The very outer reaches of the Solar System is a region of space known as the Oort Cloud, which may extend as far as a light-year from the Sun. We only know about the Oort Cloud because that’s where long-period comets come from, randomly falling into the inner Solar System from time to time.
Fraser: Astronomy Cast episode 292 for Monday, February 4, 2013 – The Oort Cloud
Fraser: Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
Fraser: My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of cosmoquest.org. Hi Pamela, how are you doing?
Pamela: I’m doing well Fraser how are you doing?
Fraser: Great. I’m not sure if anyone is going to get this but we are going to be at South by Southwest in Austin for the South by Southwest interactive exhibit with NASA. We’re going to be near the great big model of the James Wood space telescope from March 8th to March 10th and so if you’re going to be in Austin, by all means come by and say hi to us.
Pamela: We aren’t going to be doing a meet-up because our meet-up is the NASA tent so come join us. There is no bracelet required and is open and free to everyone.
Fraser: We’ll be there for three straight days so by all means come by, say hi and shake our hands.
Fraser: In the very outer reaches of the Solar System is a region of space known as the Oort Cloud, which may extend as far as a light-year from the Sun. We only know about the Oort Cloud because that’s where long-period comets come from, randomly falling into the inner Solar System from time to time. So Pamela this is a funny thing where we have an object, a structure, a thing in space that we actually have never seen right? We have to presume that it’s there but we actually can’t see it.
Pamela: It’s really annoying because there are so many different things that it might be effecting. There are people that hypothesize that this continuous shell of icy material and dust around our solar system creates reddening. There are people that say that it’s impacting our ability to measure distances and perhaps it’s impacting how we observe the cosmic microwave background. We don’t know. There are all of these things that it’s presence may or may not be effecting and that’s kinda really super annoying.
Fraser: I had no idea, that’s really interesting. You can imagine that depending on what the composition of the Oort cloud, which we’ll explain in a second but now I’m just excited thinking about this; it’s actually distorting our view of the cosmos because it’s this bubble that could be this bubble around us. So for anyone who doesn’t know lets go back again and actually talk about what an Oort cloud is.
Pamela: It’s basically this two component glob of stuff. To get scientific,
Fraser: That was very scientific.
Pamela: (Laughs) There is considered to be, for the Oort cloud, two components. One is the big sphere that everyone talks about that is roughly a light year across. .8 lights years, to be more accurate, is it’s theoretical outer limit based on how far away something can be and still be gravitationally attached to our solar system. It’s considered to be the source of some of our longest-period comets and the things out in it are considered to be building in small moon-sized chunks of ice. Because we can’t observe these suckers, we really don’t know what their limitations are. This could be just a big brother to Kuiper Belt, in which case, we have a bunch more of these dwarf planets out there; it could be limited to smaller objects, we don’t know. Nested inside this big spheroid of material is what’s called the hill zone. These are a disc light component to the Oort cloud.
Fraser: So where does the Oort cloud come from?
Pamela: Well it actually came from a variety of different places depending on who and which theories that you adopt. Most likely it came from a combination of places during the formation of our solar system. The outer part of our solar system was mingling with the outer parts of other solar systems and we stole what we could gravitationally; some of the constituency of the Oort cloud is probably stolen material from other solar systems.
Fraser: If it’s this cloud a light-year or .8 of a light-year across, that’s a pretty big bubble that surrounds the solar system. You could imagine these star systems’ Oort clouds passing through each other and material jumping ship from one place to another.
Pamela: Yeah and one of the more intriguing things is that we probably have some stolen material and occasionally this stolen material gets gravitationally bumped until it comes into our inner solar system where potentially we can sample it.
Fraser: I know that when we do meteorite samples in the solar system we find that all of the meteorites tend to have the exact same formation date. They all formed 4.6 million years ago with the formation of the earth and the sun but can you imagine if we found a sample of one of these comets that had a different age?
Pamela: It’s harder to do that with ice because you’re dealing with ammonia, frozen methane, carbon dioxide, carbon monoxide and all these frozen gasses. It does get more difficult to age date them but you can look at the compositional radius and say the composition of this object doesn’t match anything else we’ve seen. The issue is that comets have a huge variety so we’re still trying to figure out what exactly is within a normal boundary parameter for a comet. The more we explore the Kuiper Belt the more we’ll understand what something that formed with our solar system should look like. In the future as we look at these long period comets coming into our solar system, hopefully we’ll be able to say which ones are natives and which ones are the explorers of other solar systems.
