Astronomers Think They Know The Reason For Uranus's Kooky Off-Kilter Axis

an image of uranus taken using the keck observatory. The planet appears to glow blue against the darkness, with thin, gossamer rings wrapped vertically around its middle

Uranus as imaged by the Keck Observatory. (Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory)

Although it shares many similarities with our Solar System's other ice giant, Neptune, it has a bunch of quirks that are all its own.

And one of these is impossible to miss: Its rotational axis is so skewed it may as well be lying down. That's a whopping tilt of 98 degrees from the orbital plane.

And, to top it all off, it's rotating clockwise – the opposite direction from most of the other planets in the Solar System.

A new study has found a plausible explanation for this weird behavior: A moon migrating away from the planet, resulting in Uranus being pulled over onto its side. And it wouldn't even need to be a big moon. Something half the mass of our own Moon could have done it, although a larger moon would be the more likely contender.

The reasoning has been laid out in a paper led by astronomer Melaine Saillenfest of the National Centre for Scientific Research in France. This paper, not yet peer-reviewed, has been accepted into the journal Astronomy & Astrophysics and made available on preprint resource arXiv.

Scientists have come up with models to explain this weird behavior, such as a massive object that collided with Uranus and literally smacked it sideways, but the more favored explanation is a bunch of smaller objects.

However, this hypothesis raises issues that are even more difficult to explain: namely, those pesky similarities to Neptune.

The two planets have extremely similar masses, radii, rotation rates, atmosphere dynamics and compositions, and wacky magnetic fields. These similarities suggest that the two planets could have been born together, and they become much more difficult to reconcile when you throw planet-tipping impacts into the mix.

This has led scientists to seek other explanations, such as a wobble that could have been introduced by a giant ring system or a giant moon early in the Solar System's history (albeit with a different mechanism).

But then, a few years ago, Saillenfest and his colleagues found something interesting about Jupiter. Thanks to its moons, the gas giant's tilt could increase from its current slight 3 percent to around 37 percent in a few billion years, thanks to the outward migration of its moons.

Then they took a look at Saturn and found that its current tilt of 26.7 degrees could be the result of the rapid outward migration of its largest moon, Titan. This could have happened, they found, almost without having any effect on the planet's spin rate.

Obviously, that raised questions about the most tilted planet in the Solar System. So the team performed simulations of a hypothetical Uranian system to determine whether a similar mechanism could explain its peculiarities.

It's not unusual for moons to migrate. Our own Moon is currently moving away from Earth at a rate of about 4 centimeters (1.6 inches) per year. Bodies orbiting a mutual center of gravity exert a tidal force on each other that gradually causes their rotations to slow. In turn, this loosens gravity's grip so that the distance between the two bodies widens.

Turning back to Uranus, the team performed simulations with a range of parameters, including the mass of the hypothetical moon. And they found that a moon with a minimum mass of around half that of Earth's Moon could tilt Uranus towards 90 degrees if it migrated by more than 10 times the radius of Uranus at a rate higher than 6 centimeters per year.

However, a larger moon with a size comparable to Ganymede was more likely, in the simulations, to produce the tilt and spin we see in Uranus today. However, the minimum mass – about half an Earth Moon – is about four times the combined mass of the current known Uranian moons.

The work accounts for this, too. At a tilt of about 80 degrees, the moon became destabilized, triggering a chaotic phase for the spin axis that ended when the moon ultimately collided with the planet, effectively "fossilizing" Uranus' axial tilt and spin.

"This new picture for the tilting of Uranus appears quite promising to us," write 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 continue.

"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."

It's not clear whether Uranus could have hosted a moon large enough and at a high enough migration rate to produce this scenario, and it will, the researchers say, be challenging to show with observations.

However, a better understanding of the current rate of migration for Uranus's moons would go a significant way towards resolving these questions. If they are migrating at a high rate, this could mean that they formed from the debris of the ancient moon following its destruction many eons ago.

The research has been accepted into Astronomy & Astrophysics and is available on arXiv.

