This week, we’re on to the next planet in the solar system. We don’t know a whole lot about this blue gas planet, but today we’ll cover some of the neat stuff we do know, including it’s faint rings, sideways axis of rotation and its rocky core – a first in the gas planets we’ve encountered so far in our tour.
- NASA’s Uranus Factsheet – just the numbers
- Uranus’ Rings Fact Sheet
- Uranian Satellite Fact Sheet
- Uranus – Statistics, facts, links and images from nineplanets.org
- Voyager Science Summary: Uranus
- The Uranian Ring System – lots of facts, images and animations
- Welcome to the Planets: Uranus
- Space Topics: Uranus Facts and Pictures
- Uranus: Magnetic Field and Magnetosphere
Remember the survey we did a couple of months back? The results are in and have been published in the new journal, Communicating Astronomy with the Public. Our article, Astronomy Cast: Evaluation of a podcast audienceâ€™s content needs and listening habits was published in the first issue.
Fraser Cain: This week, weâ€™re not going to need the pronunciation guides in the same way, but one of the cool things is the planet weâ€™re dealing with has many different pronunciations. What are the different ways we can say it, Pamela?
Dr. Pamela Gay: The safest way, the way thatâ€™s least likely to get us made fun of by any small schoolchildren is to say Uranus, but then thereâ€™s also the famous ur-ANUS way of saying it.
Pamela: Right â€“ get the giggle out. Then thereâ€™s also URAN-us, as in â€œto pee on usâ€?.
Pamela: So, all three are out there, two are embarrassing, and when in doubt donâ€™t emphasise anything and just say â€œUranusâ€? and run quickly.
Fraser: I think there was a classic segment on the Tonight Show where Carl Sagan admitted to Johnny Carson that with Voyager reaching Uranus, that NASA wanted to sell up the â€œUranusâ€? pronunciation, but theyâ€™re all fine.
Pamela: If you really want to be formally educated about it, from the Latin itâ€™s Uranusâ€¦ but over a couple of hundred years, ur-ANUS has become perfectly acceptable. So yeah â€“ go with it. Itâ€™s all good.
Fraser: All right, well now that weâ€™ve got the pronunciation out of the way, letâ€™s talk about the planet itself. Give us the intro!
Pamela: Itâ€™s a little bit different than the previous two gas giants we talked about. It actually has a rocky core thatâ€™s surrounded by a couple of layers of ice, and its discoveryâ€™s also a bit different. It is bright enough that you can just barely make it out with your naked eye. Itâ€™s this pale blue dot at the very edge of human vision.
Even though it had been seen many times, no one actually recognized that it was a planet until 1781. It was William Herschel who finally figured it out â€“ and he thought it was a comet initially. He was going through, plotting the skies, and came across this slightly fuzzier than everything else pale blue dot, and announced that he had found a comet.
As people watched it and looked at it and made calculations about its orbit, they realised this was something that was way faraway, that was moving on a basically circular orbit, and eventually came to realise this was actually a planet.
Fraser: So with nice, dark skies, people a long time ago might have spotted it and known that it was a planet and there would have been another planet in the sky that people were well aware of.
Pamela: the thing is we actually have recorded measurements from much earlier. There were measurements in 1690 by John Flamstead, and there was Pierre Lamonier who saw it in 1750 and 1769 and they made records of this star. The star was 34 Tauri, they mapped it, they named it, they called it a starâ€¦ and it wasnâ€™t a star.
It moves very slowly on the sky. It has an 84-year period around the Sun, which means that in the course of one year, itâ€™ll move a little bit more than four degrees across the sky. Thatâ€™s about eight times the width of the moon, but from one night to the next thatâ€™s very little motion. Your casual person just chewing through the sky isnâ€™t going to notice that small amount of motion.
Fraser: But just to be clear, if you went to really, really dark skies, and knew where to look, you could just barely see it with the unaided eye â€“ and with binoculars, no problem.
Pamela: Yeah. Itâ€™s just right at the edge of vision. Itâ€™s right around 6 magnitude, which for your standard every day person (there are some insane freaks out there like Steve Oâ€™Mira whoâ€™s a great person but whose eyes are supernatural â€“ he can see things that normal people canâ€™t see), this planet is on the edge of normal-person vision, in dark sites. Itâ€™s really quite cool. It took a long time for people to figure out that the slow moving object wasnâ€™t just a star, it was actually another planet.
Fraser: Letâ€™s go back to the construction. You said it has a solid core â€“ how do they know that?
