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Sedna - Is it actually a dwarf planet?

The Origins

Sedna is one of the most distant known objects in our solar system, supposedly originating from a region of dust and ice called the Oort Cloud.

Truthfully, scientists as of yet have no idea how large the Oort Cloud might be, and even whether it exists at all, but some astronomers speculate that it could be as far away from the Sun as more than halfway to the next closest star to our own, Proxima Centauri.

The reason for its proposed existence is to explain the origin of the thousands of comets that fly into the region of the Solar System where we and the other 7 planets reside.

Sedna, as depicted in Universe Sandbox, a space simulation game that you can check out here

Sedna, a dwarf planet

A world beyond worlds

Sedna orbits the Sun at an incredibly far distance. When it comes to describing the orbit of an object, we can define two separate points on this orbit - the Aphelion (the furthest point to the parent body), and the Perihelion (the closest point to the parent body).

Like most of the planets in the Solar System, with Mercury being the only real exception, Earth's orbit around the Sun is relatively close to being spherical. While technically it is an elliptical orbit, it's stretched only very slightly in the scheme of things, with the Aphelion measuring 1.017 AU from the Sun, and the Perihelion measuring 0.9873AU. This gives an eccentricity (basically a measure of how round an orbit is), of roughly 0.0167.

For reference, 1 AU is the average distance between the Sun and the Earth, which is approximately 92 million miles.

You might be wondering how any of that is even remotely relevant to Sedna...

Well, Sedna's orbit is extremely elliptical.

In fact, it's one of the most elliptical orbits in our solar system. The Perihelion of Sedna's orbit is thought to be 76.4 AU, and the Aphelion is thought to measure an incredible 983 AU from the Sun!

Just to help put that into perspective, 1 AU is the average distance between the Earth and the Sun, meaning that at it's furthest distance, Sedna is almost 1000 times further way from the Sun than the Earth! Yet it can come as close as 76.4 AU, which is still 3 times further away than Neptune (which is the furthest known planet).

Honestly, I don't think numbers do it justice, so to help put a picture in your mind of the kind of distance and eccentricity that we're talking about, see this image below, which I captured from Universe Sandbox, a space simulation game that you can check out here.

The Solar System

The lines represent the orbital path of the celestial bodies in our solar system, and the bundle of white lines that can be seen represents the Kuiper Belt, which is a region of rock, ice and dust that lies beyond the orbit of Neptune. Apologies that it's hard to make out, but all the planets in our solar system reside inside of that bundle of lines, so you can just see how far away Sedna really is, even at its closest point...

In most of my articles where I talk about an object in our solar system, I usually prefer to stay away from just listing basic properties, and prefer to talk more about interesting mechanisms like magnetic fields, tidal forces, moons etc... But the reality is that we simply don't really know anything about Sedna. In fact, we don't even really know how large or massive it is.

Sedna is so far away that making any meaningful measurements is incredibly difficult. In particular, the fact that Sedna has no moons, and we're yet to send a space probe to orbit it, makes it impossible to know its mass within reasonable certainty, as we rely on measuring the effect it has on a body with a known mass to calculate the mass of the object at question.

Over the years, the estimations for the diameter of Sedna have been revised time and time again, from the first proposal made by Mike Brown, who co-discovered Sedna's existence, which placed an upper limit of 1800km, to the latest revision that predicts Sedna's diameter is between 900-1100 km. Either way, this puts it at roughly 0.08 (8%) times the diameter of the Earth, or 1/3 of the Moon.

Converting this then into volume, we can say that Sedna is estimated to be 0.0005 times the volume of Earth, in other words, 0.05% times the size.

The temperature

At 12 - 147 billion kilometres away from the Sun, the average temperature on Sedna is a beyond chilling -245 degrees Celsius, or -409 degrees Fahrenheit for the Americans, and doesn't experience much change in temperature at all - the lowest temperature is thought to be -243 degrees Celsius, and the highest -245.

