Wednesday, October 6, 2010

Uranian Nights, Zarminan Valleys

In the last couple of posts I’ve mentioned Jupiter as a landmark in the Eastern sky shortly after sunset. We checked out Jupiter and its moons around this same time last year. If anything, viewing Jupiter is even easier than it was when I did that post, so check out its moons with binoculars if you get chance this week.

While you’re at it, there’s something else you can very easily check out with binoculars close by. It’s another planet, Uranus (pic below right, not mine).

Tangent: Let’s get the name thing out of the way. I say “yoo-RAY-nuss”. Yes, I know that sounds like “your anus”, uranus_vg2but the alternate pronunciation , “YUR-uh-nuss” sounds like “urine us”, so I’m not sure that’s any better. I don’t know which one is “right”. Even if I did, I’m not sure I’d use it. For example, I deliberately mispronounce the word “pianist”. I say “pee-ANN-ist”, even though it’s supposed to be pronounced more like “penis” with a hint of a “t” on the end. I can’t do it. It’s not as though I talk about pianists all that often, but when I do, it’s usually not with one of my friends or peers, but rather with someone I’d be uncomfortable saying “penis” in front of, like friends of my parents or something…

Uranus is unique among the planets in that it’s just barely naked-eye visible (NEV). All the other planets are either easily NEV* or totally non-NEV (Neptune, Pluto, Eris, etc.) Uranus has an apparent magnitude of around 5.9, just inside the NEV limit of ~6.5.

*Though in fairness Mercury can be tricky just because it’s so darn close to the sun.

But you can’t see a 5.9 from an urban area like Salt Lake Valley, and even if you’re out in the boonies, you need really sharp eyes to pick it out. With binoculars of course, you can spot it easily, but it’s kind of hard to scan the sky with binoculars; you have to know almost exactly where to look. Fortunately right now Jupiter is a super-easy skymark.

Uranus Locator Find Jupiter and the plane of its 4 moons. Now look up and left/East from Jupiter about 25-30 degrees “above” the orbital plane of its moons. At an apparent distance of roughly ~10-20 times the apparent distance between Jupiter and its farthest out Galilean moon*, you’ll come to a star. That’s Uranus.

*Of course I don’t which one it’ll be when you look. The furthest out is Callisto, but it may not look that way- or even by visible- when you view it, depending on where it is in the course of its 16-day “month”. That’s why the “10-20 times” thing is so vague…

Uranus wasn’t discovered until 1781. It was almost certainly seen countless times before, but no one apparently figured out that it was a planet. Though much larger than Earth, it’s only around 1/3 the size of Jupiter and 4 times as far away, which is why it’s so much less obvious in the sky. Both planets, along with Saturn and Neptune, are generally lumped in the category of “gas giants”. In some ways this categorization makes sense; all 4 planets are massive, composed of similar stuff and lack a definite solid “surface”.

Earth Jupiter Uranus Size They each have lots of moons (Uranus has at least 27), a system of rings and a strong magnetic field. But because Uranus and Neptune are much farther out from the sun, the structure of that stuff is likely different, and so they’re sometimes described as “ice giants” instead. Uranus is believed to be composed of 3 layers. The inner layer is thought to be a rocky core of some sort, pretty small, maybe ½ the mass of the Earth. The outer layer, the atmosphere, consists mainly of hydrogen and helium. The blue “surface” visible in photographs Uranus is an unbroken planet-wide deck of methane clouds. Below this are thought to be additional cloud layers- possibly a deck of ammonia hydrogen sulfide clouds below the methane deck, then one of ammonium hydrosulfide clouds below that, and finally, down lower, a deck of water clouds.

But the middle layer, the mantle, which contains over 90% of the planet’s mass, is ice, consisting largely of water and ammonia. The ice isn’t “ice” as we think of it, but a hot and super-dense fluid of these compounds in crystalline phase.