Pamela: When we try and sort out how planets are structured, we look at a lot of different things, like how their moons go around them. It basically boils down to figuring out what the mass of the object is, how big the object is in terms of diameter, and then you start figuring out (with mathematical models) how it is that you could get that amount of mass in that size of an area. If you do very careful measurements, you can actually find the moment of inertia. I donâ€™t know if weâ€™ve actually done that with Uranus, but we can figure out the moment of inertia of an object by how that mass is distributed as a function of radius.
So it basically boils down to looking at it, figuring out what its mass is, and then you do a lot of complicated mathematical modelling to fit that amount of mass in that size of a planet.
Fraser: Right, and you can see the outer layers so you know that it should be, say, mostly hydrogen and helium, but since those arenâ€™t very dense, something much more dense has got to be going on inside.
Pamela: This is where we get to rocky core surrounded by layers of ices: water ices, carbon dioxide icesâ€¦ itâ€™s really something completely different than Saturn and Jupiter. We call these the ice giants because, just like Neptune, itâ€™s this blue colour that comes from having methane in its atmosphere and itâ€™s much colder. Itâ€™s further away from the Sun, and because itâ€™s actually lower mass than Neptune, itâ€™s even colder than Neptune â€“ it just hasnâ€™t held onto its heat, and it doesnâ€™t have as much heating from compressing down under its own mass.
So we have this planet that is the fourth largest in terms of mass, and third largest in terms of radius because it just hasnâ€™t compressed down very much. Itâ€™s quite a cold planet, and it has layers and layers of ice beneath its cloudy surface.
Fraser: So does it qualify as a failed star as well, or are we just way too small now?
Pamela: Weâ€™re getting way too small now. Itâ€™s something you start talking about in terms of size compared to earth. Itâ€™s four earth radiuses in size. Weâ€™re not comparing it any longer to Jupiter: itâ€™s 14.5 Earths in terms of its mass. Here weâ€™re starting to get to things that arenâ€™t even full gas giants. Itâ€™s an ice giant instead.
Fraser: All right. Now, itâ€™s got a few bizarre things about it. First thing is it rotates on its side.
Pamela: Uranus, actually, at different points in its orbit, looks like a target. We see it and thereâ€™s this big blue part in the centre. If we look carefully in the right wavelengths, we can see its rings wrapping around it saying, â€œHey, shoot here with your telescopeâ€?.
At other points, such as last year, we see it exactly edge on. When itâ€™s edge on, itâ€™s at its equinox in its orbit â€“ itâ€™s at the first day of spring or fall, depending on which hemisphere youâ€™re looking at. Here on earth, at winter solstice, the north pole and everything north of the arctic circle doesnâ€™t get any sunlight. On Uranus, pretty much everything north of the equator doesnâ€™t get much sunlight. Itâ€™s tilted completely on its side, and this does some really weird things that weâ€™re still trying to figure out.
Weâ€™ve only had the ability to get high-resolution images of it for a few tens of years: not even its full orbit. What weâ€™re starting to see as itâ€™s getting toward its equinox, is clouds that we never saw before. Dark storms are starting to crop up on its polar areas.
Weâ€™re also seeing neat things like the pole of the planet thatâ€™s facing the Sun is typically much brighter in colour, much whiter in colour. The pole thatâ€™s away from the sun is much darker, much greyer in colour. It looks like that colouration, that pattern, isnâ€™t associated geographically with the north pole or the south pole, but rather with just facing the Sun.
As Uranus moves out of its equinox, weâ€™re actually going to see the colours of the poles change. Weâ€™re going to see one pole go from bright to dark, and the other go from dark to bright. This is already started. The pole that weâ€™ve been staring at for the past tens of years now has what appears to be a white collar around it as it starts to darken. Thereâ€™s this white ring of clouds around that darkening pole, and we start to see cloud formations cropping up around the other pole.
Itâ€™s really quite fascinating to see this seasonal change for the very first time in modern astronomy.
Fraser: Itâ€™s quite a surprise. Most of the gas giants seem to be having these kinds of seasonal changes. Youâ€™d think itâ€™s just a great big ball of gas at really frigid temperatures and not much is going to change it. But in fact, all the planets weâ€™ve talked about already do change quite dramatically, depending on which hemisphereâ€™s facing the Sun, and what part of the orbit theyâ€™re on. So I guess itâ€™s quite amazing to watch this because, with Uranus weâ€™ve never seen this before.
Pamela: This is actually one of the worlds that we know the absolute least about. It has only had one space mission fly anywhere near it, and that was Voyager 2 back in 1986. For the general public, that mission is actually almost completely forgotten. When I looked at the date, I was trying to figure out why I didnâ€™t remember this. I remember several of the other Voyager flybys; theyâ€™re some of my earliest memories.
