Questions Show: Orbit of the Planets, Green Stars, and Oort Cloud Contamination

We’re back to a theme-less questions show. We’re right across the Universe this time. Why are the planets lined up in a nice flat plane? Why are there no green stars? And is the Oort Cloud contaminating our understanding of the cosmic microwave background radiation? If you’ve got a question for the Astronomy Cast team, please email it in to and we’ll try to tackle it for a future show. Please include your location and a way to pronounce your name.

  • Orbit of the Planets, Green Stars, and Oort Cloud Contamination
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    Why are the planets lined up in a nice, flat plane?

    Why aren’t there any green stars?

    Is the Oort Cloud contaminating our understanding of the cosmic microwave background?

    Are there any places on Earth where the moon doesn’t rise or set for a period of time?

    If we could land on a white dwarf, what would it look like?

    How do we know our galaxy is a spiral galaxy?

    Was space always dark?

    Are we expanding along with the universe?

    Transcript: Orbits of the Planets, Green Stars, and Orrt Cloud Contamiation

    Download the transcript

    Fraser Cain: Welcome to the AstronomyCast Questions Show. This is where we answer your questions about Space and Astronomy. Do you have your ‘Question Hat’ back on Pamela?

    Dr. Pamela Gay: I hope so. It was only partially on last week. I realized that I gave a very confusing explanation to the Twins Paradox. Especially to people who actually understand Relativity so I’m going to clear that one up on my blog. So, if you’re still curious about the Twins Paradox, checkout

    Fraser: You’re going to write like a big blog post about it and get into the nitty gritty detail.

    Pamela: Yeah.

    Fraser: Okay. See you never have to worry about ideas for your blog, do you?

    Pamela: No. I just need to worry about the time for my blog.

    Fraser: Yeah. Exactly. Well, alright we’re back to a theme-less question show. We’re going to go right across the Universe this time. So why are the Planets lined up in a nice flat plane? Are there any green Stars? If not, why not? And is the Oort cloud contaminated? Our understanding of the cause of microwave background radiation.

    Now if you’ve got a question for the AstronomyCast team, just e-mail it in to and we’ll try to tackle it for a future show. Include your location and a way to pronounce your name.

    Alright, let’s get on to the first question. This comes from Dan in San Antonio, Texas. “Why is it that the orbital paths of the Planets in our Solar System fall roughly within a single plane?”

    That’s true. If you were to take like a record or a plate or a Frisbee you could draw on the Planets the circles, the orbits that the Planets follow. I know they’re not circles, but they would all be in a very thin flat plane. Why is it?

    Pamela: Well the basic reason is when our Solar System formed it started out as a large blob of gas and dust. As that blob started rotating, it flattened out the same way pizza dough flattens out as you throw it in the air and spin it.

    Now, what’s more interesting is, why is it that all the Planets don’t rotate and orbit in the exact same plane? The differences from Planet to Planet have more to do with the fact that our Solar System knocked itself around for a while.

    The Planets gravitationally interacted with each other, with Asteroids, and with Comets. With all these different interactions they slowly knocked themselves out of being in a perfect plane and into a slightly badly stacked set of orbits that we see today.

    Fraser: Right. And so it’s the spinning, right? In that if you take a ball of gas and dust, and spin it quickly, it’s going to flatten out because of the Centrifugal Force.

    Pamela: Which even though everyone says it’s an imaginary Force, the fact is that things want to fly off in straight lines. It’s that tendency to want to fly in a straight line that is balanced out by a center-pointing Force that leads to everything flattening out. There is a great XK CD comic that we’ll have to point to that is related to this.

    Fraser: But you can kind of imagine that yeah, it started as a ball and as it spun faster and faster as it contracted it turned into this flattened disc. Eventually all of the little parts of the disc collected together into the Planets and then the Solar Wind on the Sun got going and blew away all the remaining material. You were left with the Planets orbiting the Sun right where they were when they were in that disc.

    Pamela: Well, what’s kind of neat is some of the models that we have trying to describe what the early Solar System looked like kind of have all of the big Planets sort of tumbling in a mass about the Solar System. They all were sort of inter-orbiting and interacting.

