Uranus's tilt

All planets in our solar system rotate on an axis tilted in relation to the ecliptic plane (the plane carved out by their orbit around the Sun). However, Uranus’s axis is tilted at such an extreme angle that the planet rotates while virtually lying on its side. Despite several theories, scientists do not fully understand what causes Uranus’s tilt.

Overview

Uranus was observed as early as 1690, but astronomers thought it was a star. Using a telescope he built himself, British astronomer Sir William Herschel observed Uranus over a series of nights in 1781. Herschel initially reported to the Royal Society that he had discovered a new comet. After tracking the “comet” for two years, astronomers finally agreed that Uranus was actually the seventh planet in the solar system.

In 1829, astronomers determined that the rotation of Uranus was unique. All of the planets rotate on an axis that is tilted with respect to the orbital plane of the solar system. The orbital plane is the imaginary surface on which the planets orbit and almost lies on the Sun’s equator (the plane is tilted at a 7-degree angle with respect to the Sun’s equator). Axial tilt is calculated by drawing a line perpendicular to the orbital plane. The rotational axis of the planet is compared to the perpendicular line. For example, the rotational axis of the Earth has a tilt of 23.5 degrees. Mars is tilted at 25.19 degrees, and Saturn’s axis is tilted at 26.73 degrees. Uranus, on the other hand, has an axial tilt of 97.8 degrees. Because of this, Uranus is often referred to as the “sideways planet.” Either the north or the south pole of Uranus is usually pointed toward the Sun. Uranus’s equator experiences day and night the same way as the Earth’s polar ice caps do. Because Uranus orbits the Sun once every 84 years, the poles of Uranus each experience forty-two years of sunlight, followed by forty-two years of darkness. Only around equinoxes is the Sun facing Uranus’s equator, causing “normal” Earth day-night conditions. Its last equinox occurred on December 7, 2007, and the next will not happen until the year 2049.

Due to its unusual orientation, scientists have conflicting methods for determining which pole is “north” and which is “south.” The International Astronomical Union (IAU) refers to whichever pole lies above the orbital plane as the north pole. Most scientists use this designation. Others use the right-hand rule from physics and the direction in which the planet is spinning to designate the poles north or south. This method contradicts the IAU’s determination, instead naming the pole below the orbital plane as “north.”

The only spacecraft to date that has visited Uranus is Voyager 2. Launched in 1977, the probe reached Uranus in 1986. Voyager 2 came within 81,500 kilometers of the planet. It discovered and photographed ten new satellites and nine rings orbiting Uranus. The spacecraft also helped scientists determine more precisely the axial tilt of Uranus.

There are two main competing theories to explain why Uranus is tilted on its side. No one knows who proposed the popular “collision” theory, which posits that Uranus formed and then a large Earth-sized object crashed into it with such force that it left the planet on its side. The accepted theory of planetary formation is the idea of nebular condensation, developed in the seventeenth century by French philosopher René Descartes. As a massive cloud of interstellar dust and debris condensed, it would collapse and start to spin. Planets slowly begin to form from clumps of matter joining together. The bigger the planets grew, the faster they would be able to attract more material through a process known as accretion. The debris cloud that the planets formed from is called the accretion disk, which became the orbital plane. The collision knocking Uranus on its side would have had to happen early in its formation. Possibly, an object struck the planet’s core before Uranus’s satellites had condensed from the debris cloud surrounding it. Another theory is that the impact left behind debris that later became Uranus’s satellites. However, there are several questions that remain unanswered in this scenario.

Why does Uranus have a nearly circular orbit, like the other planets? Would not a large impact have affected Uranus’s orbit? If Uranus’s satellites had formed before the collision, why were their orbits not changed? The satellites orbit Uranus’s equator, just like its ring system. Two very small captured satellites, however, have been found orbiting Uranus’s poles. The nebular theory also fails to explain other oddities of the solar system, such as why Venus has a retrograde rotation (rotates backward), why Mercury and Pluto have elliptical orbits, and why Uranus and Neptune have tilted magnetic fields.

In 1997, Argentinean scientists Adrian Brunini and Mirta Parisi published a paper giving plausible ways that Uranus became tilted. They believed that if a collision took place, it had to have been when Uranus was a more solid core surrounded by a planetary envelope. The impacting object would have hit the proto-Uranus from the opposite direction as it traveled around the Sun. The two scientists thought that studying Uranus’s satellites was the key to figuring out its odd axial tilt. They concluded that either the satellites of Uranus were created by the collision itself or no collision happened. Brunini and Parisi’s study found that Uranus’s satellite Prospero (S/1999 U3) set a number of constraints on any possible conclusion. Therefore, they believe it is possible that a new theory of solar-system formation is needed to explain the tilt of Uranus. A number of scientists seem to be shifting toward the second explanation: that Uranus formed tilted on its side and that the nebular theory for the formation of the solar system fails to explain how this could have happened. Researchers have been working on finding a simulation that solves this and other oddities of the solar system.