The Voyager 2 flyby of Uranus happened just 4 days before the Challenger disaster. So in most peopleâ€™s minds, itâ€™s been completely forgotten, whereas the memory from that same week of the Challenger stands out firmly. Poor NASA couldâ€™ve had all this huge, wonderful, â€œhere are all of our images of this bright blue planet, with these very beautiful, very thin ringsâ€? but instead they were trying to recover from one of NASAâ€™s greatest PR and human disasters.
And we havenâ€™t sent anything back to it. We donâ€™t even actually have any plans to send anything to it anytime in the known future, so we basically have this blue world that weâ€™re trying to study from way here on the planet Earth using the Hubble Space Telescope.
Fraser: Letâ€™s talk about the rings then. How do these compare to Saturn and Jupiterâ€™s rings?
Pamela: Theyâ€™re much darker. Theyâ€™re more like Jupiterâ€™s rings than Saturnâ€™s rings. Theyâ€™re much thinner in terms of you have these narrow, isolated rings. Uranusâ€™ rings are just tens of meters in diameter, theyâ€™re quite thin, and theyâ€™re mad up of dark particles. They were actually discovered not by seeing them directly, but because they caused a star in the background to eclipse.
Back in 1977, a group of scientists were trying to study Uranus by watching how it passed in front of a background star, causing that star to eclipse. They watched this and they got a lot of data going into eclipse of this background star. We call these occultations.
As they were reducing their data, they noticed it wasnâ€™t just a case of watching the star flicker out as it went behind the planet, but as the star moved away from the planet, it then went out of view five more times, and each of these times the star disappeared it was passing behind a ring. That was how we first found these rings. Theyâ€™re very crisp. You look at pictures of them and you can see the star field behind them â€“ theyâ€™re just these narrow rings that are very separated, very clean and scientists think theyâ€™re actually probably quite young. These are very different from what weâ€™re used to looking at with Saturn.
Fraser: Do they have ring-tending moons in the same way as Saturn? Are there little moonlets there keeping the rings all nice and organized?
Pamela: There are moons embedded in the rings. We unfortunately donâ€™t have quite as good a sense of just how small they get, but at this point weâ€™ve found twenty-some-odd different moons tending the rings. Some of them are actually starting to creep up in size, but none of them are as big as the ones we saw around Saturn and Jupiter: we donâ€™t have ay Ganymede or Titan hanging out being planet-sized. Here weâ€™re starting to get moons that are more moon-sized â€“ more the size of our own moon.
They also have neat names. Here we start naming the moons after characters in Shakespeare and Alexander Pope. We have Miranda, Titania and Oberon. Again, like Saturn, these are ice-rock conglomerates. Theyâ€™re about 50-50 in general. We donâ€™t have a lot of really high-resolution pictures. Weâ€™re dealing with just images we got from Voyager 2, but as near as we can tell, theyâ€™re about 50-50 ice and rock. They also contained some ammonia and carbon dioxide ice mixed in there. Theyâ€™re out tending the rings.
The largest of these, Titania, is actually half the size of our moon. That we can see these at all is kind of incredible. Weâ€™re looking clear across the solar system, and on a good day with a fair-sized amateur telescope, you can make out the largest of these moons.
Fraser: But if these moons were magically moved into the inner-solar system, weâ€™d have comets right?
Pamela: Theyâ€™re not exactly comets â€“ comets arenâ€™t generally 50% rock, but yes, theyâ€™re more like comets than asteroids.
Fraser: But theyâ€™re not almost 100% ice like some of Saturn’s moons are.
Pamela: Right, so here weâ€™re getting a little bit more rocky, though we have a planet thatâ€™s more icy. Itâ€™s neat to see how you have the trade-offs between these two worlds. Saturn, itself: not icy. Uranus: icy. Moons of Saturn: much higher in ice. Moons of Uranus: (as near as we can tell) a bit higher on average in the rock content.
Fraser: You brought this up already, which was the Voyager 2 flyby back in 1986. What did Voyager see as it went by? What were some of its discoveries?
Pamela: The main thing it discovered was back then, it was during a period in Uranusâ€™ season where it was just this basically featureless blue object. It was mesmerising in how featureless it was. It was just this clear shade of blue and nothing else. No storms, no spots, nothing. Just this vast blueness. Thatâ€™s kind of unusual; everything else we look at we see storms on. Even Neptune, which is very similar, has these big white storms that seem to be fairly persistent. We just happened to catch Uranus during a period of time where its surface was utterly boring. Sometimes boring is remarkable.
Fraser: I know that recent images from Hubble have shown something different.