    It was through these different interactions that the Planets got thrown into different orbits. At one point we probably had Saturn and Jupiter in some sort of a resonance. Our Solar System did not start off with the Planets in the places they are currently located. We’re not quite sure what happened, but there are some really neat models for all of those different orbital dynamics.

    Fraser: Very cool. Alright, let’s move on. This one we’ve got audio for it and it comes from Mark Buxton. “Hi. This is Mark Buxton. Greetings from Australia. So, I’m a science teacher and one of my students says to me when we’re talking about Stars why are there no green Stars? That got me thinking. We do all this talk about red Stars and blue Stars, and we know that the color is a direct relationship to the temperature of the Star because it behaves like a Black Body, so why is there no green Stars? It would seem that the peak emission of the green color is not possible. I’m wondering whether that particular temperature is unstable or if there is something else that affects it?”

    Okay. So, Pamela, is Mark right? Are there no green Stars?

    Pamela: Well, there are plenty of green Stars. The problem is that our eyes aren’t keen on seeing them as green. When we go outside and we look up there’s lots of very red Stars that we can make out with our eyes. Betelgeuse in the Constellation Orion is probably one of the best examples of this that you can see this time of year. There are Stars that we can start to make out that are blue.

    But in terms of trying to separate out the different colors of the Stars our eyes are pretty good at going that’s kind of red, that’s kind of yellow, that’s kind of blue, and we don’t really make out any other colors.

    And so while there are green Stars, by the time the light has gotten through our Atmosphere, and gotten to the retina in the back of our eye, our brain has sort of gone white.

    Fraser: Well, what causes the color of a Star in the first place?

    Pamela: It has to do with the temperature. It’s if you’ve ever seen the old generation “Star Trek” episode where Captain Kirk is trying to warm up so he uses a phaser to heat up a rock.

    When that rock gets heated up it turns red, and in fact with your stove if you have an electric stove when the coils heat up they get red.

    If you could turn up the temperature even further, the stove burner would go from red to orange to yellow, and again your eye wouldn’t perceive green. It would perceive it as going blue and white hot.

    Fraser: Right. That’s why a light bulb is giving off white light. The actual part of the bulb is that temperature; it is that temperature to give off that color light.

    Pamela: And here, white isn’t really a color, it’s just what we perceive. Temperature, we call this Black Body Radiation, and a hot object that is emitting what we call continuum of light is giving off light in all different colors, but it’s not giving light off equally in all different colors.

    So a cooler object is going to give off the most light and the red in our eyes is going to perceive this is red. A really hot object is going to have its peak luminosity, the peak amount of light that it gives off coming off in the blue. We can shift where the peak wavelength is coming by changing the temperature of something.

    If you have a dimmer switch with an incandescent light bulb you can actually see the light bulb go from a reddish ruddy color when it’s barely turned on to getting yellower and then getting whiter.

    Even though it’s not really white, that’s just what we perceive as you heat it up, as you put more energy through it as you turn the dimmer switch to not dim. And the key is about perception.

    If you go out and use tools to measure what color the peak light is coming out there are green Stars. It’s our human eyes that cause us not to see them as having a peak color that we perceive as green. Instead we start perceiving things as white.

    Fraser: And a green Star then would be partway in-between a main sequence Star like our own and a hot blue Star. So, are they probably massive and short-lived?

    Pamela: They will be bigger than our Sun. They will be shorter lived then our Sun. And they’re out there but they’re not really common. By numbers the most common Stars are the red Stars.

    And the bigger Star the fewer and further in-between they are. If you’re out hunting for green Stars the types of places you want to look are going to be Star-forming regions.

    You’re going to want to go look at the Orion Nebulas. You’re going to want to go looking in open clusters, places where you still have these big, hot, young Stars.

    Fraser: Well, so I think that answers the question. There are green Stars, they are just rare. When the light gets through the atmosphere into our eyes, we don’t perceive them as green, as much as we are able to see red, yellow and blue Stars. Awesome.