Double Uranus Expand-O There are a lot of weird things about Uranus. It’s bigger than Neptune, but less dense and less massive. It’s also- for reasons that are not clear- much colder than Neptune, possibly the coldest planet in the solar system. It has a strong magnetic field* that’s weirdly aligned. It’s titled at 59% relative to the axis of rotation (the ultimate weird thing which we will get to momentarily) but that’s not the weird thing. No, the weird thing (but, again, not the ultimate weird thing- I am getting there) is that it is asymmetric, meaning that a straight line between the magnetic poles doesn’t pass through the center of the planet, but is shifted toward the planet’s South** pole by a distance of roughly 1/3 the planet’s radius. It’s unknown why this is, but suspected that it might have something to do with the huge, highly-conductive ammonia-water ice-mantle. (Neptune has a similar structure and a similarly misaligned magnetic field.)

*I explained magnetic fields- specifically the Earth’s magnetic field- in this post.

**North and South are loaded words on this planet, as I will explain in a moment.

Side Note: I’ve mentioned Neptune a couple of times, and in fact I located it as well with binoculars last week. But it’s dimmer and not near an obvious skymark, so I’m leaving it for another post…

Earth Uranus Magnetic But the really weird thing about Uranus is its axial tilt. Planets in our solar system generally have axes of rotation that are not perpendicular to the plane on which they orbit the sun. Earth’s axis is tilted at 23 degrees, which is why we have seasons. Mars is tilted at 25 degrees, providing similar (if longer*) seasons. Jupiter has almost no axial tilt (3%) and so no real seasons. But Uranus has an astounding 98 degree axial tilt, meaning that its axis of rotation is roughly parallel with the solar orbital plane. Think about what this would mean if Earth were tilted 98 degrees. For any latitude North of say, Costa Rica, Winter would include multiple entire days of total darkness. Here in Salt Lake City, we wouldn’t see the sun at all between around the 2nd week of November and the 2nd week of February. Around the Spring and Fall equinoxes, we- and the whole planet- would experience “normal” day/night cycles. But the days would grow way longer way faster. Around the 2nd week of May, the sun would come up and stay up- circling round and round the sky- until the 2nd week of August. Down near the equator during these same times, “day” and night” would still be experienced, but it would be a constant dawn/dusk, with the sun never ranging more than a few degrees above or below the horizon.

*Because its year is longer.

Of course on Uranus, the timeframes are completely different, because the Uranian year is 84 Earth-years long. Uranus passed through Equinox 3 Earth-years ago; its next solstice will be in 2028. So right now things are fairly “normal”, but in another decade or so one hemisphere will be firmly in daylight, the other in darkness, for the next couple of decades.

Extra Detail: So how come we say Uranus is tilted at 98 degrees? Why don’t we say it’s tilted at 82 degrees, and the other pole is “North”? Because viewed from “above”- that is North of the solar orbital plane relative to the North pole of the Sun, Earth, and every planet except Uranus, Venus and Pluto- Uranus spins the wrong way- that is clockwise. The planets all orbit, and mostly rotate, counterclockwise; those that don’t are called retrograde. So Venus and Uranus are spinning the “right” way if they’re considered to be upside-down.

OK, so why is Uranus “upside-down? The favored hypothesis is that a collision between it and some other planetary-type body knocked it sideways.

Detail-Tangent: I’ve noticed that this collision-hypothesis is used to explain a lot of things in astronomy, perhaps the most significant to us on Earth being the formation of our abnormally large moon. Other examples include the current orbits of the Martian moons, the fate of the prehistoric hypothetical Planet V*, and the apparent debris disk** surrounding the star Vega. I’m not an astronomer, and so far be it for me to question current hypotheses, but it sometimes seems like the collision hypothesis is invoked whenever there’s a not-so-easily-explained orbital or rotational anomaly…

*Supposedly existed in the asteroid belt, got knocked out of orbit by repeated asteroid collisions, fell into the sun. Pretty far out hypothesis, but would make an awesome Superman plot.

**Which I learned about when researching that star for the Summer Triangle post but elected not to work in to an already marathon post.

It’s also weird to think about what Earth would be like if our year were 84 years long. Ursula LeGuin explored this in Planet of Exile; the planet Werel had a 60 Earth-year-long year, which led to all sorts of oddities; hardly anyone for example lived to see Summer twice. Which, come to think of it, is something we all take for granted, but which a majority of living creatures here on Earth probably never experience. Even most mammals live less than a decade. Of course for such a long year, the Earth would have to orbit a hotter star at a farther distance. But since big, hot stars don’t last all that long, it’s questionable whether one would sustain an Earth-like world long enough for complex multicellular life to come about.