Fraser: So we stole material from other solar systems…
Pamela: That’s part of the source of the Oort cloud
Fraser: Right… Maybe… Probably… Who knows?
Pamela: The rest likely came from, what can best be described as, the angry dance of Saturn, Jupiter, Uranus, and Neptune in the early days of the solar system. As Jupiter and Saturn passed through, having resonate orbits, they basically flung material in all directions. Some of the material, roughly a quarter of it, got flung into the inner solar system and roughly a quarter of it got flung into the outer solar system entirely. Probably about half of this material ended up getting sent into extremely elliptical orbits; orbits that will mirror what we see for the long-period comets. Because of the nature of the interactions once that stuff gets out there the material does spend the bulk of its time in it’s most distant points in its orbit. This is true of every orbiting object. It’s the Kepler equal area and equal time law where when you’re in close you sweep out this very fast angle so you can sweep out an equal area to the amount that you… when you’re further out, sweep out and in the same amount of time you move much much slower. Now once the objects are out there they have the potential to gravitationally interact with other stars, with dark molecular clouds and with all these different things that we see as our solar system passes through the galaxy. All of these different interactions, even an Oort belt object on Oort belt object interaction, can work to smooth out the orbits to make them more and more spherical and more and more and more circular over time to create this distributed spheroid material.
Fraser: I guess that was leading into my question which is that if there is these interactions of the giant planets in our solar system kicking all of these objects out into the outer solar system, you would think they would all be on these parabolic orbits where they are going out and are all going to come back in and keep doing orbits like the comets do. I can see that once they get out there they have interactions and maybe interactions with other stars and it changes their orbits and free body interactions and eventually you get whatever remained is a cloud and everything else was gobbled up by the sun.
Pamela: The things that are on parabolic and hyperbolic orbits might pass through the inner solar system once and then they’re gone. It’s the elliptical ones: some of them remain as comets, some of them did have circular orbits and then got disrupted again and became comets but most of them the orbits relax over time and then we end up with this nice spherical component. We theorize it seems to match all of the comets that we see coming in; they have to come from somewhere but we don’t have certainty.
Fraser: So we’ve got an idea of maybe the source of this cloud, what about the discovery and the name? Where did that come from?
Pamela: The name comes from the person who finally got the theory listened into. This is one of those horrible examples of science that things get named after who has the most loud way of presenting the information. Back in 1932 Estonian astronomer Ernst Opik… I think I pronounced that one correctly, theorized that all of these long period comets that are coming at us from all different directions. Unlike most of the things that we see in our solar system, their orbits aren’t confined to the plane of our solar system. They don’t come from the same place that we see asteroids coming from rather they’re coming from all different directions, completely randomized. The only way to get to this completely randomized origins is if you have an orbiting cloud at the outer-most edge of our solar system. He put forward this theory and then in the 1950’s it was re-theorized a second time or rediscovered if you will by Jan Hendrik Oort. It was a way to try and understand where all of these comets are coming from and trying to understand why all of these comets that we see have such unstable orbits. When you see these sun grazers or when you see these clearly parabolic and hyperbolic orbits, which means they come in once and then are gone forever, those clearly aren’t things like the planets which are orbiting over and over and over. Where are they coming from that they can keep getting renewed? The only way to solve this paradox was to create a reservoir of comet material in our outer solar system so later in the 1950’s Oort made this postulation. Let’s face it, the word Oort is much more fun to say than Opik. Oort ended up getting to have his name associated with this cloud of material in the outer solar system.
Fraser: So what about the Hills cloud then?
Pamela: That again is a theorist came along worked on postulating how to explain all of the distribution of material and since it’s a mostly random but not entirely random distribution where there is this preference towards things being in the plane. That preference can get explained by a second component that overloads the plane of the Oort cloud.
Fraser: Is it one of those situations where there are extra comets coming from that region and then that would be explained by this gravitational…
Pamela: …Second component.
Fraser: Yeah, to give you this hills cloud. Again this is pretty tricky right because there is no observable evidence for this cloud at all.
Pamela: The way to think of it is if you’re getting sprayed with water you know there is going to be either a cloud or a hose involved. Depending on how you’re getting sprayed with water you can start to figure out what characteristics the source of the water must have. Here we’re seeing the outcome, the water falling on us so we have to put together the pieces of what must be some of the characteristics. If you can detail how far away the water is coming from you start to be able to place boundary conditions. The comets are the frozen water that is falling on us that places the boundary conditions on the physics that helps us describe this unseen frozen water faucet in the sky.