The rest of Uranus, chaff removed (most of it anyway), mostly sorted, but with some pertinent ones emphasized to lubricate the discussion.

Uranus is belching X-rays and is weirder than we ever thought [03/31/2021]
Jupiter Used to Be Four Times Farther from the Sun, Study Claims [03/26/2019]
Science Says: A big space crash likely made Uranus lopsided [12/21/2018]
A big space crash likely made Uranus lopsided [12/21/2018]
Study of Uranus Suggests Some of its Moons are on a Collision Course [07/04/2018]
A massive object devastated Uranus a long time ago and it never fully recovered [07/04/2018]
Did Something Massive Smash Into Uranus? [07/04/2018]
'Cataclysmic' collision shaped Uranus' evolution [07/03/2018]
How Did Uranus Form? [03/09/2018]
Why is Uranus on its Side? [08/06/2016]
The Many Mysteries of Uranus [02/02/2016]
Did Jupiter Bumped The Giant Planet From Our Solar System? [11/02/2015]
Astrophile: Mighty Trojan found marching with Uranus [08/29/2013]
Uranus Is Being Chased By Asteroids! [06/18/2013]
Migrating planets caused meteor storm [03/26/2013]
Giant planet ejected from the solar system [11/11/2011]
Something Has Exploded In a Spectacular Fashion On Uranus [10/31/2011]
Series of bumps sent Uranus into its sideways spin [10/10/2011]
Neptune may have eaten a planet and stolen its moon [04/03/2010]
Solving Solar System Quandaries Is Simple: Just Flip-flop The Position Of Uranus And Neptune [12/30/2007]
Uranus moons seen overtaking each other for first time [05/18/2007]
Death Spiral: Why Theorists Can't Make Solar Systems [03/29/2006]
Uranus remains mystery despite new discoveries [12/01/2004]
Enigma of Uranus solved at last [03/10/2004]
A Diamond the Size of Earth - is this Jupiter's core? [12/28/2018]
It rains solid diamonds on Uranus and Neptune [08/25/2017]
Uranus may have Melting and Resolidifying Diamond Oceans [01/19/2010]
Diamond Oceans Possible on Uranus, Neptune [01/17/2010]
Diamonds unlikely in gas giants like Uranus [08/06/2007]
In a Rare View, Rings on Uranus Show Their Changes [08/28/2007]
The Dark Side of the Rings of Uranus [08/28/2007]
Uranus Ring Turns Out Blue [04/10/2006]
Seventh planet has a blue ring [04/08/2006]
UC Berkeley scientists report new rings found around Uranus (a blue ring and a red ring) [04/06/2006]
The Second Ring-Moon System of Uranus: Discovery and Dynamics [02/23/2006]
Hubble discovers new rings and moons around Uranus [12/29/2005]
Hubble spots Uranus' moons, rings [12/24/2005]
Two more rings discovered around Uranus [12/22/2005]
Images of Uranus reveal ring, atmospheric fireworks [11/29/2004]
Uranus: Whacky weather, odd rings [11/13/2004][01/28/2004]
China wants to probe Uranus and Jupiter with 2 spacecraft on one rocket [09/23/2022]
What would you name a Uranus probe? The internet's answers are about what you'd think [09/13/2022]
NASA really wants to probe Uranus and could start planning next year [08/24/2022]
Uranus to begin reversing path across the night sky on Wednesday [08/24/2022]
Now We Know Why Jupiter Doesn't Have Big, Glorious Rings Like Saturn [07/25/2022]
Telescopes reveal why Neptune is more blue than Uranus [05/31/2022]
Probing Uranus Is Top Priority This Decade, U.S. Science Advisors Say [04/19/2022]
Astronomy Picture of the Day - In Motion: Uranus and Moons [11/30/2021]
WHERE COULD ALIENS BE HIDING TECHNOLOGY IN OUR SOLAR SYSTEM? [10/13/2021]
Mushballs -- Giant, Slushy Hailstones -- Stash Away Missing Ammonia at Uranus and Neptune [10/06/2021]