Pamela: As the seasons have changed, weâ€™ve started to get banding, weâ€™ve started to get storms, and weâ€™ve started to see both white spots and dark spots. Thereâ€™s been what we can only liken to thunderstorms going on as the seasons change.
As you heat one part of the planet that hadnâ€™t been heated for a long time, you end up with flows. You end up with currents in the atmosphere that start driving storms that werenâ€™t there before, so it seems to be that as we flip which pole is warm and which is cold, weâ€™re moving into stormier times that lead to a much more interesting to look at surface of Uranus.
Fraser: So what spacecraft is planned to go back there.
Pamela: Well, itâ€™s one of these things where itâ€™s not one of those worlds thatâ€™s looking to show us a lot of life, a lot of interesting geophysicsâ€¦ you go and visit Saturnâ€™s moons and you have titan that has this amazing geology and it has fluvial systems with flowing methane and ethane on its surface, and you have ice volcanoes and all this other really cool stuff going on. You go to Jupiter and you have plasma streams and volcanism and tidally disrupted icy Europa that has liquid water underneath a few kilometres of ice. All these things are really fascinating and you look at Uranus and see: plain, blue, boring world with some fairly boring, mundane moons.
So yeah, itâ€™s kind of low on the priority list right now.
Fraser: But why was it on the priority list for Voyager?
Pamela: Because we knew nothing about it. Voyager 2 and Voyager 1 (which were launched in that order, but Voyager 1 escaped the solar system first) were out as our first stab at exploring the solar system. We didnâ€™t have Hubble at that point to get high-resolution images. We didnâ€™t have Keck and the Very Large Telescope. We didnâ€™t have any way of getting detailed images of these distant planets.
Fraser: They were also lined up quite conveniently, right?
Pamela: Everything was lined up perfectly. We couldnâ€™t launch those missions today and do the same amount of science they were able to do.
We had this wonderful alignment of planets, we had this opportunity, NASA had good fundingâ€¦ so we set off and took our first space-look at all of these planets. We went, â€œthis oneâ€™s interesting, this oneâ€™s interesting,â€? (Saturn and Jupiter), and we sent missions to each of those planets. We got out to Neptune and Uranus and went, â€œokay, thatâ€™s kinda cool â€“ move on.â€? And we moved on.
There just arenâ€™t the same scientific draws in those two systems. Theyâ€™re intermediate mass. They arenâ€™t failed stars, they arenâ€™t rocky worlds with the potential for man to go explore on foot, so yeah. They have cool weather, they have moons that are kind of cool to look atâ€¦ but theyâ€™re hard to get to, and the science returns arenâ€™t what we can get for to easier-to-get-to Jupiter and Saturn.
Fraser: Would you say there are any outstanding mysteries that still need solving?
Pamela: How you tilt a planet on its side. Thatâ€™s always going to be one of those interesting things to think about. If you take a good look at how everythingâ€™s organized in the solar system, for the most part you get everything orbiting in the same direction, you get everything rotating about their axes in the same direction (thereâ€™s a couple of exceptions, Venus for instance), but here you have Uranus just totally tipped over.
There are some solar system models that say Saturn, Jupiter, Uranus and Neptune were all gravitationally interacting with each other, and with all of the asteroids and all of the cometsâ€¦ and in the process they were gravitationally torquing and twisting and flinging each other to completely new orbits. It couldâ€™ve been during this period of violence in the solar system that somehow Uranus got spilled over on its side such that it sometimes only shows light to half of the planet, and thatâ€™s kind of cool to think about, but itâ€™s not something we can answer by visiting it. Itâ€™s something we need to answer by better understanding computer models.
Fraser: Right, I think Iâ€™ve seen that mathematicians are working on various models to figure out what kind of collision or gravitational interaction it would take to turn it over on its side and keep it where itâ€™s at instead of tumbling over and over again.
Pamela: One of the things a mission could maybe help sort out is it has this really wonky magnetic field. Its magnetic field is pointed at an angle relative to its tilt, so you have this planet that is almost, but not exactly, tilted over on its side. Relative to that rotation axis, you have the magnetic axis tilted over roughly 60 degrees. That makes the north magnetic pole closer to the equator than to the rotational pole.
It also doesnâ€™t go through the centre of the planet. If you draw a line from the north magnetic pole to the south magnetic pole, that line doesnâ€™t go through the centre of Uranus, it instead misses by about a third the radius of Uranus, which is just really confusing. Weâ€™re not quite sure what explains this, but that is something that perhaps could be modelled better if we could go and orbit it with something that could better map out that magnetic field.
Fraser: There you go NASA (or ESA), thereâ€™s a reason to send a spacecraft.
This transcript is not an exact match to the audio file. It has been edited for clarity.