    Alright. Well, the next question comes from John Robbe from San Antonio, Texas. John had provided a whole bunch of wonderful calculations but I’ve shortened his question down to one quick sentence, so I hope we can give it justice. John wants to know “how can the WMAP detect the Cosmic Microwave Background Radiation without it being contaminated by the Oort cloud background?”

    Now I think we’ll have to explain those a bit. The Oort cloud is a theoretical cloud of Comets surrounding the Solar System. That’s where the long period Comets are thought to come from. It’s like 50,000 times the distance of the Earth to the Sun far away. So, we can’t see any of the objects there.

    And of course, the WMAP map was the Probe that NASA sent to carefully measure the temperature of the Cosmic Microwave Background Radiation. So, is it possible that this cloud of ice surrounding the Sun is messing up the measurements made by the WMAP map?

    Pamela: Sort of, kind of, yes. There’s actually, I have, if you go back to May 2007 I have a large blog post about this. There is a researcher at Harvard by the name of David Babbich who has been working to try and understand how light from the Cosmic Microwave Background and light from the Oort cloud can interact.

    We might actually be able to detect variations in the distribution of stuff in the Oort cloud by looking at third order of facts in the distribution of light from the Cosmic Microwave Background.

    Now these variations are things that are actually at such a low level that we can’t detect them with our WMAP data. We actually have to start waiting for new missions to get launched. Plank might be able to see some of this.

    The idea is you have two different effects going on. First of all you have the Oort cloud which is a bunch of little tiny objects out there hopefully in a spherical distribution, hopefully with a nice smooth distribution across the Sky, and aren’t taking up a lot of space. You don’t have to worry about them eclipsing large chunks of anything.

    But, the Oort cloud objects are probably around 8.5 degrees Kelvin. Cosmic Microwave Background peaks around about 2.728 degrees Kelvin. Objects that are these cold temperatures all emit light in the microwave. That means that the Oort cloud objects which have a peak color that corresponds to their 8.5 Kelvin (well, if they’re black bodies that we were talking about a minute ago) are going to be giving light off in a variety of colors, including the exact same colors that Cosmic Microwave Background light.

    So there’s this diffused glow perhaps coming from the Oort cloud that is part of what we see as a background noise in the Cosmic Microwave Background. If we carefully look for these distortions caused by this extra light, this background light from the Oort cloud that is superimposed against the even more background light of the Cosmic Microwave Background, we might be able to start looking for well, there’s a hole in the Oort cloud over here. This part is denser in this area down here, and these sorts of variations are things that Babbich is interested in looking for.

    Fraser: So, I guess that just confirms rule No. 1 for AstronomyCast.

    Pamela: And what’s that?

    Fraser: You can find out everything from the Cosmic Microwave Background Radiation.

    Pamela: Exactly. You can even find out the shape of the Oort cloud.

    Fraser: The shape of the Oort cloud. Who knew? [Laughter] Alright, let’s move on. This question comes from Dan Bortellini. “We all know that the Sun never sets at very high and low latitudes for parts of the summer. Are there any places on Earth where the Moon never sets for several days and nights, and if not, why not?”

    Now, we experience this quite a bit in Canada. I’m not anywhere near the North Pole where I live, but one summer I was down in the U.S, sort of in the southern U.S., and I was amazed how early the Sun set.

    It was like 9 o’clock in the summer, 8 o’clock in the summer and suddenly it was dark and I couldn’t believe. Because here in Vancouver, on Vancouver Island, it’s light until 10 o’clock at night in the middle of summer or even later.

    So, we definitely know that as the summer, as you get towards June 21, you get short nights, and the light, the Sun stays up the most it can during the day. So, does the Moon kind of go through similar cycles over the course of anything?

    Pamela: If it didn’t move so fast it might. The thing with the Moon is it has to make it all the way around our Planet once a month. So while its orbit is inclined to the Earth such that if it just stood still in one place for 24 hours, or more than 24 hours as the case might need to be, yeah, there would probably be places on the planet Earth that would be able to see the Moon up for 24 hours.