New neon mural lights up Uranus [09/29/2021]
SCIENTISTS INTRIGUED BY STRANGE BLOBS ON URANUS [09/20/2021]
What if Planet Nine is a baby black hole? [03/17/2021]
Are there aliens hiding around Uranus? [12/31/2020]
A Plasmoid 22,000 Times Bigger Than Earth Erupted On Uranus During The Voyager 2 Mission [12/18/2020]
Uranus may leak gas into space at least once a day [11/29/2020]
A Puzzling Uranus Reversal [10/03/2020]
Pluto Has a Surprising Similarity to The 5 Biggest Moons of Uranus [09/15/2020]
How to See Uranus Without a Telescope This Week [09/11/2020]
Experts believe Neptune and Uranus 'primarily' composed of water [08/16/2020]
Scientists probe the weird, alien water inside of Uranus [08/14/2020]
Old gas blob from Uranus found in vintage Voyager 2 data [03/27/2020]
Uranus is leaking gas [03/26/2020]
The Discovery of Uranus [03/13/2020]
Astronomers see 'warm' glow of Uranus's rings [06/20/2019]
Hubble Reveals Dynamic Atmospheres of Uranus and Neptune [05/04/2019]
Uranus Makes a Surprise Appearance [03/15/2019]
Who Discovered Uranus? [ March 13th, 1781 ] [03/13/2019]
Whoa, Uranus Looks Totally Messed Up Right Now [02/08/2019]
Voyager 2 spacecraft enters interstellar space [12/10/2018]
Uranus will be visible to everyone in the UK tonight [10/26/2018]
Uranus discovery fulfills cosmic punchline [04/24/2018]
Uranus Smells Like Rotten Eggs [04/23/2018]
We Might Have Just Discovered 2 Dark Moons Hidden Near Uranus [12/22/2017]
Want To See Uranus With The Naked Eye? Tonight Is The Night [10/19/2017]
Uranus up close: Giant ice planet set to be its closest to Earth tonight and will be visible [tr] [10/19/2017]
HUBBLE JUST SPOTTED SOMETHING MASSIVE COMING OUT OF URANUS [10/14/2017]
IS TIME TO GO BACK TO URANUS AND NEPTUNE? REVISITING ICE GIANTS OF THE SOLAR SYSTEM [07/03/2017]
NASA wants to probe Uranus in search of gas [06/17/2017]
Hubble Spots Auroras on Uranus [04/11/2017]
Any SoCal FReepers see the meteor a few minutes ago? [04/10/2017]
Uranus Smells Like Farts [03/29/2017]
How Did Uranus Get its Name? [03/20/2017]
Planetary Rings Defy Long Ages [11/02/2016]
Centaurs Keep Their Rings From Greedy Gas Giants [06/26/2016]
Astronomy Picture of the Day -- Cirrus over Paris [06/22/2016]
Planet Nine's profile fleshed out [04/09/2016]
Monstrous Cosmic Gas Cloud Set To Ignite The Milky Way [01/29/2016]
Will We Go Back to Uranus? [01/25/2016]
Earth-Smashing Space Rocks Undercounted [12/23/2015]
Why Earth is so much bigger than Mars: Rocky planets formed from 'pebbles' [10/27/2015]
The Gas (and Ice) Giant Uranus [08/27/2015]
Giant Methane Storms On Uranus [03/10/2015]
Astronomy Picture of the Day -- Solar System Portrait [02/14/2015]
Two more planets in our Solar System, say astronomers [01/20/2015]
Astronomers are Predicting at Least Two More Large Planets in the Solar System [01/15/2015]
Monster Methane Cloud Hovers Over Western U.S. [12/31/2014]
Half-Moon Makes Dramatic Pass at Uranus Tonight [12/28/2014]
Uranus might be full of surprises [11/14/2014]
Astronomy Picture of the Day -- Moons at Opposition [10/11/2014]
Stalking Uranus: A Complete Guide to the 2014 Opposition Season [09/15/2014]
Astronomy Picture of the Day -- Two Rings for Asteroid Chariklo [04/09/2014]
Asteroid Found with Rings! First-of-Its-Kind Discovery Stuns Astronomers [03/26/2014]
The Obama Legacy in Planetary Exploration [01/06/2014]
THE TOO-YOUNG RINGS OF URANUS [11/21/2012]
