Spring comes to the planet Uranus

Astronomers had assumed for decades that the planet Uranus was featureless. But closer study has revealed that it’s a dynamic world with changing seasons.

Spring comes to the planet Uranus 1

When the Voyager 2 spacecraft visited Uranus in 1986, it saw a beautiful, but almost featureless planet. Then, in 1994, astronomer Heidi Hammel of the Space Sciences Research Institute happened to see a Hubble Space Telescope image of Uranus . . .

Heidi Hammel: They got some pictures of Uranus that showed these bright clouds on it. And I asked the people what are these pictures of Uranus that show these clouds? And they said, ‘That’s just what Uranus looks like.’ And I said, ‘Well, no, that’s not what Uranus looks like. Uranus doesn’t have any bright clouds like that!’

Spring comes to the planet Uranus 2
Uranus in true and false colors.

Voyager visited Uranus in its northern hemisphere winter. Now Uranus has moved in its 84-year orbit around the sun — its northern hemisphere spring equinox is due in 2007. There are more clouds in the atmosphere of Uranus — and bands encircling the planet are changing in size and brightness — as sunlight strikes parts of the planet for the first time in over two decades.

Heidi Hammel: It just goes to show you in science you don’t always know what you think you know, and sometimes you get surprised, and it’s following up on those surprising things that really makes science interesting.

Hammel now wants to see if parts of Uranus that were shrouded in darkness will develop more active weather, after a few years of sunlight.

Seasons on Uranus are quite different from Earth’s for several reasons. First of all, because it takes Uranus 84 years (more or less) to circle the sun, each season lasts 21 years. On Earth, we get different seasons because of the tilt of our planet.

Imagine the earth is a large bead on a stick. The bead spins on the stick — which gives us our 24-hour day. The stick travels around the sun. But the stick isn’t straight up and down relative to the sun. Instead, it’s tilted a little bit off the vertical. Scientists call the stick the “axis” of the planet.

But the position of the stick, or axis, stays the same as it goes around the sun. Imagine that at one point, the tilt points the bottom half of the earth more directly at the sun. This is when it’s summer in the Southern Hemisphere and winter in the Northern Hemisphere. When the planet has traveled to the other side of the sun, the situation is reversed. Now the Northern Hemisphere is more directly lit and the Southern Hemisphere is less directly lit.

Now think about what happens at the poles. You probably know that during the hemispheric winter, days are really short near the North and South poles. Sometimes the sun barely comes up at all in midwinter.

The situation at the poles is the situation most similar to days and seasons on Uranus. Basically, if you imagine Uranus on a stick (and BTW, about 64 earths could fit inside Uranus), the tilt is so large that the stick is almost horizontal. This means for two 21 year seasons out of the 84 year journey, the poles are pointed more or less at the sun, meaning that even as the planet rotates in its approximately 17 hour day, the side of the planet away from the sun will NEVER see the sun — at least, not until the planet has traveled into the next section of its orbit, where the axis is no longer pointing directly at the sun.

Excerpts from a talk with Dr. Hammel:

Q: Why do you enjoy studying planetary science?

I think I came back to studying planets, because they change, they’re dynamic. And also because they’re real, they are solid, we can send spacecraft to them.

[25] To me, things like galaxies, Galaxies, If you look at it today or tomorrow or 1000 years from now 10,000 years from now, its basically going to be the same galaxy. It’s going to look more or less the same. But that’s not true with planets, if you look today you’ll see one thing, if you look tomorrow, you’ll see something different. Because there’s weather and things change.

You have to be more on your toes, when you’re doing planetary astronomy, you can’t just sit back and say, ah well, I missed it this year, and I can see it next year.

[26] You really have to be on the ball, and especially the things that are changing. Like when you study a season on a planet, you can’t really skip fifteen years. You have to look at it, and you have to monitor while its changing. That’s the only way you’re going to understand it. So it adds a little bit of excitement or edge to your observations when you’re studying a planet. I like that about planetary astronomy. It’s fun.