    But the truth is the thing is chugging through its orbit. As it’s moving it’s constantly changing it’s height above the horizon. It’s changing its rise and set time. It’s also changing where it is relative to where it is relative to the Celestial Equator.

    All these changes add up to you don’t end up with the Moon staying above the horizon for more than 24 hours straight. In fact, it rises and sets every day no matter where you are on the Planet.

    Fraser: Right. So even if I’m standing on the North Pole, and the Sun is down for a long time, I’ll still see the Moon rise.

    Pamela: Yeah. It’s kind of sad.

    Fraser: There you go. It’s kind of cool. Okay, so let’s move on. This question comes from Thomas W. P. We didn’t get the last name so… “When a small white dwarf eventually cools down it will be a dark cold object with a large surface gravity. Assuming that we had the technology and we could send a probe, would it settle down on a desert of carbon granules, would there be a wind? What would it look like?”

    What Thomas is talking about here, when a Star like our Sun dies it becomes a white dwarf. It starts out very hot but eventually cools down over billions and even trillions of years. And you might get to some time when it is cooled down to a nice comfortable room temperature. [Laughter] Now we know that it’s going to have very high Gravity, so you know you’re going to have a very special ship that can land on the surface of a white dwarf, but what would it look like?

    Pamela: And I gave this one a good hard think. I have to admit it’s a bit confusing to think about because you’re dealing with something that does indeed have a huge surface gravity. Our current models have most white dwarfs having some sort of layer of Hydrogen or Helium on top of the surface. So I believe you would have a surface of some sort of gas or liquid.

    This is where I started to get confused as I’m not quite sure what and I need to do calculations and I didn’t, this is what I get for reading the questions 20 minutes before the show. You would have some short of sheen of Hydrogen or Helium that is neither a liquid or gaseous state on top of basically a giant diamond.

    Fraser: Right. It’s the giant diamond, the biggest diamond in the Universe. You’re not going to get granules. The gravity has pulled it into the tightest possible form for Carbon. And so you’re going to get that diamond.

    Pamela: It’s basically giant diamond covered in Hydrogen or Helium or perhaps a little bit of both. If you wait for it to cool off enough then you can probably get all sorts of fascinating phase transitions with the gases.

    Fraser: And I guess what the question is about is, would there be a wind?

    Pamela: No.

    Fraser: But, what if there was a binary companion?

    Pamela: Well, if there’s a binary companion it’s not really going to have a chance to cool off because it’s going to be perhaps gravitationally pulling material off of its Companion Star. Even if that sort of violent cataclysmic variable behavior isn’t happening, if it’s getting warmed up by its neighbor then it’s never going to cool off all that much. You’re still going to end up with something that’s warm.

    Fraser: Cool. Alright, let’s move on. This question comes from Ian Dodd from Culver City, California. “How is it that we know the Milky Way is a Spiral Galaxy and how have we been able to determine that there are four major arms and maybe a couple of minor ones including the Orion Arm where our Solar System is located?”

    “Obviously we can see the structure of other Galaxies, Spiral or Elliptical, but we can’t see the shape or size of our own because we’re right in it?”

    So how do we know that we’re in a Spiral Galaxy?

    Pamela: Well what information that doesn’t come from the Cosmic Microwave Background seems to consistently come from the Sloan Digital Sky Survey. And in this case, the information is coming from the Sloan Digital Sky Survey.

    There is a telescope down in New Mexico at a place called Apache Point that is slowly scanning through the skies over and over and over again, making a series of measurements. Some objects it’s measuring both with spectra and measuring the brightness in a variety of different colors and we’re using this information to figure out the three dimensional structure of our Galaxy In fact of our Universe because it’s also doing this for Galaxies.

    So you look at all the Stars, you plot their locations, and you can start to build up 3D models using these plots. You can actually build a model of our Galaxy and essentially fly through it inside of a computer visualization system. It’s really quite amazing work.

