Sedna Dwarf Planet: Top 6 Essential Facts

Sedna was discovered in 2003 by astronomers Mike Brown and Chad Trujillo, the same team that has also discovered the dwarf planets Eris, Makemake, Quaoar, Salacia, Orcus, and Gonggong. Named after the Inuit goddess of the sea who, in legend, lived at the bottom of the Arctic Ocean, Sedna has been an enigma since its discovery, as there is no proven explanation as to how it got where it is, and its distance makes it difficult to study.

Sedna Discovery Image
Sedna Discovery Image of 2003.

Sedna currently hovers around 20th magnitude in apparent brightness (about 100 times dimmer than the limit for an experienced astronomer looking through an 8” telescope under ideal conditions)  and will never get significantly brighter than 19th magnitude, putting it outside the visual range of pretty much all amateur telescopes. However, it is bright enough to be imaged with a large telescope and lots of exposure time;  this has in fact been done by multiple amateurs (1) (2)

Fact 1. It is one of the reddest objects in the Solar System

Artist Concept of Sedna
Artist Concept of Sedna. Credit – NASA/JPL-Caltech

Sedna is one of the reddest objects in the Solar system, besides Mars. Unlike Mars, however, the dwarf planet’s coloring comes not from iron oxide (rust) but rather from tholins, an organic slush created over billions of years of bombardment by cosmic rays. Sedna’s surface is probably an upper layer of tholins covering a methane and water ice crust. 

In 2019, NASA’s New Horizons Spacecraft flew by the tiny Kuiper Belt object known as Arrokkoth. While only around the size of a large city, Arrokkoth is covered in a pristine surface of tholins much like Sedna is, and they are probably similar in color and in terms of overall geological smoothness and features (though Sedna is rounded unlike the two-lobed Arrokkoth). 

Fact 2: It is extremely far from the Sun

Sedna spends most of its time extremely far from the Sun – it happened to be discovered near its closest approach when it is the easiest to observe, which means there are probably many unobservable Sedna-like objects out there. Currently, Sedna is around 84 astronomical units from the Sun, or about 3 times the distance of Neptune! However, numerous dwarf planets, including the massive Eris and Gonggong, as well as the Voyager and Pioneer probes, are farther from the Sun than Sedna is as of the present day. Over the three years since we’ve updated this article, Sedna has moved about 1 AU closer to the Sun. Sedna’s perihelion will occur in July 2076.

Even at its perihelion (closest approach to the Sun), Sedna is still about twice as far from the Sun as Pluto, at about 76.2 astronomical units away. This means Sedna spends the majority of its time in “interstellar space” outside the bubble of particles and extremely thin plasma known as the heliosphere. 

Sedna’s distant perihelion means it cannot have been put there via a gravitational “slingshot” by any of the gas giant planets. We now know of many Sedna-like objects with similar orbits. It is possible that they are all the result of a close approach by another star shortly after the formation of the Solar System, but the leading explanation is that a fifth gas giant planet in a highly irregular orbit has been providing an unseen gravitational influence that tugs these objects into their current orbits.

Fact 3:  It has a strange orbit

One of the most intriguing aspects of Sedna is its remarkably elongated orbit. Sedna follows a highly eccentric path that takes it incredibly far away from the Sun at its most distant point, known as its aphelion, approximately 936 astronomical units (AU) away. An AU is the average distance from the Earth to the Sun, about 93 million miles or 150 million kilometers. At its closest approach, or perihelion, Sedna comes no closer than about 76 AU, still a significant distance compared to the orbits of the eight recognized planets in our Solar System.

Sedna’s unique positioning (being positioned for a perihelion, which will occur during the lifetime of many of our readers), its mysterious origins, and its fascinating surface composition make it ideal for a space probe to visit. The enormous SLS or Starship rockets would be needed, along with an immense nuclear power system (at twice as far from Pluto, a lot of power is needed just for communication with Earth) – along with assistance from the gravity of Jupiter, and possibly additional flybys of the Sun or another one of the giant planets to gain speed – all for a few hours of data collection as such a probe would fly by Sedna at a faster speed than anything launched by humans to date. 

Braking even a tiny spacecraft into orbit around Sedna for a landing or extended survey would require a level of enormous scale or technological sophistication that is unlikely to be possible by the time of launch, and with a launch ideally timed to occur in the 2050s, we’ll likely see scientific papers and increasing proposals over the coming decade as plans begin to coalesce for a mission. However, it’s completely possible that the world’s space agencies may simply choose to skip Sedna as it swings around the Sun. There are dozens of other icy bodies beyond Neptune that are equally worthy of exploration and perhaps more easily accessible or simply more tantalizing than Sedna.

