Once Around Sedna

[00:00:02]Once around Sedna.

[00:00:06]So, Sedna is a small icy world in the outer solar system, 8 billion miles from the Sun. At least at the moment, that's where it is.

[00:00:17]And out there in the cold, it's just 12° Kelvin. So, just 12° above absolute zero. Very cold indeed.

[00:00:27]This is the actual discovery image of it on the right. We're not seeing a huge amount of detail there. It's just a tiny little blob in the image.

[00:00:40]It was discovered by the team at the Palomar Observatory, Mike Brown and his crew.

[00:00:47]And what they did was photograph the night sky many times and track a small moving object. It's indicated by the yellow-green arrow in the diagram there.

[00:00:59]Just a tiny little dot that's hardly moved at all. Just 4.6 arc seconds over a period of 3 days of imaging.

[00:01:10]Hardly moved at all.

[00:01:13]So, when this was discovered, it was announced and the media got hold of it as the 10th planet.

[00:01:21]Uh the ninth planet at the time was Pluto.

[00:01:24]And this was a new world in the outer solar system. So, they immediately leapt upon it and decided that it must be the 10th planet. Um and I love the image of the 10th planet, Captain Future, on the right there.

[00:01:40]Now, it does seem to fit the dwarf planet designation. It seems to be large enough to be round. It orbits the Sun, and it has largely cleared its orbit.

[00:01:52]There aren't a lot of other objects trailing around in exactly the same sort of orbit.

[00:01:58]So, while it's not yet officially deemed a dwarf planet, I think most astronomers would probably give it that designation.

[00:02:08]Obviously, there's a certain amount of debate over these things.

[00:02:13]The name Sedna is officially adopted and is from an Inuit goddess of the sea and marine animals, following on from the sort of nautical theme of Neptune, I suppose, out there in the outer solar system.

[00:02:30]But, when it was discovered, Mike Brown and his team gave it a nickname of the Flying Dutchman because it had been in the images but not noticed for a very long time, in fact, as we shall see.

[00:02:47]And, of course, the Flying Dutchman sailed the oceans unseen. So, this seemed very appropriate.

[00:02:56]It's located 90 astronomical units from the sun, at least at the disc time when it was discovered. And again, there's another image there just of the little tiny dot circled in green, so you can tell which one we're talking about.

[00:03:12]It's three times the distance of Neptune, 30 AU for Neptune.

[00:03:18]Even further away than the dwarf planet Eris that was discovered beyond Pluto.

[00:03:24]And is was at the time the most distant member of the solar system to have been found, much further out than anything else.

[00:03:36]What always happens when something like this is located is that we look back through previous photographs over the many years, looking for a pre-covery, a a of the image in previous images that have been made and it was traced all the way back to an image on the 25th of September 1990, so giving us 30 years of tracking of its movement through a whole series of images.

[00:04:11]As we've said, the Flying Dutchman Sedna had gone unnoticed just because it is so far away that the relative movement against the background of stars had been missed when people were looking and searching for other objects. They were always looking for things that had moved to a greater extent.

[00:04:32]Now, this orbit might be 90 astronomical units at the moment, but that's because it's quite close to us in terms of its long thin elliptical orbit.

[00:04:44]You can see in the diagram there just about the orbit of Sedna is the huge ellipse and then the small purple orbit that's Pluto and then the green one is Neptune and all the other planets crammed together near the sun there.

[00:05:02]The uh period of its orbit is enormous. Pluto takes around 250 years. This takes 11,400 and it goes from where it is now near the sun and it's not quite at its nearest point yet. It's 90 AU right now. It will swing into 76.2 astronomical units, the Earth-Sun distance, as it passes perihelion and it's still twice as far away as Pluto's orbit.

[00:05:37]But then it will swing all the way out to an aphelion, the outer point of its orbit, the furthest distance from the Sun of 1,000 astronomical units and it will be moving incredibly slowly.

[00:05:52]Kepler's laws of planetary motion tell us that things move in ellipses and that they move quicker when they are nearer to the central point and slower as they climb away. Obviously, the total energy of the system stays the same. As you climb out of the gravity well, more of that energy is potential energy and so the kinetic energy of motion falls away.

