Uranus and Neptune: two different worlds

Strange things happened in the outer Solar System when he was born. Tea ice Giants, Uranus and Neptune are the two outermost major planets in our Sun’s family, and in size, volume, composition, and great distance from our Star, they are very similar. 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. Ithese giants They are planets that contain elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. Although the two planets should be nearly 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 believed they had figured out why.

“There are… striking differences between the two planets that require explanation,” commented Dr. Christian Reinhardt in February 2020. pr planetsess liberation 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 of Computational Sciences.

Dr. Stadel commented in the same PlanetS press release that one of the striking differences between the two planets is that “Uranus and its primary satellites are tilted about 97 degrees in the solar plane and the planet is effectively rotating retrograde with respect to the Sun.”

Also, the distant duo’s satellite systems are different. Uranus’s main satellites are in regular orbits and inclined with respect to the planet, suggesting that they formed from a disk, similar to Earth’s Moon. By contrast, Triton, Neptune’s largest moon, is highly tilted and is therefore considered a captured object. Triton also shows important similarities with the distant ice dwarf planet, Pluto, suggesting that the two may have been born in the same region–the Kuiper belt which lies beyond the orbit of Neptune, and is the frigid, dimly lit home to myriad comet nuclei, small minor planets, and other icy bodies. Planetary scientists predict that, in the future, Triton’s orbit will decay to the point of colliding with its adoptive parent planet.

In addition to other differences, Uranus and Neptune may also differ with respect to 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 100 K. These compounds include water, ammonia, and methane, with freezing points of 273 K, 195 K, and 91 K, respectively. In the 1990s, scientists first realized that Uranus and Neptune are a different kind of giant planet, very different from the other two giant inhabitants of our Sun’s family, Jupiter and Saturn. The constituent compounds of the duo of ice giants they were solid when they were primarily incorporated into the two planets during their ancient formation, either directly as ice or encased in water ice. Currently, very little water remains on Uranus and Neptune in the form of ice. Instead, water exists primarily as a supercritical fluid at the temperatures and pressures within them.

The general composition of the duo of ice giants it’s only about 20% hydrogen and helium by mass. This differs significantly from the composition of the two gas giants in our Solar System. Jupiter and Saturn are more than 90% hydrogen and helium by mass.

Modeling the formation history of the terrestrial planets and gas giants that inhabit our Solar System is relatively straightforward. It is generally believed that the quartet of terrestrial planets came into being as a result of collisions and mergers of planetesimals within protoplanetary accretion disk. Tea accretion disk The environment around our newborn Sun was made of gas and dust, and extremely fine dust specks possessed a natural “stickiness.” The tiny dust particles collided with each other and coalesced to form bodies that gradually grew in size, from the size of a pebble to the size of a rock, the size of a moon, and finally the size of a planet. 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 abundant population of rocks and metals. planetsimals which 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 that orbit stars beyond our Sun, are thought to have evolved after the formation of solid nuclei weighing about 10 times the mass. from the earth. Therefore, the nuclei of gas giants, such as Jupiter and Saturn, formed as a result of the same process that produced the terrestrial planets…while accumulating heavy gaseous envelopes from the ambient solar nebula over the course of a few to several million years. However, there are alternative models of core formation based on pebble accumulation that have been proposed more recently. Alternatively, some of the giant exoplanets may have arisen as a result of gravitational pull. accretion disk instabilities

The birth of Uranus and Neptune through a similar process of central accumulation is much more complicated and problematic. The escape velocity for the small primordial protoplanets (small planets in formation) located 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 could well have been sent on hyperbolic trajectories that sent them howling out of the family of our Sun entirely and into the frigid darkness of interstellar space. Alternatively, such bodies, trapped by the gas giant duo, would likely have been accreted on Jupiter or Saturn, or launched into distant cometary orbits beyond Neptune. One A is equal to the average distance between the Earth and the Sun, which is about 93,000,000 miles.

Since 2004, despite problematic modeling of their formation, many extraterrestrials ice giant Candidates have been observed orbiting distant stars. This suggests that they may be ordinary inhabitants of our Milky Way.

Given the orbital challenges of protoplanets located 20 A 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 gravitationally dispersing to the more distant, darker, 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 February 2020. PlanetS press release. This would likely explain their similar compositions, the mean orbital distances from our Sun, and their cognate masses.

But how can their differences be explained?

Our primordial Solar System was a “cosmic shooting gallery,” where impacts from colliding objects were frequent, and the same is true for extraterrestrial planetary systems beyond our Sun. For this reason, a catastrophic impact was previously proposed. giant as the source of the mysterious differences between Uranus and Neptune. However, previous work only studied impacts on Uranus or was limited due to strong simplifications regarding impact calculations.

For the first time, the team of planetary scientists from the University of Zurich studied a variety of different collisions in both Uranus and Neptune using high resolution computer simulations. Starting with a very similar pre-impact ice giants showed that an impact of a body with 1-3 times the mass of the Earth in both Uranus and Neptune can explain the differences.

In the case of Uranus, a grazing collision would tilt the planet but would 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 in regular orbits like those seen in Neptune. Such a catastrophic shock, which stirred up the traumatized planet’s deep interior, is also suggested by the larger observed heat flux on Neptune.

Future NASA and European Space Agency (ESA) missions to Uranus and Neptune may provide important new 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 a formation path initially similar to that of Uranus and Neptune can result in the observed dichotomy in the properties of these fascinating outer planets,” Dr. Ravit Helled told reporters in February 2020.

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

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