On the other hand, if an Earth-like world orbited a much smaller star, at a closer distance, then it might well experience a much shorter year. And that smaller star could burn steadily for a long, long time- far longer in fact than our own sun, and plenty long enough for all sorts of interesting things to evolve.

In case you’ve been living in a cave for the last week, astronomers may have recently found such a world.

A Completely Different Planet Than The One I Started This Post About

Gliese 581g is close by as stars go, closer than Vega, at just 20 light years away. But you won’t see it naked-eye or with binoculars or even a small telescope. It’s a Red Dwarf. Red dwarves are smaller than main sequence stars (like the sun) but larger than brown dwarves (which are too small to achieve stellar ignition, and which I explained in this post.) They range from about 40% the mass of the sun down to about 8%. Because of this smaller mass, they’re not as hot in the core, and so behave differently. Specifically they move heat and energy between core and surface in a fundamentally different way. The method used by red dwarves- convection- uses more of the available hydrogen and so “burns” longer, than the method- radiation- used by larger stars such as ours. Most of the stars in the galaxy are red dwarves; they’re just not the ones we see.

Sol Gliese Extra Detail: When plasma gets hot enough, it becomes transparent, so energy from fusion in the star’s core can radiate through to the surface of the star. After a while, an excess of helium (the product of hydrogen fusion) builds up in the core, leading to the end of the main sequence fusion. A red dwarf’s mass isn’t sufficient to bring about core temperatures high enough to induce plasma-transparency, so heat and light are transferred to the surface via convection- that is by stuff moving around inside the star. This “moving stuff around” has 2 effects that extend the life of the star dramatically. First, it mixes things up and more efficiently fuses the available hydrogen throughout the star, and second, it prevents excessive helium from building up prematurely in the core. Convective red dwarfs have life spans in the trillions of years.

Trillions of years from now, long after our sun and all the stars like it have burned out, and the galaxy has stopped making new ones, the night sky will still be filled with stars, but “you” won’t see any them; they’ll be all red dwarves*.

*Eventually, around 100 trillion years from now, the last red dwarves will burn out as well, and the universe will enter the Degenerate Era, which is a whole other story and way beyond the scope of this post.

Gliese 581 is already between 7 and 11 billion years old. (Our own sun is only ~4.5 billion.) It has at least 6 planets. The 4th planet out, Gliese 581g, is thought to be roughly 1.5 times the size, and 3.5 times the mass of Earth. It orbits Gliese 581 at a distance of just 13.5M miles, or around 15% the distance of the Earth to the sun.

Earth Zarmina In short, it’s at the right distance from its sun for liquid water to exist, and is the right size to hold onto it, and an atmosphere likely denser than ours, and it’s been that way for longer than Earth has been its way. It appears to be the best candidate for an Earth-type planet yet. Co-discoverer Steven Vogt has nicknamed it Zarmina’s World, after his wife.

Tangent: This only works if your wife has a Wicked Cool Name. If I for example discovered a new planet, and named it, let’s say, “Sue’s World”, the name would totally not stick. But “Zarmina” is an awesome name; it absolutely sounds like a planet or something else Mysterious and Cool and Important. I’m trying to think of a way to broach the idea to AW of changing her name to something weird and cool. Like “Veldareth”, or “Stralmandar” (accent on the 2nd syllable) or something. That way if I ever discover a planet* I can name it after her.

*Skeptical readers might question the likelihood that I would ever discover a planet, given that I am not an astronomer, and in fact don’t really even have a working telescope at the moment. Perhaps so, but I could maybe name something else I discovered, like a good campsite or a fun trail or a new way of flipping pancakes or something.

If life does exist on Zarmina, it’s strange to think of what it might experience. The sun would appear red, and bigger. Even though the volume of Gliese 581 is only about 1/3 that of our sun, it’s 6 times closer to the planet, so should appear roughly twice as big in the sky.