[00:06:18]And out at the extreme, it's only going to be doing 377 m/s.

[00:06:24]So, if it were at the anywhere out there on its orbit, not only would it have been tremendously fainter and hard to see because the light from the Sun spreads out as it moves away over the surface of a sphere and the surface of a sphere is pi r squared, so the brightness of the light hitting Sedna will drop as the inverse square of the distance.

[00:06:52]And then the light reflected back has to cover the distance as well and so that will be reduced by the square of the distance on the way back.

[00:07:01]So, essentially, the light that we see coming back to us from these objects drops as the fourth power of the distance. It's quite incredible. So, at aphelion, we just wouldn't have been able to detect it.

[00:07:16]But we wouldn't have noticed it because the movement would have been so slow that we probably wouldn't have picked it up. It would have hardly changed from year to year.

[00:07:28]Now, we have been tracking its year-to-year motion and as a little diagram here of a couple of years of that from March 2003 through to 2005 and it's been watched moving in these loops, this a looping motion across the sky. And then this strange pattern is called retrograde motion. We see it in all the outer objects from Mars onwards.

[00:07:57]They seem to move backwards for a period across the sky and then move forwards again.

[00:08:05]Even the ancient Greeks had spotted this and the reason for it is the perspective, the parallax against the distant stars as the Earth overtakes it on the inside.

[00:08:20]And the relative motion between the line of sight from the Earth to the object against the distant stars swings in this loop because the Earth is moving rapidly from coming up to passing and then pulling away from the direct line between the objects and the Sun.

[00:08:43]So physically, the diameter is quoted as 995 km when you go and look it up, but actually this rather depends on how you choose to make your measurements.

[00:08:56]The margin for error goes from about 800 to about 1,100 km. So that 995 is a little bit too precise for my liking. It's somewhere in that range.

[00:09:11]And that makes it smaller than our moon.

[00:09:14]It makes it considerably smaller than Pluto and a little bit smaller than Pluto's moon, Charon. And maybe about the same or a little bit larger than the dwarf planet Ceres. So definitely in the dwarf planet size range.

[00:09:33]Trying to estimate its mass is difficult.

[00:09:38]There's no known moon orbiting it to allow us to use Kepler's laws to figure the amount of central mass from the orbital distance and the period, a very straightforward calculation.

[00:09:52]So, we don't really know how strong its gravity is. No spacecraft have been anywhere near it. We've not been able to track anything being affected by its gravity. So, it's a bit of a guess.

[00:10:06]Now, based on its approximate size and likely composition of mixture of ice and rock, it's around 10 to the 21 kg, roughly the same as Pluto's moon Charon, but really we are guessing based on very limited information.

[00:10:28]If it had a moon, we would certainly be able to start to pin that down. And the question of whether or not it has a moon has been looked into.

[00:10:39]Many of the small worlds, the dwarf planets and trans-Neptunian Kuiper Belt objects, Eris, for example, and uh beyond Pluto, Pluto itself, several of the other objects have moons that we have seen, and those have been very helpful letting us pin down the masses from those laws of planetary motion.

[00:11:04]Sedna, when it was initially watched carefully, was varying in brightness, and it seemed to be on a period of around 40 days.

[00:11:16]And this could either be that it was spinning on its axis in 40 days, which seems quite slow. Many of the objects seem to spin faster than that if they're of this sort of size.

[00:11:30]It could therefore possibly be that there was a moon being seen in a 40-day orbit, perhaps, disappearing behind Sedna into eclipse, and no longer contributing to the light flux that we were seeing, and then reappearing again.

[00:11:48]Um perhaps uh tidally locked with Sedna, who knows?

[00:11:54]But recently, further measurements have suggested that actually the true rotation period of Sedna is just 10 hours. Now, that's pretty fast, and uh so maybe the 40 days is, in fact, an orbiting satellite.

[00:12:14]So, we turned to the Hubble Space Telescope to it to have a look and to try to see if we could make any measurements to find any satellites, any moons of Sedna.

