, despite the problematic modeling of their formation, many have been extraterrestrials ice giant Candidates orbiting distant stars have been observed. This suggests that they are frequent residents of our Milky Way.

Considering 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 scattered by gravity into the more distant, dark and frigid reaches of our solar family.

Two different worlds

“It is often assumed that both planets formed in a similar way,” noted Dr. Alice Chau in February 2020 PlanetS press release. This would likely explain their similar compositions, mean orbital distances from our Sun, and their related masses.

But how can their differences be explained?

Our original solar system was a “cosmic shooting gallery” where impacts from colliding objects were common – and the same is true of extraterrestrial planetary systems beyond our sun. For this reason, a catastrophic giant impact has previously been suggested as the source of the mysterious differences between Uranus and Neptune. However, previous work only examined the impact on Uranus or was limited due to severe simplifications regarding the impact calculations.

The team of planetary researchers from the University of Zurich examined a number of different collisions for the first time both Uranus and Neptune with high-resolution computer simulations. Starting with very similar pre-impact ice giants they demonstrated as an impact from a body 1-3 times the mass of Earth both Uranus and Neptune can explain the differences.

In the case of Uranus, a grazing collision would tip the planet but not affect its interior. In dramatic contrast, in Neptune’s past, a head-on collision would affect its interior but not form a disk. This is consistent with the lack of large moons in regular orbits, as seen with Neptune. Such a catastrophic crash, which churned up the deep interior of the traumatized planet, is also suggested by Neptune’s greater observed heat flux.

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

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

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

Thanks to Judith E Braffman-Miller | #Uranus #Neptune #worlds

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