Earth Zarmina Orbits And the Zarminan sun would appear even redder, because Zarmina’s atmosphere may well be denser than ours and, more importantly- and I am getting to the absolute weirdest-of weird things here- it would always be- at least for where living creatures were most likely to view it- setting. Or rising.

Side Note: I should say that it would look really, really red to us. I suspect it would look a lot different to any creature with anything like eyes that had evolved there. Red dwarves have strong infrared light, so it’s likely that the eye of any native Zarminan would be adapted to a longer-wavelength visible spectrum. Conversely BTW, Red dwarves emit relatively little ultraviolet light. Between the negligible UV, the dense atmosphere and the low sun angle, future astronauts could probably leave the sunscreen at home.

Due to its orbit and age, Zarmina is believed to be tidally locked*, that is the same side always facing the sun, as our own moon always presents the same side to Earth. Venus Split The habitable zone, if there is one, may be at or near the ribbon of eternal sunrise encircling the planet. But such a “ribbon”**, if it exists, may be wobbly or discontinuous, depending on the nature and topography of the surface. Ocean currents could move warmth into the dark or cold into the light. Mountain ranges might cast shadows extending the cold/dark zones, while sunward-opening valleys, fjords or canyons might extend the habitable zone dark-ward for some distance. The dark side might host massive glaciers, which would melt as they expanded sunward, feeding mighty rivers, seas or proglacial*** lakes.

*I explained tidal locking in this post.

**I actually described a hypothetical “Ribbon World” 2 years ago in this post, way back before anyone ever read this blog.

***I explained proglacial lakes in this post, and returned to the topic in this post. I am telling you- a post for everything.

Zarminan Valley I’m guessing you wouldn’t get much of a night sky from the Zarminan “Ribbon”; the perpetual twilight would allow darkward glimpses of only the few brightest stars, if any. To really see the stars, a Zarminan would have to venture far into the dark and cold of the endless, frozen Zarminan night. If he/she/it did, it wouldn’t have much trouble making out our sun. It wouldn’t be the brightest star in the sky, but it sure would be easier to spot than Uranus.

Note about sources: My best initial source for red dwarf stars, convection and stuff was reader Doug M., who, as I have noted previously, is the Smartest Reader Ever of this blog. More info came from the Astronomical Society of the Pacific website, one of the University of Michigan’s online astronomy courses and Wikipedia. Additional info on the discovery of Gliese 581g came from the dozens of popular print and online articles I’ve read since news of the discovery over the last week and completely neglected to make note of.

7 comments:

Anonymous said...

Awesome post! You have outdone yourself with the double expando graphics.

That red dwarf along with "ribbon of life" would have to produce some very intesting creatures, ones that have to survive both extremes or have limited mobility. Plus, with a continually setting/rising sun, sleeping patterns would have be dramatically different.

How about suetherland, zetasue, suecasa or sueplaneta/planetadesue?

mtb w

rabidrunner said...

So this has nothing to do with your post, but do you read the Boston Globe's Big Picture? It's fantastic. A particular post seemed right up your alley. For some reason.

http://www.boston.com/bigpicture/2010/09/animals_in_the_news_1.html

rabidrunner said...

Link was truncated. For some reason. Here's the post.

Ski Bike Junkie said...

"More info came from...one of the University of Michigan’s online astronomy courses..."

Go Blue!

Watcher said...

mtb w-I like "Zetasue". Thanks, this will save me having to pitch AW on the name change!

Rabid- cool shots, thanks for the link.

SBJ- Yeah, you took that class when you were there, right?

KanyonKris said...

Lots of fascinating info in this post. And excellent graphics. As usual, well done making what could be just dry facts interesting and relatable.

Enel said...

Very cool! You even anticipate my "Why 98 degrees and not 82 degrees?" question.

I just now saw the moons of Jupiter with binoculars for the first time. I did not know it was possible.

I may have seen Uranus, but hard to tell. The "star" was about the magnitude of Jupiters moons and right around where you said it would be so I guess I will take the credit.

I have been seeing Mercury a lot lately near dawn. It is easier once you have seen it a few times and understand its behavior.

Thanks.