Fraser: Lets talk about the comets that are coming out of the Oort cloud because that is what we do have direct experience with. What kinds of comets do we get from this cloud?
Pamela: Well for the most part they are long period comets; things like hail bop that have orbits that are measured not in lifetimes but in generations or in the rise and fall of empires. These are thousand year periods in some cases. We also get a few interesting exceptions like Haley’s comet which through interactions that it had with Jupiter in the past, at least we thought it was Jupiter, it’s orbit got changed so that it’s now a shorter period comet but its crazy orientation indicates that it’s not one of the ones that originated in the Kuiper belt.
Fraser: The Kuiper belt object, where are we going to be looking at? What kind of period? These are the short period comets and they are measured in dozens of years.
Pamela: There’s tens of years, hundred years-ish, it’s that order of magnitude but with the long period comets you’re looking at thousands of years.
Fraser: Thousands and hundreds of thousands. So every comet is completely unique. The first time you see one of these long period comets you’re never going to see it again.
Pamela: That’s one of the frustrations and one of the other frustrations is even the Oort cloud objects that begin to dip their way into our solar system; they have such extremely long orbits that they may not spend very much time in an observable part of our solar system. A lot of scientists think that the dwarf planet Sedna is about roughly 1500 km across. It comes into just 75 astronomical units from the sun and that’s an extremely large distance. Compare that to the 35 to 45 of most of the objects that we’re looking at. That’s its nearest approach.
Fraser: And then it goes out to…?
Pamela: It goes out to 1000 astronomical units
Fraser: Yeah and takes 10,000 years or something like that to do it’s orbit. It’s crazy.
Pamela: We have other objects with less beautiful names like 2006SQ372. It’s a 100 km-across object and we were able to find it because it came all the way in to about 25 astronomical units so that’s inside the orbits of the outer-most planets and then it will go out to about 2000 astronomical units.
Fraser: By any other name these would be comets. If they got closer in to the sun they would…
Pamela: …but so would Pluto.
Fraser: So would Pluto, true. So would Enceladus. They would grow a tail and could you imagine if Sedna or one of these got within Mercury’s distance or Venus’ distance of the sun it would grow a tail. It would be unbelievable. Most comets are only, what, 10-20 km right? They’re small?
Fraser: So these would be the brightest objects ever seen, it would be unbelievable.
Pamela: That would be kind of cool.
Fraser: Wouldn’t it??
Pamela: Unfortunately the bigger an object is the more force is required to disrupt its orbit. The likelihood the big ones are going to get jostled enough to come in and pay us a visit is fairly low. Luckily the small ones are fairly easy to jostle.
Fraser: But the small ones are also very dangerous.
Pamela: Yes, Tunguska experienced that back in the early 1900’s out over Siberia where roughly 1000 square miles of trees got damaged and/or flattened. Windows shook for thousands of miles; it was a big event. We try to avoid getting too close to comets but they’re not something we can move our planet out of the way of.
Fraser: Yeah we’ve talked about this a bit in past shows. With an asteroid you can predict 100 years in advance that it’s in a dangerous orbit and you can take that time to research it and study and move a space craft out and try to use a gravity tractor, paint it, shoot it with nukes or whatever you’re going to do, you’ve got time. Even with the short period comets you’ve got time but with the long period comets you’ve got months and then POW.
Pamela: If that. One of the unfortunate things is that because they do have such highly elliptical orbits you basically are making a nice edge pass through the solar system. Depending on unfortunate geometric circumstances it could be that something comes in from behind the sun that we don’t notice until it’s making a pass, basically out of the part of the sky that the sun is located in, straight at the planet Earth.
Fraser: That is the worst case scenario…
Pamela: Essentially what happened with the small asteroid that blew up over Russia is it came out of the direction of the sun and we just didn’t have early warning and thus a billion rubles worth of broken windows and other damage.
Fraser: I’m getting surprised how big that asteroid was compared to what originally people were saying it was 70 tons and now it’s like 7000 tons. It’s actually a pretty big rock. So now you actually started off in the beginning of the show how this cloud might actually be effecting our view of the universe so can you talk about that a bit more? You’re freaking me out!
