The first planet to be discovered using a telescope has an oddly featureless view. Most of our limited knowledge of Uranus (and the incredible featureless picture below) comes from a single flyby mission by the Voyager 2 spacecraft in 1986.
Uranus has some surprising features that are not well understood:
#1: Uranus is tilted on its side
While most planets go around the Sun like a spinning top, Uranus goes around like a rolling ball. Uranus is titled 98 degrees on its side making the poles directly face the Sun.
Tilt of Earth compared to Uranus. Source: Wikipedia
It is suspected that a past collision with an Earth-sized planet caused Uranus to tip on its side. A more recent proposal that’s more promising is a collision-less scenario where a close encounter with a large body causes the tilt.
#2: Uranus has very little heat of its own
Uranus emits about as much heat as it absorbs from the Sun, meaning it has very little heat of its own. Other planets like Jupiter and Saturn for example, emit more heat than they receive from the Sun, as shown in the figure below:
Heat absorbed (from the Sun) vs. heat emitted by the planet. Source: Planetary Atmospheres by F. W. Taylor
Even the similar-sized Neptune has much more internal heat than Uranus does, making Uranus the coldest planet in the Solar System. As to why Uranus has so little internal heat is not known yet. It is possible that the supposed past collision with an Earth-sized planet would have expelled a lot of Uranus’ heat in the process.
#3: Uranus has unusual weather patterns
With very little internal heat, it is expected that weather patterns on Uranus are driven primarily by sunlight. Increase in atmospheric activity and storms is expected during the equinox — when the equator of Uranus faces the Sun directly. To everyone’s surprise, 7 years after the equinox in 2007, 8 massive storms were observed on Uranus. The largest of those storms was about half the size of the Earth.
Infrared images of Uranus taken on Aug 6, 2014, using adaptive optics on the 10-meter Keck II telescope. The white spot is an extremely large storm that was brighter than any feature ever recorded on the planet. Source: NASA
Such unexpected activity clearly shows that something is happening on Uranus that we don’t understand. That Uranus is more active than what our current models tell us.
#4: Uranus has a weird magnetic field
The magnetic field of Uranus does not originate from its geometric center. Moreover, unlike the Earth’s magnetic field which mostly aligns with its rotational tilt, the magnetic field of Uranus doesn’t.
The unusual magnetic field of Uranus, as determined by Voyager 2 during the 1986 flyby. Source: Wikipedia
Having the planet spin in a different direction to the magnetic field causes the magnetic field of Uranus to tumble every day, acting as a repeating ON/OFF switch for charged particles from the Sun. As to why the magnetic field is displaced off-center and titled with respect to the planet’s rotation axis is not known yet. But the fact that Neptune too has a displaced and titled magnetic field means that the answer might have to do with the general composition and internal structure of such Ice Giants.
Why we need an orbiter around Uranus?
A 5-day flyby mission can only tell you so much about a planet. Moreover, remote observations are not sufficient to help understand the various mysteries surrounding Uranus. Putting an orbiter around Uranus (like the ones around Jupiter and Saturn) will help us not just understand Uranus but also the past of our Solar System. The fruitfulness of understanding Ice Giants like Uranus/Neptune goes beyond that. Have a look at this graph showing the number of exoplanets discovered of each type:
A histogram showing the number of exoplanets discovered by each type. The blue bars represent previously verified exoplanets and the orange bars represent Kepler’s newly verified planets as of May 2016. Source: Wikipedia
The category of planets called Sub/Mini-Neptunes are the most common type as per our observations. Understanding Uranus & Neptune is thus the key to understanding how a giant fraction of all planets form and behave.
The good news is that a Uranus orbiter+probe is proposed to be launched in the 2030s.
An artist’s concept of the Voyager 2 spacecraft approaching Uranus in 1986. Source: The Planetary Society
Look here it comes to the ringed cyan-blue,
Telling us about the world anew..
On its side are worlds very few,
Oh Galaxy, its such a lovely view.
rhboomer on September 23rd, 2017 at 16:47 UTC »
We should have orbiters around every planet in the solar system by now...
Blogger32123 on September 23rd, 2017 at 16:21 UTC »
Put more money into funding space projects in general. Space funding has given us so much.
Astromike23 on September 23rd, 2017 at 16:17 UTC »
PhD in astronomy here. I wrote my dissertation about original research I've done about Uranus.
Not sure who the author is, but they should have done their research a little more thoroughly - there's some pretty bad info in this article:
This hypothesis started waning about 15 years ago when impact simulations were getting good enough to show that it's exceptionally difficult to produce an impact that's large enough to tilt Uranus but not completely obliterate the planet. It's a little more likely to do this with multiple impacts, but still not exactly easy.
The most likely scenario at this point is that Uranus had some kind of gravitational near-miss, enough to induce a tidal torque that could turn its axial tilt. There's also some evidence that this scenario would require ejecting some mass in the process, possibly a big moon. The remaining moons would eventually fall in line with the new inclination angle of Uranus' equator due to tidal forces acting over billions of years. This explanation also has the neatness that it may explain why Uranus doesn't have a big moon, which we'd expect from most formation scenarios; moreover, there are at least some formation scenarios that suggest Uranus and Neptune swapped orbits early on, providing ample opportunity for this gravitational near-miss to occur.
It's true that Uranus emits very little heat, but extremely presumptuous to assume that means it doesn't have any.
Normally heat escapes from the interior of a giant planet because there's enough of a temperature gradient between interior and exterior that convection starts up, bring bubbles of hot air to the the cloud tops. In the case of Uranus, though, we're pretty sure it has to fight against double diffusive convection. There might be a temperature gradient steep enough to enable normal convection, but there's also a density gradient that makes it much harder for convection to get started.
In essence, we think this density gradient acts as a lid on the internal heat, preventing it from escaping.
It's pretty well-known.
Unlike Earth (where the magnetic field is generated from a liquid iron core) or Jupiter & Saturn (where the magnetic field is generated from liquid metallic hydrogen), we believe that Uranus and Neptune have their magnetic fields generated by a relatively shallow superionic slushy ocean of water and ammonia.
The shallow field generation means there are higher order magnetic harmonics (quardupole and octopole) leaking out to the exterior that end up shifting the usual magnetic north and south.
That's not to say we shouldn't have an orbiter mission - we absolutely should! There are tons of mysteries we still don't understand about Uranus...but this is some pretty crappy science writing that an hour's worth of reading Wikipedia would've cleared up.