The lowest theoretical temperature in the universe is -273 degrees Celsius (to 3 s.f), and it's theoretical because our current understanding is that it can never be achieved - to find out why, feel free to check out this article.

A dwarf planet or an asteroid?

So how about we answer the question of this article - is Sedna classified as a dwarf planet?

Well, technically, no.... or at least not by the IAU (International Astronomers Union), who are responsible for the declaration of names and classifications of celestial bodies.

However, the reason for its rejection is not because of Sedna's size, or distance from the Sun, as you might expect, but rather the strict set of rules that were introduced in 2006, which essentially said the following:

In order for to be classified as a dwarf planet, a celestial body must:

  • Orbit the Sun

  • Achieve Hydrostatic-equilibrium (we'll talk about that in a minute)

  • Not be a moon and;

  • Must not have cleared it's neighbourhood (what in the...?)

The 1st and 3rd ones are a tick for Sedna, but it's the 2nd and 4th that cause some controversy, and we'll start by addressing the 2nd point first.

Hydrostatic-equilibrium essentially means that an object is massive enough for it's gravity to force the object into a round shape. Unlike asteroids, which often come in very irregular shapes, the centre of mass of a dwarf planet must be massive enough for it to accelerate the surrounding matter to form a roughly uniformly round shape.

While the general consensus is that Sedna is massive enough for this to be the case, because it's simply so far away, we don't have the pictures as of yet to be able to assess whether it's in hydrostatic-equilibrium (ie it's round) or not.

At this present moment in time, Sedna is approaching its perihelion, at roughly 84 AU from the Sun, although that's still too far for our current equipment to be able to assess its shape in any detail.

To see this for yourself, here's the latest publicised picture of Sedna, which was taken by the Hubble Space Telescope. Image Credit: NASA

Image taken of Sedna

So what about the 4th point?

It means to say that in order for an object to be classified as a dwarf planet, it can't have cleared the surrounding area, essentially saying that it must not have total gravitational control of its orbital region. It appears as if Sedna does, but we're yet to actually observe Sedna for any reasonable length of time over its orbital period, which, speaking of, is thought to be around 10,000 years, with some estimations going as high as 12,000!

Just to clarify, the "orbital period" of an object refers to the time that it takes to complete one revolution around its parent body. In this context, we're saying that Sedna is thought to potentially take up to 12,000 years to complete one orbit around the Sun. In other words, a "year" on Sedna is potentially equal to 12,000 Earth years!

The reason why it takes so long to complete an orbit around the Sun is because at 78-983 AU from the Sun (or 12 - 147 billion km), the acceleration due to the Sun's gravity is extremely weak, which gives Sedna an average orbital velocity of merely 1 km/s.

Now, obviously that's still incredibly quick if we're comparing that to a car, but if we up the ante a bit, and start comparing that to say a rocket for example, which needs to travel at around 11 km/s to enter an orbit around the Earth, then we can quickly see how comparatively slow Sedna really is.

To take this a bit further, the Earth has an average orbital velocity of around 30 km/s, and Mercury, which is the closest planet to the Sun and hence the quickest (because it feels a greater gravitational acceleration due to it's proximity to the Sun), has an average orbital velocity of 47 km/s. So yeah.... Sedna moves pretty slowly.

At this point, we've pretty much covered all the basic, fundamental properties that, to be honest, aren't really that exciting, but you'll glad to hear that there are a few areas of conversation right now that are much, much more interesting.

The interesting stuff

They have to do with:

  1. Why Sedna's orbit is so eccentric and;

  2. The possibility of more "asteroids" like Sedna (remember that technically it's not a dwarf planet, yet...)

Addressing why Sedna's orbit is so eccentric is a difficult yet exciting practice among the astronomy community, because objects don't randomly fall into these very eccentric orbits.