Pamela: (Laughs) So it’s probably only a few tens of earth masses worth of material so order 50-100 earth masses at most. That material in some cases is a sphere of basically dust. You have chunks of ice that are colliding with one another that are letting off a fine-grain particle as they crash into one another. If you’ve ever seen the images of geysers coming off of Enceladus there may be material like what you see coming off of the geysers created from the collisions of these objects over the billions of years. We don’t know that for sure. It’s could be that they’re just just nice chunks of ice that because they’re so far apart from one another, collisions are so remarkably rare that it’s a land of no dust… but we don’t know. If there this fine-grained particulate out there, this dust, it could be acting to scatter the blue light that is trying to travel its way into our inner solar system creating this reddening effect on everything we see as we try to look beyond the edges of our solar system.
Fraser: So what impact would that reddening have on our science and our understanding of the universe?
Pamela: It would mean that our understanding of the temperature of everything is just a little bit off. Not a lot but there have been people that have tried to explain some of the cosmic microwave background as perhaps being caused by effects of the Oort cloud. Adding a little bit of clarity or removing a little bit of clarity, people are making guesses and trying to understand what could be possible and we really don’t know. That’s one of the awesome things and horrifying things at the exact same time.
Fraser: For example, specifically, like with the cosmic microwave background radiation. The temperature changes that they are trying to detect are very minute.
Pamela: The nice thing is that this would be a constant effect assuming, and this is another huge assumption, that the Oort cloud has a perfectly smooth distribution. People have taken the time to try and look at that and try and find some sort of a variation that the largest scales could be explained by thickness variations in the Oort cloud.
Fraser: Like when you’re looking through the Hills cloud you’re looking through the plane, if ecliptic through the Oort cloud, maybe you’re going to get a different reading.
Pamela: Right and so these are all things that people are trying to understand. The data that we have so far hasn’t been a in a high enough resolution that we can actually make out any effects to the polarimetry or reddening that could be explained exclusively with the Oort cloud being the cause.
Fraser: We’ve talked about how the Oort cloud is invisible so what would it take to actually get out there and observe objects?
Pamela: A lot of time. It sounds like I’m being sarcastic but we’re talking about .8 light years distance. The best we can really do is wait for object after object to do like Sedna has done and come for a visit and over time build up the orbits of a small catalog of objects that come to us rather than us going to them. That’s the best we can do. More power to folks like Mike Brown who are out there trying to discover the largest objects in the Kuiper belt and the nearest objects of the Oort cloud.
Fraser: I remember, and this is just coming to me now, someone had put together a mission concept to be able to actually get a space craft out in to, at least, the near part of the Oort cloud. It would essentially be a space telescope sent out into the Oort cloud and it would just be observing objects and maybe fly past one if it could find one. It would take, as you said, 100 years to get out to a place where it could some science.
Pamela: Even more problematic than that is, well first of all you have light travel time so the signal time is going to take months to get back to earth if you do get a telescope out there. You’d have to launch a really large telescope though because the Oort cloud will be extraordinarily diffuse This isn’t Han Solo’s asteroid belt. Even our own asteroid belt isn’t Han Solo’s asteroid belt. We’re talking about tens of earth masses scattered over an entire spherical area that is .8 light years across.
Fraser: The kind of civilization that is able to study the Oort cloud is the kind of civilization that could send space craft to other stars.
Pamela: No actually that’s not true because it’s easier to see it from outside our solar system than it is inside. If there is an Oort cloud out there we can detect things like this around other solar systems. Detecting an Oort cloud from outside the system so you can get the distance so everything is compacted down and you are looking through the thickness at the edges. Just like looking at a Nebula, you can’t really see the Nebula if you are inside it. If you want to see the planetary Nebula you fly away. You see this ring where you are looking through the most parts of it. Detecting an Oort cloud you really want to be that other solar system not too far away.
Fraser: So do we see any Oort clouds around other stars?
Pamela: We have seen things that resemble the Oort cloud around other stars so this is where we continue to think that our understanding should be perfectly reasonable. Unfortunately things like nailing down things like exactly what it’s mass is, exactly what is its furthest limit and exactly what its inner limit is and those sorts of details we’re not there yet. In terms of a perfectly rational theory to explain the comets and to explain objects like Sedna, we’re on to something. We’re on to something that’s fairly typical to see within our galaxy.
Fraser: That’s awesome. Well thank you very much Pamela.
Pamela: My pleasure.
This transcript is not an exact match to the audio file. It has been edited for clarity.