[00:12:28]Now, it's really quite difficult because even with the Hubble Space Telescope, with its uh fairly modest-size mirror compared to the largest ground-based telescopes, the brightness might be very good and the stability very good because it's not affected by the Earth's atmosphere, but the resolution is somewhat limited by comparison these days, and we've really only got sort of one pixel of decent information for Sedna.

[00:12:58]So, we didn't find anything.

[00:13:03]What we can do is see the color, and it's been described as the reddest object we have ever seen. Unfortunately, that label has been attached to other objects like the moon, uh the fifth moon of Jupiter, Amalthea, and uh various other objects have been described as the reddest object we've ever seen, but certainly very red in color.

[00:13:26]Fairly common in the outer solar system and due to photochemically produced carbon hydrogen oxygen chemicals which we term tholins for a sort of mixed bag of strange organo nitrate and nitrile and amino compounds that are created by the action of the ultraviolet light or radiation on the surface chemistry of these objects and the result is this mixture of chemicals which usually has this reddish color absorption pattern and gives the red color to it.

[00:14:09]Now you can see that in the spectra.

[00:14:12]The spectra are very complex because of the complex mixture of the tholins.

[00:14:19]But the James Webb Space Telescope which is really an infrared telescope, remember, turned its eye to Sedna and had a look and found that there was certainly some peaks there for a little bit of water and hydrocarbons, some organic molecules, a bit of methane, but relatively less methane than ethane.

[00:14:45]The uh CH4 versus C2H6 the next of the alkane organic molecules. So interesting that maybe the smaller size than than the like the likes of Pluto gives it lower gravity and the methane is able to escape whereas the slightly heavier molecules don't achieve enough speed to be able to get away. Certainly the temperature of 12 Kelvin helps to keep down any sort of kinetic energy of the molecules and therefore a to escape for the heavier ones.

[00:15:25]Now, we're not sure, but it's likely that a body of this size has undergone a certain amount of differentiation, would have formed, probably had a molten core, at least initially, and then the heavier materials, any metals, and the heavy rock would have sunk to the center, and the lighter volatile materials, particularly water and ice, would have floated over the top of that, being lower density. And so, there is a possibility that we have a core, maybe even a liquid water ocean under a thick ice crust. And uh this seems to be quite a pattern in outer solar system objects. They seem to end up being these sort of snowball worlds.

[00:16:16]This leads to a question about where Sedna has come from, and how a comet's in its orbit.

[00:16:24]We have a lot of objects beyond the Kuiper Belt. Pluto is really the prime object in the Kuiper Belt, a second asteroid belt that lives beyond Neptune.

[00:16:36]And further out than that, we find what's called the scattered disc with objects like Eris in rather more elliptical orbits. And these were probably hurled there, scattered by the gravity of Neptune early on in the history of the solar system.

[00:16:57]During the formation period of the solar system, as the giant planets were growing, particularly Jupiter growing to its massive size, sweeping up a lot of the material in the solar disc, growing and spiraling in towards the sun, the uh the laws of angular momentum conservation and the resonance between the planetary orbits probably caused Neptune to migrate outwards carrying away some of that angular momentum.

[00:17:28]And that disturbed the Kuiper Belt certainly preventing the formation of another large planet there and probably scattered a lot of objects that Neptune came close to as it migrated outwards hurling them further out into these weird elliptical orbits.

[00:17:48]But Sedna, its orbit is somewhat disconnected from that. It's twice as far away as Pluto during its closest approach to the Sun and goes way way way out to 1,000 astronomical units. So it seems too extreme to be part of this scattered disc.

[00:18:08]So this does raise the question, could it be that it's an interloper from the Oort Cloud?

[00:18:15]The Oort Cloud lies way out in the outer solar system right out at the boundary millions of icy objects left over from the creation process of the solar system and the source of many of the long period comets that seem to come in on thousand or million year long orbits looping in on very long thin ellipses perhaps having undergone some gravitational interaction with other bodies in the Oort Cloud and being deflected inwards.