There's a few possible explanations for the eccentricity of Sedna's orbit, but I'm only going to talk about 2 of them in this article, as they're the most convincing (and interesting).

In no particular order, the first explanation is that Sedna interacted with an object hundreds of millions, if not billions of years ago that either: no longer exists, or has moved on so far away that we can't reasonably say that it would've come into close proximity with our solar system.

We're pretty sure that none of the known planets could have interacted with Sedna whatsoever, as even Neptune doesn't come anywhere near close enough to substantially accelerate it with it's gravitational field.

However, a possible scenario could be that the Sun didn't form alone, but rather as a cluster of protostars (very small, young stars) that dissociated with each other over time, due to varying different gravitational interactions. While these protostars existed, they could've interacted with Sedna in a way that forced it into this highly eccentric orbit that we see today.

What's even more interesting though, is that Sedna could've originated from outside of our own solar system...

Perhaps Sedna originally belonged to the next nearest star system, the Alpha Centauri system, but was slingshotted so far from those stars that its acceleration back to them was too small, and it ended up travelling 4.3 light years to our own solar system. It's proximity with the Sun now could've tugged it into this highly eccentric orbit.

Obviously it's hard to prove whether that could be true or not, but it makes finding out the elementary composition of Sedna all the more important, as it could potentially hold material from a completely different star system.

So that's the first interesting topic of conversation - why Sedna's orbit is so eccentric, but what about the 2nd - could Sedna be alone?

It very much follows on from what we've just talked about, as I mentioned that Sedna couldn't have interacted with any of the known planets in our solar system, as it simply doesn't go anywhere near close enough to even Neptune, but there is another possibility - perhaps Sedna isn't alone.

Going back to my very first point in this article, at the time of writing this, Sedna is the furthest known object from the Sun that's within our solar system, and the theory goes that it originated from the Oort Cloud, which is a hypothetical region of the solar system that could extend as far as even 75% of the way to the next closest solar system, the Alpha Centauri system.

It's thought to comprise of billions of comets, many of which end up intruding into our more local region of the Solar System - basically anything within the Kuiper belt (a region of dust, rock and ice that lies beyond Neptune).

Nobody knows whether it exists or not, but the point is that Sedna is the only known object that's distance from the Sun suggests that it could've come from there. If there are more objects like Sedna out in those regions, then they could've very well interacted with Sedna in such a way to slingshot it inwards, towards the Sun, which is where we see it today (not close to the Sun, but making it's way over in that direction). Remember from earlier that Sedna's orbital period is thought to be over 10,000 years, which means that humans have never observed it beyond a fraction of the distance of where it lies now.

This very theory begs the question of if there could be a planet, or perhaps even multiple planets beyond Neptune that we're yet to discover that pulled Sedna in, before slingshotting it into the more inner regions of the Solar System - a potential planet 9, 10, 11 and so on....

Something that might seem off-topic at first, but does relate to this, is that Sedna is very uniformly the same colour, which is a deep red, similar to that of Mars.

It's thought that this colour could come from a surface coating of hydrocarbons that reflect the wavelength of red. More importantly though, the uniform colour suggests that Sedna hasn't been involved in any collisions that would've removed regions of hydrocarbons, leaving dark patches behind.

Why is that important?

Well it might throw a spanner into the works, if we're considering that Sedna would've spent a substantial amount of time in what is theorised to essentially be a ball pit of comets (the Oort Cloud), but perhaps either the Oort Cloud just isn't very dense, or that Sedna spent relatively little time inside of it, and managed to escape unscathed...

I think that just about wraps things up for all-things Sedna...

It can't be reiterated enough that this is an extremely distant world that we really don't know much about.

What's there though is extremely fascinating, and discovering more about Sedna and its origins could open up a whole host of new theories for the formation of our solar system all together.

Thanks for reading, and if you found any of this interesting, then I'd recommend sticking around and perhaps becoming a Hero (signing up), here at Expansive!

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