Fact 4: It is very cold & inactive

As a result of being so far from the sun, most of the time the surface temperature on Sedna hovers slightly above absolute zero, so cold that any thin nitrogen atmosphere would remain almost always frozen. There is probably not much geological activity, and Sedna’s remoteness means there are probably few impact craters. As a result, Sedna’s surface may be quite smooth and uniform – probably one of the most pristine surfaces in the Solar System, untouched since its formation. For 200 years or so around perihelion (which it is heading for in 2076), Sedna does warm up to above 35 degrees Kelvin and could host an atmosphere of nitrogen and other evaporated gases from its surface slush; however, none has been detected at this time. 

Oddly enough, Sedna could still have a subsurface ocean, kept warm by the residual heat of its formation combined with the decay of radioactive elements(!) in its core. However, determining this would require at least a flyby probe, if not a near-impossible orbiting mission.

Fact 5: It is a member of the Oort Cloud (sort of)

The origin of Sedna is one of the ongoing mysteries in the study of the solar system. Sedna is an Oort Cloud object, a vast spherical cloud of icy bodies believed to surround the Sun at a distance of up to 100,000 AU. Sedna is, specifically, a member of the Inner Oort Cloud, or Hills Cloud, a very sparse torus-shaped cluster of icy bodies beyond the outer reaches of the Kuiper Belt. While the main Oort Cloud is a massive spherical shell of comets that could stretch over a light-year out from the Sun, the Hills Cloud seems to consist of larger and rockier bodies like Sedna. As we’ve mentioned previously, Sedna and objects like it are probably pristine samples of the Solar System’s earliest days. However, these objects did not form in their present-day positions. Sedna might have been perturbed from the Kuiper Belt, where Pluto resides, by gravitational interactions with a passing star or a yet undiscovered planet in the outer solar system. Sedna and the dozens of objects like it with similar orbits and compositions could not have been put in their present-day location by Neptune or another one of the gas giants, at least not by themselves. The gravity of a passing star or an as-yet-undiscovered ninth planet (in some form, be it a small gas giant or multiple solid bodies in a distant orbit around the Sun) had to have interfered. 

Sedna could also have been captured by our Sun from another star system entirely during a close encounter, but this would be extraordinarily unlikely. However, there certainly has to be something responsible for the existence of Sedna and other Hills Cloud objects in highly elongated orbits, as the probability of a coincidence is astronomically low.

Fact 6: It is the largest dwarf planet without a moon, and one of the larger ones without an accurate measurement of mass or size

Most of the large dwarf planets, such as Pluto, Eris, and others, have at least one moon – Sedna does not. It is possible that one has simply evaded discovery, but for now, it is more likely than not that Sedna lacks a moon of any sort. Without a moon, scientists cannot determine the mass of Sedna, and its exact size is difficult to determine accurately – estimates range from a diameter of 915 to 1160 kilometers, about half the size of Pluto or a bit bigger than Ceres.

The surface gravity of Sedna, as with any celestial body, is determined by its mass and radius. Accurate measurements of Sedna’s mass and radius remain uncertain due to its great distance from Earth and the challenges of observing such a distant and small object, especially without the presence of any moons. However, given that it is considerably smaller than Pluto and has a lower density (being composed mainly of ice with some rock), it’s safe to conclude that Sedna’s surface gravity is much weaker than Earth’s. This would affect everything from the shape and behavior of any potential atmosphere to the maximum size of surface features like mountains and cliffs. 

If Sedna is similar in density to Pluto or Ceres, which it most likely is, the gravity on the surface is somewhere between 2-4% of that of Earth, or ⅓ to ⅙ of that of the Moon, and the escape velocity of the dwarf planet would be about 500 m/s (about 1000 mph). You could easily jump over an apartment complex. Depending on Sedna’s rotation period, you could probably drive an F1 car (electrically powered, since there’s no atmosphere for combustion on Sedna) off a sloped race track and achieve orbital velocity should its rotation give you enough of a boost to get started (orbital velocity is somewhere around 200-300 m/s, and at the equator, Sedna’s rotation might get you a quarter to half that speed already).

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME Magazine, National Geographic, Sky & Telescope, La Vanguardia, and The Guardian. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars.

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