[27] And the fact that you can send spacecraft to these planets to provide a completely different view, and to provide some ground truth, makes it a little more tangible, makes it a little more immediate. You can speculate from now until doomsday, about what a black hole is like, but we’re never really going to go inside one, at least not in our lifetime. But we may send a spacecraft to Neptune. And so the work that I’m doing might be proven right or wrong in my lifetime. So I better make sure I do it right.

[28] Astronomers love that sense of studying things that are far away, but bringing them right into their own computer. Many of us who do astronomy like that — we project ourselves out to where we are working, whether we’re working on the planet Neptune, whether we’re working on the crab nebula, or working on a distant galaxy, its a way of your putting yourself in that place.

[29] Maybe that’s why astronomers, so many of us like science fiction. Because we think in terms of science fiction a lot. We’re trying to project what your data are telling you , and try to get a picture of what that environment is really is like. Try to understand it in a tangible, physical way.

Q: How did this project on Uranus get started?

[Some scientists were using Hubble to take some pictures in the vicinity of Uranus. They were looking for dim satellites, and just happened to get in the vicinity of Uranus and got a picture. And it had these bright clouds, on it. And I asked what are these pictures? And they said, “That’s just what looks Uranus looks like.” And I said, “No its not! That’s not what Uranus looks like. Uranus doesn’t have any bright clouds!”

That really catalyzed this whole project, starting to watch as it approached its equinox that it was really just a serendipitous picture that the planet itself was changing its appearance.

[14] It just goes to show you in Science you don’t know always know what you think you know, [17] and sometimes you get surprised, and its following up on those surprising things that really makes science interesting.

[19] So it was just accident, I mean, no one ever proposed to look at Uranus with the Hubble space telescope because we all knew that there was nothing to see.

[7] No one’s been able to do the kind of work we’re doing now, studying Uranus because we simply didn’t have the tools — until now. So its kind of exciting to be watching the planet as its undergoing these changes.

Q: What has changed in Astronomy to make this project possible?

[8] The tools that we use now are really some of the biggest and best telescopes that we’ve got at hand. With them, we can see the most distant planets in our solar system. We use the Hubble space telescope to get imaging of Uranus at visible wavelengths and we use the Keck 10-meter telescope in Hawaii to get infrared images of the planet.

And it’s really critical to use these very good telescopes, because the planets are so far away. Even though they are big planets, many many times larger than the earth, when see them from the earth, they are basically just tiny specks — almost points in the sky. They are disks, but the disks are so tiny, the earth’s turbulent atmosphere smears them around so much that we can’t even take a picture of them. You have to have really good clear skies, or use the Hubble, which doesn’t suffer from atmospheric effects.

With all the telescopes that we use nowadays, nobody looks through a viewfinders anymore. Professional astronomers never look through telescopes or lenses, we always use computers to take our data. With Hubble, or the Keck, or any modern professional astronomy telescope, you are sitting at a computer monitor, running an instrument, which has an electronic camera, that takes the pictures, and then the pictures are displayed on the computer screen.

[10] What we can see with Hubble and with Keck when we look at Uranus is we can see the planet and we can see bands on the planet, sort of like Jupiter or Saturn has bands, and that was sort of a surprise for some people. Because when Voyager II flew by Uranus in 1986 and took pictures with its cameras, at Uranus, it basically saw no bands on the planet, it was a bland, smooth, planet.

Q: How did you get interested in Astronomy?

[24] When I went to college I didn’t really plan to go into astronomy. It just was not on the radar screen. I knew about astronomy, and it was OK, but it was nothing I ever set out to have a career in. I took an astronomy course when I was a sophomore simply because I had an elective to fill, and that looked interesting. I really enjoyed it. Again, its not something I felt I desperately had to do, but it was fun. I liked working with the telescope and taking data. I kept on taking classes in it and eventually I started working with a professor as a researcher for him, and one thing led to another I kept getting deeper and deeper into it. And eventually I decided to go to graduate school in that. I had been doing planetary science as an undergraduate, studying planets, so I decided when I went to graduate school that I would diversify. When I went, I went to a place where they taught a lot of basic astronomy: stars and stellar interiors, and galaxies, galactic structure and evolution, cosmology, just to get a broader background in astronomy. But when it came time to pick a thesis topic for my Ph.D., I came back to planets. And I think I came back to studying planets, because they change, they’re dynamic.

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