Caoimhín's Content

Today we will be continuing our look at various objects in the Solar System, which now means continuing with dwarf planets. I do of course want to continue reminding people about the currently visible comet, C/2023 A3, as well. It’s still there, just barely visible as we come to the end of October. This is more so true in the countryside, but we will stick with a city view for now.

The target of observation for this today is the dwarf planet Eris. Dwarf planets are often a little bit more eccentric or more highly inclined than the planet planets, or major planets. For this reason, we’re not guaranteed to get dwarf planets along the line of the ecliptic. Eris, at the moment, is thankfully pretty close to it. This means you can use the visible planets to help approximate where it is. If you wait until around midnight, after Jupiter has risen, Eris is almost in between Jupiter and Saturn. If you were to draw the curve of the ecliptic, Eris would be a fair bit below it, but close to between Jupiter and Saturn if you drew a straight, rather than curved, line. It’s in a region of the sky that looks pretty empty if you’re looking at it from the city, but of course Eris wouldn’t be visible to the naked eye even from the darkest countryside location. It’s in the constellation of Cetus, whose stars are mostly too faint to see from the city.

Eris is the dwarf planet. Eris was the object that motivated the reclassification of planets into planets and dwarf planets, the reclassification of Pluto. I have a piece and accompanying video on the distinction and possible issues with it, if you take a look back through the older posts. Eris is a little bit smaller than Pluto when it comes to diameter. It was listed at just 2,600 kilometers, but it is other sources give the even smaller value of 2,326 kilometers, with Pluto at 2,376 kilometers. Neither are particularly big, comparable to cutting out Russia or the continent of South America from the Earth’s crust and folding it into a hollow ball, that’s the kind of area these dwarf planets have. They are incredibly small objects, but Eris is a little bit more massive than Pluto, and because it’s more massive but smaller it must be more dense. It must be tightly aggregated, this is definitely a solid ball of rock, which has the implication that Eris has achieved hydrostatic equilibrium. Its current distance from the Sun is 95 AU, almost 100 times further from the Sun than we are. O course 95 isn’t exactly 100, but I’m rounding up. Eris is incredibly distant from the Sun and so of course it takes a very long time to orbit the Sun, we’re seeing a sidereal period of over 500 years, 556 years to be exact. Eris at the moment, it’s about magnitude 18.72, which makes Eris of course impossible to see with the naked eye, and despite varying with our respective orbits, it never gets bright enough to see unaided. It’s very difficult to see even with telescopes. Heading out into the countryside will make things better, you will get a slightly better view of Eris from the countryside, but it’s still going to be incredibly incredibly difficult to observe.

Eris was discovered by comparing the little bright dot, which you can see with a powerful enough telescope, against the background stars. In Stellarium, zooming in too close might remove any background stars behind Eris, but stars are visible around it at magnifications where it is visible, and more light sensitive equipment like cameras would pick out stars that are too faint to be included in Stellarium as well. By watching Eris over the course of, originally just a few hours, it visibly moves in the way an object orbiting around the Sun would be expected to move. Watching Eris over the course of a couple of days, you can see that it’s definitely moving compared to surrounding stars, it’s definitely orbiting the Sun. If a nearer object, such as an asteroid in the asteroid belt, that’s much much closer to the Sun, is also visible, you’ll see how much quicker it moves. An asteroid would appear to move way faster, zipping across the sky compared to Eris. Nonetheless, its motion be slow, it might be gradual, but Eris is moving around the Sun.

From a dark location and with a massive telescope, Eris is visible. The telescope I use as an example is a Meade LX200, a telescope which is 16-inches wide across the top. The LX200 is a Schmidt-Cassegrain, a type of reflecting telescope, but a modified reflecting telescope. These telescopes do not have an eyepiece at the side, but in the back like a normal refracting telescope. However, they do still have a secondary mirror, it just sends light straight back through a hole in the primary mirror. This keeps these telescopes from being too long, but still incredibly wide. Eyepieces, or oculars, help to focus on a particular object, they can add magnification but cut down the amount of sky by changing your field of view. The simulated example in the video also has a 3 times Barlow lens. This adds more magnification, making Eris even more visible. Even with a telescope this massive, if we didn’t have the help of those lenses it would barely be visible at all, and if we were using a different eyepiece, to try and get a wider view of the sky, that could also make Eris difficult to see. Some eyepieces have built in crosshairs or astrometric guidelines, and these can almost block it out the tiny dot that Eris usually appears as.

