Skip to Content
Uncategorized

Collision-Free Theory Explains Why Uranus Is Lying on Its Side

Astronomers have always assumed that Uranus must have been knocked onto its side by a collision. Now a new idea suggests that the planet’s remarkable tilt could have another explanation.

One of the great mysteries of our Solar System is why Uranus is tilted on its side. Surely, if the solar system formed from the same rotating cloud of dust and gas, then all the bodies within it should rotate in the same way. And yet Uranus’ axis of rotation lies at 97 degrees to the plane of the solar system.

The standard explanation is that Uranus must have been involved in some kind of interplanetary collision with and earth-sized protoplanet in the early days of the solar system. That’s a tempting idea but it has some shortcomings. For example, it doesn’t explain why the orbits of the moons of Uranus are similarly tilted, not that of its rings.

Today, Gwenael Boue and Jacques Laskar at the Observatoire de Paris in France put forward another idea. They say that Uranus may have become tilted during the period soon after formation when the planets were migrating to the orbits we see now. They point out that the presence of satellites around a planet can increase its rate of precession, if it has a high initial inclination of more than say 17 degrees. This increase can be by as much as a factor of 1000 if the mass of the moon and the radius of its orbit have certain values. For Uranus, this is for a moon of 0.01 Uranian mass and at 50 Uranian radii.

The problem, of course, is that Uranus does not have such a moon. Its most distant companion is Oberon with a mass of just 10^-5 Uranian masses and an orbit of 23 Uranian radii.

Boue and Laskar’s idea is that Uranus once had a moon of the required size and orbit, which caused the planet to tilt during the planetary migration, but that this moon was ejected during a close encounter towards the end of the migration.

To study whether this idea is feasible, they simulated the process of giant planet migration in the early solar system some 10,000 times. They then discarded all scenarios in which the planets collided or did not end up in the correct final order. They then selected only those outcomes in which Uranus had an inclination of more than 17 degrees and also rejected any simulation in which Uranus came within 50 Uranian radii of another planet, since that would be likely to eject Oberon as well as the additional hypothesised moon. That left 17 simulations.

Boue and Laskar then added the additional moon to see how it would effect the tilt of Uranus and repeated each of these 17 scenarios a further 100 times. In 37 cases, the new moon helped Uranus onto its side and then ended up being ejected after a close encounter with another gas giant.

That’s an interesting result and not just because of the tilt: some models of planet formation predict that Uranus ought to have had another moon (albeit somewhat smaller than the one Boue and Laskar introduce). Consequently, this idea has the elegant property of explaining two mysteries for the price of one, never a bad thing in science.

Ref: http://arxiv.org/abs/0912.0181: A Collissionless Scenario For Uranus Tilting

Keep Reading

Most Popular

Workers disinfect the street outside Shijiazhuang Railway Station
Workers disinfect the street outside Shijiazhuang Railway Station

Why China is still obsessed with disinfecting everything

Most public health bodies dealing with covid have long since moved on from the idea of surface transmission. China’s didn’t—and that helps it control the narrative about the disease’s origins and danger.

individual aging affects covid outcomes concept
individual aging affects covid outcomes concept

Anti-aging drugs are being tested as a way to treat covid

Drugs that rejuvenate our immune systems and make us biologically younger could help protect us from the disease’s worst effects.

Europe's AI Act concept
Europe's AI Act concept

A quick guide to the most important AI law you’ve never heard of

The European Union is planning new legislation aimed at curbing the worst harms associated with artificial intelligence.

Stay connected

Illustration by Rose WongIllustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at customer-service@technologyreview.com with a list of newsletters you’d like to receive.