    Now there are chunks that are harder to see and harder to map just because we’re dealing with gas and dust, but in general it’s the Sloan Digital Sky Survey that is allowing us to do things like find tidal streams that are wrapped around the Milky Way Galaxy. It can find the shapes of arms; find the distributions of Stars in this direction and that direction.

    It’s just a matter of taking image after image after image and figuring out how the Stars are moving relative to the Earth and where they’re located relative to the Earth.

    Fraser: So, this Sloan Sky Survey is really looking at the distance to various Stars around us and then starting to map out the distributions. I guess if you just took each Star and if you had a 3 dimensional map and just keep dropping them into your map you start to go “oh, there’s an arm” right.

    Those Stars are all collected together into an arm and that’s how you know it’s an arm. Now, it’s not four arms anymore, right? I mean the most recent research is that it’s two arms.

    Pamela: Yes. And that actually starts to make a lot more sense because we think our Milky Way Galaxy has a bar in the center. It’s hard to figure out where the extra arms are coming from if there’s a bar.

    So, when we look out at other Galaxies whenever we see a bar, we consistently see two arms coming off the bar. Now our own Galaxy is starting to make a bit more sense.

    Fraser: Right. So I guess the quick answer is that Astronomers are painstakingly calculating the distance to every Star that they can see and then mapping that out in 3 dimensions until they start to get a sense of what the Galaxy looks like.

    Pamela: And I’d be remiss if I didn’t say the nearest Stars we’ve measured their positions with the highest accuracy using the Hipparcos Mission to measure Parallax. This is a many step process where there are estimations involved, but we have the nearest Stars nailed down using Hipparcos and proper motions and Parallax. Then we’ve worked our way outward using a variety of different methods.

    Fraser: We did a whole show on the various ways that you measure distance in the Universe so refer back to that. Alright, let’s move on. Graham Collins wants to know “Was Space always dark? If the Cosmic Microwave Background is now in the range of microwave radiation, but was once in the range of gamma rays, would it have at one point pass through the range of visible light? And if humans existed at that time in the Universe, would Space have appeared any different to them?”

    Pamela: Well, to start with. The Cosmic Microwave Background was never really the color of gamma rays. There was a point and time in the Universe when everything was so amazingly hot that the temperature would have corresponded to gamma rays. But that time came back before our Universe became transparent.

    Fraser: Right. That was the time when the whole Universe was like a Sun.

    Pamela: Exactly. Well, it was after the Sun part, but while the whole Universe was basically a hot dense plasma.

    Fraser: Right. But there was a later time where it was invisible light.

    Pamela: Right, exactly. So, this Universe did go through a phase where our current Cosmic Microwave Background would have been a Cosmic Optical Background.

    I don’t know what the luminosity of that was, but presumably if you had a big enough telescope you would be able to look out and see this constant glow in all directions. That’s just kind of cool to think about.

    Fraser: Wow. I never thought of it that way. But you’re right. If the CMBR was visible light at one time, there would have been a time when you could have seen it. But would it have been sort of before or after the Dark Ages? Right, this is the time when the Atoms in the Universe got to the point that they were transparent before that everything was opaque and you couldn’t see. And I can imagine you standing in the middle of it when it’s all light everywhere. You’d see light in all directions but if it was at the point if it got transparent.

    My question is, you know, if you’re standing say on the Moon and you’re looking off into Space, and the Sun is off to your left, you don’t see white beams of light coming off from the Sun. Right because it’s not bumping into anything. So even if there was, you know, you wouldn’t see the light everywhere, you’d have to see the light coming from something. Does that make any sense?

    Pamela: Yeah. Okay, so let’s step backwards and just think about the origins of the Cosmic Microwave Background. So, I’m trying to remember the exact color and I can’t.

    I believe when the Universe first became transparent and the Cosmic Microwave Background was released to run wildly across the Universe, when it was released it was off in the ultraviolet. The Universe at that point was completely transparent sort of. You had neutral gas everywhere. There were no Stars; it was now a different type of opaque.