Going with a smaller telescope, such as going down there to one of the less wide modified reflecting telescopes will render it invisible. Even with a telescope 10 inches across, without Barlow lenses to help at least, it isn’t visible, but the addition of Barlow lenses can bring it into view, which certainly demonstrates the difference these things can make. To give a quick comparison, Eris is invisible using a telescope that would let you see Jupiter or Saturn, quite well. You would certainly be able to see many of their moons, even some of the smaller moons would clearly visible. Eris wouldn’t be visible at all, even with a telescope that large, mostly due to its distance rather than its size. Ceres is the closest dwarf planet to us, a dwarf planet that is technically visible through binoculars and telescopes. Ceres isn’t big, yet you can see it without using a crazy big telescope, whereas Eris is completely out of view, unless you’re a small observatory yourself.

Given that Eris is incredibly difficult to spot, it’s no surprise that it took a lot of observations for it to be spotted and confirmed. It took people deliberately hunting for something after Pluto, which of course they were. A large part of why Pluto was discovered is because Neptune’s orbit seemed a little bit strange. Neptune itself was, at least in part, discovered because Uranus’s orbit was a little bit strange, and people were looking for something big outside Uranus that could help explain the strangeness of it’s orbit. They found Neptune, and found that Neptune’s orbit was also a little bit strange, so people kept looking. They then found Pluto, but Pluto was too small to explain the changes in Neptune’s orbit, so people kept looking, and they found Eris. Eris of course is also quite small, and some astronomers believe that there is still another, larger, planet to be found.

Some other objects now classified as dwarf planets and other distant objects were discovered before Eris. Haumea, which is an official dwarf planet, and Sedna, which is at least a strong candidate. However, it was Eris’s estimated mass, being greater than Pluto’s, that really spurred to changes in classification. Its size is also very close to Pluto’s and this let people know that something was going to have to change. We were either going up to 10 planets in the solar system, with the outer ones being tiny, or we would need some other definition. With big round objects like Ceres already discovered and new discoveries coming in so rapidly in the early 2000’s, it seemed like the number could climb quite high, it would be 13 by now. When Eris was being discovered the astronomers knew that it was going to shake things up, which is one of the reasons it was called Eris. Eris was the goddess of strife and discord. Discord really means the inability to communicate, of course the communication software is a pun on this.

Dysnomia does not seem to be included in Stellarium software. Dysnomia is the moon of Eris. Eris has a moon along with many of the small dwarf planets and even smaller distant objects. Being way out towards the Oort Cloud and Kuiper Belt, there’s a lot of small objects flying around, a lot of objects that have not been consolidated into planets. Pluto of course has a bunch of moons, and some of these smaller objects may have more moons as of yet undiscovered. This may seem odd, given that much larger objects that are closer to the Sun, like Mercury and Venus, have none. This is partly because there’s less available little bits of rock. There’s just less available free objects to grab here in the inner solar system, whereas way out in the outer solar system there’s a lot of small objects flying around.

We’re going to shift our perspective to the Solar System Observer again. If you’ve read some of my other pieces, you’ll know this is a point high above the solar system, where you can look down and see the orbits of the planets. With the orbit of Eris visible, we can see that it’s far outside the planets. With the Sun in the center you can see that it Eris’s orbit isn’t very eccentric, but it is a little eccentric, and it is also reasonably inclined with regards the ecliptic. Moving through any small amount of time won’t let us see much of a difference in the position of Eris, but moving through years at a time will let us see a noticeable shift. At this rate we can see the inner planets looping around the Sun quite quickly, while Eris is just inching away along its orbit. As I said, it takes hundreds of years for Eris to complete and orbit, so it will take a lot of time for Eris to noticeably move, but year by year we can see it creeping along its orbit, just a little bit, nice and slowly.

As it orbits, its eccentricity causes its distance to the Sun and its distance to us to change as well. Going back a few a few decades its distance drops down to just 90 AU from us, 92 AU from the Sun, and that will keep changing as we go through the years. Of course, thanks to our orbit, our distance to Eris will change by about 2 AU over the course of the year. We’ll be 1 AU further away from it when we’re on the far side of the Sun and 1 AU closer to it when we’re on the near side of the Sun, so a range of 2 AU depending on the time of year. However, thanks to it’s inclination it can also vary a little bit more. The difference between it’s distance to us and its distance to the Sun can be greater than an AU. Eris could be way above us on the plane of the ecliptic or way below us on the plane of the ecliptic and that will influence its distance to us. Continuing through the years until we’re way back in the 1700’s, and we’ve barely gone through a quarter of Eris’s year. Jumping back to today causes a noticeable jump in Eris’s position, but we’d need to go all the way back to 1468 to see a complete orbit to today.

So that is Eris. The one, the only, the dwarf planet that’s caused all of the issues, it’s a big part of the reason why the category of dwarf planet is a thing. We discovered Eris, and we realized that Eris and Pluto have a lot more in common with each other than they do with the rest of the objects in our solar system. I do hope you enjoyed this little look at the cause of so much strife and discord. If you did, please do like it and if you’d like to support future videos and pieces you can also subscribe to this website and my YouTube channel. Hopefully, I’ll see you back here next time.