Our solar system is home to any number of unexplained phenomena, including our lack of a “hot Jupiter,” the size of the moon relative to Earth, and the odd distribution of icy dwarf planets out beyond Neptune. This distribution is why some astronomers believe a so-called “Planet Nine” may exist. Out of all the oddities we know of, one of the oddest is Uranus. Instead of orbiting right-side-up like every other planet, Uranus orbits practically lying on its side. And it orbits while spinning in the opposite direction as every other planet but Venus. How did it get this way? Now, thanks to computer modeling and a lot of math, a team of astronomers may just have an answer.
It all started with Jupiter. Melaine Saillenfest is an astronomer who studies orbital dynamics in the outer solar system. A few years ago, Saillenfest and his colleagues noticed something curious in their orbital models of Jupiter and its dozens of moons. As they ran out the clock on their model, they saw Jupiter wobble on its axis. Due to gravitational forces between the gas giant and its moons, Jupiter’s spin axis tilted seemingly by itself. The planet’s axis swung from just a smidge off perpendicular to leave Jupiter canted over on its side at almost 45 degrees, within a few billion years.
Juno captured these dancing geometric storms above Jupiter’s pole. Storms like these may exist around the poles of all the gas giants. Image: NASA
Intrigued, Saillenfest and his colleagues turned their focus to Saturn. This time, instead of looking into the future, the astronomers looked into the past trying to explain Saturn’s current inclination of 26.7 degrees. They found that the rapid outward migration of Saturn’s largest moon, Titan, could explain the gas giant’s current tilt. This could have happened, the team wrote in its 2021 report, with little to no effect on the planet’s spin rate.
Taken together, these studies suggested that Uranus might behave like its gas-giant peers. However, Saturn and Titan make up a neat two-body system — three if you count the Sun. Uranus has dozens of moons (although fewer than Jupiter). To get their models of Uranus to agree with what we see today, not only did the astronomers have to account for all those moons, they had to invoke a distant, mystery satellite. Building on their prior studies, Saillenfest and colleagues found that the reason Uranus is off kilter is that it captured — and then destroyed — an entire moon.
To produce the tableau we see today, the astronomers write in their report, Uranus must have captured a satellite in an off-axis, retrograde orbit. That gravitational drag upended Uranus’ axis of rotation, and the ice giant‘s existing moons imposed other gravitational forces in multiple directions. In the resulting mayhem, Uranus and its moons ended up creating a “chaotic zone” that messed with the orbits of everything inside.
The astronomers don’t identify a specific hunk of rock as the mystery satellite, although they do suggest it may have been one of the “swarm of planetesimals” that were still condensing into our planets. But they say that a satellite with just half the mass of Earth’s moon could have done the job. Moreover, there’s a lot more small debris in space than there are rogue moons, let alone rogue planets. There’s no outward sign that a planet-sized object hit Uranus and knocked it over. That makes it unlikely that Planet 9, a hypothetical trans-Neptunian object five to eight times the mass of Earth, is a contender.
Uranus actually has rings! X-ray emissions from Chandra, in pink, are superimposed over an optical image of Uranus from the Keck telescope. Image credit: X-ray: NASA/CXO/University College London/W. Dunn et al; Optical: W.M. Keck Observatory
Uranus would have captured its mystery satellite in a retrograde orbit. Then, it did the exact opposite of a gravitational slingshot. Orbital resonance between the planet and the moon created a destructive feedback loop. That loop siphoned away energy from the planet to its satellite. In their report, the authors write, the resulting chaos eventually destroyed the satellite with tidal forces.
Once the satellite disintegrated, its debris may have crashed into Uranus. Some of the debris may also have been flung off into deep space. But once it disintegrated, it no longer had the same “organized” gravitational influence on Uranus. Consequently, the gas giant’s axial tilt stopped changing, “freezing” the planet at its current 98 degrees of axial inclination.
“This new picture for the tilting of Uranus appears quite promising to us,” wrote the researchers.
“To our knowledge, this is the first time that a single mechanism is able to both tilt Uranus and fossilize its spin axis in its final state without invoking a giant impact or other external phenomena. The bulk of our successful runs peaks at Uranus’s location, which appears as a natural outcome of the dynamics,” they continued.
“This picture also seems appealing as a generic phenomenon: Jupiter today is about to begin the tilting phase, Saturn may be halfway in, and Uranus would have completed the final stage, with the destruction of its satellite.”
Saturn has a gigantic polar storm in the shape of a perfect hexagon. Here it is, as it would appear to the human eye. Image: NASA/JPL
Humans sent the Cassini and Juno probes out into deep space to take pictures and readings, in order to learn more about our gas giants. Data from those probes allowed scientists to discover that Jupiter may have absorbed a giant, ancient impactor in the distant past. Recent studies suggest that Jupiter’s core is more of a smear. To get that way, something must have moved it. But we can’t tell without more data.
Likewise, all kidding aside, think of what we could learn from a deep space probe that visited Uranus, like Juno for Jupiter. Juno has told us so much about Jupiter’s magnetosphere. A similar probe could explain why Uranus has such a disorganized, “non-dipolar” magnetic field. It could confirm or correct this idea that Uranus orbits this way because of a captured moon. We could even confirm or put to rest the Grand Tack hypothesis. With more data, we can begin to unwind these and other mysteries from the early history of the solar system.