How to See Uranus in Telescopes This Weekend [09/30/2012]
Auroras Seen On Uranus For First Time [04/14/2012]
Uranus auroras glimpsed from Earth [04/13/2012]
Look Up! Uranus Opposes the Sun [09/28/2011]
Astronomy Picture of the Day -- Hurricane Irene Forms [08/28/2011]
Starship Fuel from the Outer System (gas mining operation on Uranus) [06/03/2011]
Project Icarus: The Gas Mines of Uranus [06/02/2011]
25 Years After Voyager 2, Here's What We Know About Uranus [02/01/2011]
Night Sky Query [01/30/2011]
Voyager celebrates 25 years since Uranus visit [01/24/2011]
Scientists plan mission to probe Uranus [01/11/2011]
Giant Stealth Planet May Explain Rain of Comets from Solar System's Edge [12/04/2010]
On the Fringe: Astronomers look to the Kuiper belt for clues to the solar system's history [01/14/2010]
Evening Lectures on Migrating Planets, Hazardous Asteroids Search [09/19/2009]
Suicidal planet seems on death spiral into star [08/27/2009]
Asteroid Belt Loaded with Former Comets [07/16/2009]
Surprise! Saturn has small moon hidden in ring [03/03/2009]
Dim but Visible: Seeking Out Uranus [10/06/2008]
Melting ice under pressure [09/26/2008]
An Unknown Planet Orbits in the Outer Solar System [08/05/2007]
Ocean Planets on the Brink of Detection [02/03/2007]
Uranus Has a Dark Spot [10/31/2006]
New insights into composition of giant planets [10/18/2006]
Hubble spots dark spot on Uranus (picture) [10/06/2006]
Hubble takes first image of solar eclipse on Uranus [09/02/2006]
We're going on a planet hunt [04/05/2006]
Hot Jupiters do not rule out alien Earths [03/31/2006]
Moongazing reveals the chaotic world of Uranus [12/23/2005]
Uranus is 'responsible' for sea quakes (Funny Title of the Day Award?) [03/31/2005]
New Images Reveal Clouds on Planet Uranus (Unusual weather = global warming?) [11/10/2004]
Uranus alert [08/30/2004]
Spotlight on Uranus (It's Getting Closer) [08/27/2004]
Probe To 'Look Inside' Asteroids [07/28/2004]
Venus clouds 'might harbour life' (Global Warming Apparently Not An Issue) [05/26/2004]
New Hubble Photos of Uranus [01/28/2004]
Regarding Uranus... [01/15/2004]
Reworked images reveal hot Venus [01/14/2004]
Planet list includes hard-to-find Uranus [01/13/2004]
TINY MOONS DISCOVERED AT URANUS [10/21/2003]
Hubble Spots Two Tiny Uranian Moons [09/26/2003]
Americans are from Mars, Europeans are from Venus [02/01/2003]
Astronomy Picture of the Day 1-15-03 [01/15/2003]
Planet Uranus Caught On VLT (Very Large Telescope) [12/31/2002]

Our own Moon is currently moving away from Earth at a rate of about 4 centimeters (1.6 inches) per year. Bodies orbiting a mutual center of gravity exert a tidal force on each other that gradually causes their rotations to slow. In turn, this loosens gravity's grip so that the distance between the two bodies widens.

Typical lack of scientific understanding by a journalist. The tidal force doesn't "loosen gravity's grip"...gravity of earth and moon are constants. The tidal effect causes the bodies to accelerate and move to higher orbits around their center of gravity, causing them to move apart.

The smaller Moon is already tidally locked with Earth...the orbit and rotation periods are the same so it always shows the same "face" towards us. But the Moon is gradually slowing the Earth's rotation, so in a few billion years our "day" will be the same as the Moon's orbit period.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.