In 2006, Brunini published a new theory of the formation of the solar system in Nature magazine. His mathematical model is based on the idea that Jupiter and Saturn once had a 1:2 orbital resonance. This means that in the time it took Saturn to orbit the Sun once, Jupiter went around twice. The gravitational effect of Jupiter and Saturn gradually changed the orbits of Uranus and Neptune. Brunini’s simulation shows that his model would take about a million years for the outer planets to reach the orbital positions we now observe. He argues that during the close encounter of Saturn and Uranus, the angular momentum of the planets shifted, which, over time, caused their axial tilts to change. This scenario, Brunini argues, can explain the orbits of Uranus’s rings and satellites, which would have slowly changed their orientation along with Uranus. Unlike a collision, Brunini’s scenario would have taken hundreds of thousands of years to play out.

No definitive answer has been found for what caused Uranus’s unique tilt. Only Voyager 2 has visited Uranus; new spacecraft would be able to provide more data but cannot be sent until either the planets are again aligned for a “slingshot” (gravity-assist) approach (more than a century away) or the necessary nuclear propulsion systems are developed. Until then, researchers are left making mathematical and computer models in their efforts to solve the mysteries of Uranus’s axial tilt. In 2022, NASA announced plans to make its Uranus Orbiter Probe (UOP) a priority within the space program. This probe would not only orbit Uranus but also dive into its atmosphere, producing valuable information about the planet and possibly shed insight into the reasons for the planet’s tilt. The UOP is slated to launch in the mid-twenty-first century.

Methods of Study

Uranus can be observed from Earth with telescopes, and on dark, clear nights can be viewed with the unaided eye. Scientists have also taken photographs of the Uranus system using the Hubble Space Telescope. In late 2002, astronomers in Chile were able to image Uranus, its rings, and some of its satellites. The pictures were taken with the Very Large Telescope (VLT) at the European Southern Observatory (ESO) Paranal Observatory. The rings that are normally unable to be viewed from Earth, along with seven satellites, appeared in the image because it was taken at near-infrared wavelengths. Launched in 2021, the James T. Webb Space Telescope has imaged incredibly clear pictures of Uranus’s rings, storm clouds, and polar caps, providing increasing data about the workings of the planet.

The Voyager program is the only spacecraft that has visited Uranus. Launched in 1977, Voyager 2 came within 81,500 kilometers of Uranus on January 24, 1986. Voyager 2 was equipped with more than a dozen scientific instruments, including cameras, television cameras, magnetometer, and spectroscopes. Voyager 2 viewed Uranus’s “south” pole (located south of the orbital plane), which was pointed toward the Sun. At Uranus, Voyager 2 discovered ten satellites and two rings. The spacecraft also studied the planet’s five largest moons (Oberon, Umbriel, Titania, Ariel, and Miranda), taking the first close-up photographs of them. Voyager 2 provided the first close-up photographs of Uranus and detailed information about its magnetic field, ring system, weather, and unusual axial tilt. By the early nineteenth century, scientists knew that the planet was tilted, but it was not until Voyager 2 arrived at Uranus that astronomers knew precisely how tilted it was.

Computer modeling can be used to explore the dynamics of complex systems over time, where geologic time is essentially replaced by computation time. Modern computers allow a tremendous amount of computational power, and the magnitude of that computational power is continually increasing. Basically, a computer modeling effort such as that used by Brunini and similar researchers seeks to begin with certain basic assumptions about the initial conditions of a complex system such as Uranus in its interaction with larger bodies such as Jupiter and Saturn and then introduce the gravitational interactions between all of these bodies and allow the computational cycle to mimic the passage of time as each of these bodies orbits the Sun and continues to interact with the others. This sort of thing cannot be easily done by hand. Sir Isaac Newton, in presenting his development of mechanics in Principia Mathematica (1687), provided a means of quantifying the gravitational interaction between two bodies. That relatively simple problem can be solved in closed form in both spatial and temporal coordinates. However, the three-body problem requires numerical analysis, which is largely done at present by computer programming or software packages since it cannot be solved in closed form. The more bodies are involved in a calculation, the more computing power is required.

Context

Scientists may never know the real reason for Uranus’s axial tilt. Further study of the planet Uranus by spacecraft, and even possibly by humans, could lead to the answer. Computer simulations and mathematical models can help scientists speculate what might have happened. Maybe the accepted nebular theory for how the solar system formed is incorrect. Maybe the gravitational effects of Jupiter and Saturn slowly caused Uranus to lean to its side. Maybe Adrian Brunini and his colleagues are correct, and the only way to make this determination is to study Uranus’s satellites. In 2022, using computer models, scientists introduced a new theory behind Uranus’s tilt, which implied that a massive moon fell into the planet’s orbit, causing a permanent gravitational disruption. The quest to explain Uranus’s extreme axial tilt could lead to a new view of how Earth and the solar system formed.

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