    Instead of having the light not able to decouple from the Electrons and the Protons before the Cosmic Microwave Background was released a light beam as it tried to move from point A to B would get absorbed and remitted and absorbed and remitted constantly over and over and over. Then the Cosmic Microwave Background was released. That occurred at the moment when the Electrons coupled with the Atomic Nuclei and we ended up with a Neutral Universe.

    So now we have the Cosmic Dark Ages. There were no Stars hanging around, Galaxies were dark, and now we have a Universe that is again opaque but for different reasons.

    Fraser: But I guess, but just like then as in now, you can see the Cosmic Microwave Background Radiation as the afterglow of the Big Bang. You would see it then too, right? If you were standing in the middle of it, if you looked in all directions, you would see that afterglow. Because we still see it today.

    Pamela: Right. And it was a different color back then. Now what I don’t know because I haven’t run these calculations is how that color changed over time. There was a period when the Dark Ages ended where the Universe was flooded with ultraviolet light mostly coming from the Quasars basically just streaming light off as they were absorbing gas and dust.

    Most Galaxies went through a Quasar Phase. There was a huge number of Quasars early in the Universe that essentially re-ionized all of the gas and made the Universe completely transparent again. So, our Universe has gone through a variety of phase changes. What’s always been constant is this background flux of light. But it’s changed too, its color changed.

    Somewhere there is some theorist who has figured out at what point in the Universe that color was what color. If he’s out there listening, or if anyone out there listening knows that theorist, we’d love to know exactly at what time marker in Space and Time the Universe passed through say, blue.

    Fraser: Right, but what you’re thinking, and I’m sure we’re going to get a bunch of letters, that there was a time in the early Universe when you were in the middle of it, and the background to the Universe in all directions was blue.

    Pamela: And the question is was it a bright enough blue that the casual observer would be able to see it with their eyes? How big a telescope would they need if it was low flux? And when did that moment occur?

    Fraser: I think I hear someone’s research paper. [Laughter] That is a great question, Graham. Thank you. I think we’re going to be revisiting it. Let’s move on.

    Mark Trenchant asks “The Universe is expanding at roughly 70 kilometers per mega parsec but what is actually expanding? If I had a steel bar one mega parsec long, would it be getting longer at 70 kilometers per second or is Space stretching around it? Are the particles getting bigger or the gaps between them?”

    And Mark really wants to know is he’s getting bigger?

    Pamela: You know, this is a question that disturbed me when I was a small child because someone told me about the expansion in the Universe when I was far too young to grasp the problem. And I was like terrified that eventually that my head would get so big that the Neurons couldn’t communicate to each other anymore.

    Now, this isn’t happening. In fact, what’s expanding is the framework of Space in time. The mile markers in Space are moving apart. But things that are either electromagnetically held together like tables, chairs, human bodies, or gravitationally together like Galaxies, Galaxy Clusters, Stars, things like that, things that are held together with Forces they aren’t changing in size.

    It’s the framework of Space that is expanding. You can sort of imagine of taking a piece of graph paper that is made on elastic fabric. I don’t know why you would make elastic fabric into graph paper, but play along with me.

    If you set a hairbrush on that elastic graph paper, and stretch the graph paper beneath it, the number of boxes that the hairbrush takes up is going to get smaller and smaller as the boxes expand. But the hairbrush stays the exact same size. Well, within our Universe, as the Universe expands Galaxies are staying the same size. You’re one mega parsec long steel bar would stay the exact same size; it’s this graph paper of our Universe that is doing the expanding.

    Fraser: Right. So if you had a mega parsec bar sitting in one Galaxy and another mega parsec bar sitting in another Galaxy, the distance in between them would increase, but the bars themselves stay exactly the same size.

    Pamela: Or more to the scale factor if you had a mega parsec bar stretching between two Galaxies and you somehow nailed the Galaxies to either end of the mega parsec bar, all of that would stay the same size.

    Fraser: Yeah. It would stop expanding apart from each other because they were held together.

    Pamela: Right.

    Fraser: Cool. Alright. Well I think we are out of time for this week. So we will pick up more questions next time and we’ll do our next show in a couple of days. Alright, thanks Pamela.