[00:18:51]Now the Oort Cloud starts at about 1,000 AU from the Sun and that's roughly the outer point of Sedna's orbit. So it seems possible that it was a member of the Oort Cloud and was deflected inwards towards us. So being able to study it would perhaps give us a handle on some of what is going on out in that far outer reaches of the solar system.

[00:19:20]Perhaps a slightly wilder couple of ideas are that it may be an interloper from another star system. We've seen a couple of objects plow through our solar system, uh Oumuamua and Atlas and uh Borisov have all hurtled into the solar system on long hyperbolic orbits and then gone on their merry way. They essentially are interstellar comets.

[00:19:47]And perhaps Sedna was uh such an object that came in and became captured somehow.

[00:19:55]Perhaps a rogue planet or brown dwarf passed through the outer solar system scattering objects in its wake. Um perhaps Sedna was the planet of a passing star and got captured by the sun's gravity during a near approach.

[00:20:14]Whatever it was, we don't know about it.

[00:20:16]The culprit, if there was one, is now long gone and Sedna has been left in its orbit for perhaps a very long time. So, all rather speculative.

[00:20:29]Now, Sedna is not actually alone in its uh unique orbit. There are a whole group of bodies called the Sednoids. You can see the sun and the orbit of Sedna there and these other purple ellipses with their objects' catalog numbers like 2012 VP113 and others.

[00:20:54]And there's something rather curious about these in that they are all clustered such that their close approach, their perihelion to the sun, is almost in the same place.

[00:21:08]They all seem to cross their orbits on one side of the sun, and then their aphelions of their long ellipses are all over to the left-hand side of the diagram, pointing away to that direction, none of them to the right.

[00:21:25]Now, this could be a chance event, but the statistics suggest it isn't, and that there is probably some common common cause to this.

[00:21:37]It could be that there is an object that is corralling them into this, but whatever it is, it tends to rule out the idea that Sedna was just a random capture, because if it did happen, it seems to have happened a dozen times or so with all the different Sednoids there.

[00:21:58]The idea is perhaps that these objects are being scattered into this pattern by a ninth major planet in a very distant orbit from the sun that we've not detected, and there are lots of reasons to suggest that this might be the case.

[00:22:18]Long computer simulations of how these objects end up in this orbit really work best if there is a ninth major planet out there in an orbit as shown in orange.

[00:22:33]Now, we found a couple of new objects that kind of break the mold. There's this one perhaps in a similar path, but this one here, 2013 FT28, that seems to stick out on the wrong side of the the sun. So, maybe it wasn't a uh complete pattern, and maybe we were just lucky in finding a dozen objects all the same way. The jury is still out on this one. And I think the way that we are going to solve it is through the Vera Rubin Survey Telescope, the Large Synoptic Survey Telescope renamed after the excellent astronomer Vera Rubin.

[00:23:17]This telescope has a unique design to it, which allows it to scan 3° of sky and image then night sky in great detail over just a period of a few nights and then do it time and time again and it enables us to look for moving objects.

[00:23:41]And so maybe even finding the ninth planet will be one of its achievements one day and certainly tracking a lot more of these scattered disc and Kuiper Belt objects we are going to discover many, many more of them as a result of the data from the Vera Rubin telescope.

[00:24:02]And finally, perhaps we should remember that Sedna is coming to perihelion and it will be 2076 when it is at its closest passing the Sun and so perhaps now is a good time to start putting together a 25-year long journey for a spacecraft to go and meet it before it starts heading away back out and becomes so distant that there is little chance of doing that. I think we should get our act together and create a fleet of probes that we can send out to the outer solar system. I think we should build them all the same and like a class one probe and be able to therefore keep the cost of the design and the manufacture of these down that the Henry Ford probe and get it such that we can launch a whole fleet of them perhaps on one of SpaceX's giant rockets and really set to exploring all of these strange worlds of the outer solar system.

[00:25:12]So, that was once around Sedna and thanks very much for listening. I hope you enjoyed it and I hope you'll listen to other videos as they come up.

[00:25:22]Bye for now.