Comparison of the size of the Earth and Uranus. The gas giant’s bluish color is caused by the methane in its atmosphere, which absorbs the red component of the solar radiation spectrum. Credit: created by Orange-kun, Wikipedia from NASA images (free work).
At an average distance of 2.87 billion kilometers from the Sun, that is, more than 19 times farther than Earth, the seventh planet of the order – Uranus – orbits our central star. It is third in size after Jupiter and Saturn, in the minimum density ranking with the value 1.27 g / cm³ ranks second behind Saturn with less than one 0.7 g / cm³.
Uranus was discovered in a telescope by William Herschel as early as 1781, but it took two years to confirm that it was not a star, but a planet. Astronomer Johann Elert Bode made a significant contribution to this, and even suggested naming it after the Greek god of the heavens.
Due to its composition (mainly hydrogen, helium, methane, a few other hydrocarbons) and temperature (68 K on the surface, only 49 K in the atmosphere), Uranus belongs to the gaseous ice giants. It is surrounded by 13 faint rings and orbited by 27 small moons. The peculiarity of Uranus is its great inclination of the rotation axis. Let’s try to visualize this anomaly: the Earth’s rotation axis is deviated by 23.5 ° from the perpendicular to the orbital plane. In a thought experiment, he rotates our planet so that its inclination increases by a right angle, that is, from 23.5 ° to 113.5 °. The geographic poles, or the axis of rotation, will move to the current plane of the equator, the southern one will become the northern one. Due to the sun’s rays, the earth will rotate in the opposite direction to the current one, i.e. retrograde. The current east will change to the west and vice versa.
The German astronomer Johann Elert Bode (1747–1826) was instrumental in identifying a new space object discovered in 1781 by William Herschel. He discovered that it was a planet and proposed to name it Uranus Credit: Wikipedia, freelance work
Apart from Venus, only Uranus has such a backward (retrograde) rotation in the Solar System (axis tilt less than 3 °). The inclination of its axis of rotation is 97.86 °, so it points only 8 ° below the plane of the orbit. Therefore, this planetary giant appears more like a giant bluish ball rolling on its journey around the Sun. When we become aware of the periodic weather fluctuations caused by the tilt of the Earth’s rotation axis, we can better understand the extreme changes during an orbital cycle. of Uranus. It lasts 84 Earth years, so an “annual” period is 20 of our years. A very long and dark polar night and a bright polar day.
The largest moon of Uranus – Titania. The composite color was created from Voyager 2 images taken on January 24, 1986 from a distance of approximately 500,000 km. Titania has a diameter of about 1,600 km and a mass of one twentieth that of our Moon, which is also more than double the diameter. Credit: NASA / JPL, free work
The 27 known moons of Uranus are named after characters from the works of William Shakespeare and Alexander Popea. Most likely they are captured asteroids. And they may also be responsible for the interesting “lying” position of the giant planet’s axis. An interesting theory arose from the collaboration of six Italian and one American astronomers explaining the peculiar inclination of Uranus and its opposite rotation. According to prevailing ideas, probable, but not confirmed, it is the result of a gigantic or a series of collisions of various sizes with other bodies at the end of the formation of the planet. But the Italians propose another solution: the influence of the tides of the ancient migrating moons of Uranus.
A few years ago, team members noticed that Jupiter’s tilt was increasing due to gradual changes in the orbits of its moons. Mathematical calculations predict that this slope will change dramatically over the next billion years. At Saturn, they discovered similar mechanisms associated with the migration of its largest moon, Titan. Isn’t the explanation of Uranus’s great tilting axis of rotation hidden in the mutual gravitational interaction with one or some of its larger ancient satellites? Astronomers created computer simulations of the consequences of tidal forces for different sizes and speeds of hypothetical moons on the dynamics of the rotation axis and came to an interesting discovery. Gaseous tilt, given its volume light Uranus, from the original small angle relative to the perpendicular to the orbit up to 90 ° is said to lead even a single gravitationally captured planet (moon in orbit) with a minimum mass of 4 ten thousandths of the planet’s mass, if it moves for billions of years along a certain orbit gradually decreasing for a distance equal to 10 times the radius of Uranus. Converted, it should therefore be a body with a mass of at least 3.5 x10 ^ 22 kg, which is – surprisingly – comparable to our Moon (less than 7.35 × 10^ 22 kg). However, it is ten times larger than the mass of Titania, at the same time the largest of Uranus’ moons (approximately 3,526 × 10 ^ 21 kg). However, for the parent planet to rotate sufficiently in less time than the age of the Solar System itself, the drift rate of a sufficiently massive ancient natural satellite would have to be comparable to the current rate of expansion of our Moon’s orbit. It moves 3.7 cm away from the Earth every year.
The simulation showed that under such conditions, Uranus will tilt with relative ease by at least 80 °. Beyond this limit, tidal forces destabilize the satellite’s orbit and the planet’s rotation to such an extent that the movement of its rotation axis becomes more chaotic. Its inclination will eventually increase to over 90 °, so the planet’s rotation will go in a retrograde direction. This final chaotic phase ends with a collision: the impact of the moon on the planet. After that, the situation stabilizes and the great tilt of the planet is preserved.
Researchers have published the study on the arXiv prepress server and, in case of positive reviews, it will be published in the journal Astronomy & Astrophysics. Although in computer simulations they have modeled the influence of a hypothetical captured body sufficiently massive, which then orbits Uranus along a descending path influenced by tidal forces, they expect the possibility of also studying the interactions between several presumed moons, possibly existing in the future.
Video: National Geographic presents the planet Uranus
Literature: arXiv, news on phys.org
