Trans-Neptunian Objects

This post was originally published in February 2019, in the CNSE Herald, a student newspaper from my undergraduate institution.

Left: The bold and blue Neptune, the last planet in the solar system.
Right: The icy red Pluto, once the last planet, now a dwarf planet.

As children, we learned about the solar system, from the flaming Sun at its center, to the far reaches of the blue gas giant Neptune, along with the former planet called Pluto. But it turns out, there are more mysterious objects within our solar system beyond the reaches of Neptune, comprised of what astronomers call “trans-Neptunian objects.” Pluto is one example; it is obviously beyond Neptune. This special category of minor planets contains many astronomical objects in our own solar system that we still don’t know much about. And enigmatic celestial bodies are always worth investigating.

Pluto, one of the “closest” trans-Neptunian objects, is anywhere from 2.66 billion to 4.67 billion miles from Earth. In comparison, the Earth is 93 million miles from the Sun; roughly two orders of magnitude smaller. Keep in mind, that at that distance, light from the Sun takes a little more than 8 minutes to reach the Earth. In addition, light, in one second, can traverse the entire equator approximately 7.5 times. Needless to say, these Trans-Neptunian objects are quite far out: it took New Horizons, the fastest spacecraft to leave Earth, around 9.5 years to reach Pluto, the closest of these Trans-Neptunian objects. From here, these far-flung bodies will be abbreviated as TNO.

Let’s take a look at select TNOs. Tens of new TNOs have been discovered since Pluto’s discovery in the 1930’s. We will only cover four in this article. You might also want to review the planets of the solar system, just to get an idea of the astronomical scale of celestial objects and their orbits.

Note: Day and year are defined as they are on Earth.


Charon, as imaged by New Horizons.

This TNO is known as the largest of Pluto’s five satellites (more general term for moon). Discovered in 1978, it is named after Charon, a ferryman in Greek mythology who carried souls of recently deceased people into the underworld. It orbits around Pluto about every 6.4 days. Pluto itself revolves around the sun in an oval-like, elliptical orbit every 248 years or so. It is one of only three TNOs imaged up-close by a satellite (the others being dwarf planet Pluto and asteroid Ultima Thule).

Compared to other moons, Charon is quite large in proportion to its parent body, Pluto. Specifically, its diameter is only about half (50%) of Pluto’s, and has about one-eighth (12.5%) the mass. In comparison, the diameter of Earth’s moon is a little more than a quarter (27%) that of the Earth itself, and has about one-hundredth (~1.2%) the mass.

Charon’s north pole contains components of Pluto’s thin atmosphere. These include tholins, organic molecules created by cosmic radiation that may play a role in creating life. Ammonia hydrates and water crystals observed on the surface may be signs of active cryovolcanoes and cryogysters: geologic outlets that erupt volatile compounds like ammonia rather than lava. Such exotic volcanoes can only be found on the coldest reaches of the solar system.


Haumea (center dot) as observed by the Keck Observatory in Hawaii.


Artist’s impression of a rotating Haumea

Haumea is an unusual TNO discovered in 2004. It is named after the Hawaiian goddess of childbirth. Its mass is about one-third of Pluto’s. Haumea is farther out from the sun than Pluto; it takes 284 years for Haumea to orbit around the sun, unlike Pluto, which takes about 248 years. This TNO is best known for its highly oblong shape for a celestial object. More formally called an ellipsoid, Haumea’s major axis is about twice as long as its minor; in other words, it is two times as long in one dimension as it is in the other. This grape-like shape is believed to be caused by its rapid rotation: a day on Haumea is a little less than 4 hours. This is faster than any known stable object in the solar system.

Like many TNOs, Haumea has a very icy surface. No ammonia hydrates have been detected on it, suggesting an absence of cryovolcanism compared to Charon. Haumea has a small ring surrounding it, somewhat like the (much!) larger planet Saturn. In fact, it was the first TNO observed that had a ring. Like Pluto, Haumea has moons; two in fact.

A colony on Haumea would be the closest mankind will ever get to living on a grape*. Surgery and plasma generation will be insignificant milestones once this is achieved.

90377 Sedna

Left: Sedna as observed by the Hubble telescope. Right: Sedna’s extreme orbit relative to the planets of our solar system.

My personal favorite, Sedna is another icy TNO. It is named after the Inuit goddess of the sea. Sedna’s intrigue comes not from the object itself, but its orbit around the sun. It takes an inconceivable 11,400 years to complete its extremely elliptical orbit. To put that into perspective, 11,400 years ago humans were still in the Stone Age (more specifically the Neolithic era), about 9400 BC. According to telescope observations, Sedna apparently is one of the reddest objects observed, rivaling Mars in regards to color. It’s believed to be smaller than Charon.

The astronomer Michael Brown, who co-discovered Sedna (as well as Haumea), thinks Sedna has immense scientific value due to this extreme orbit. By understanding how its orbit was formed along with the planet itself, scientists could learn a lot about what the early solar system looked like. Sedna’s orbit has also been mentioned in the context of some rather interesting theories about the outer solar system, referring to the space beyond even Sedna’s exceptionally long reach. One such theory proposes the existence of an even more mysterious “Nemesis” companion star to our Sun (thus making our solar system a binary system), responsible not only for Sedna’s formation but also Earth’s mass extinctions from asteroid or comet impacts.

Sedna will reach perihelion (or the closest something ever reaches the sun) around 2076, where it will be about 7.10 billion miles out. Recall that Pluto is 2.66 billion to 4.67 billion miles away from the sun. A mission to Sedna could take about ~25 years at the shortest; the satellite would launch anytime between 2033 and 2046. Such a mission may be one of the most profound scientific investigations into space conducted yet. Only once every 11 millennia would humanity be able to directly observe an object that usually resides inconceivably far from our own planet Earth. Who knows what stories Sedna has yet to tell?

One thing’s for sure, Sedna would be a great place to put a time capsule. Or a prison.


Left: Eris (center dot) as observed by the Hubble telescope.
Right: Artist’s impression of Eris.

Eris, named after the Greek goddess of strife, is technically the most massive TNO observed to date. Discovered in 2005, Eris is very similar in size to Pluto (its diameter is only a few tens of kilometers smaller) and has a elliptical orbit like Sedna (though not to that extreme). It orbits the sun every 558 years. Eris’ previous name was Xena, named after the protagonist in Xena: Warrior Princess. Eris has a moon named Dysnomia, after the Greek goddess of lawlessness.

At the time, Pluto was considered a planet, so Eris was, for a time, the solar system’s tenth planet. After 2006, Eris joined Pluto as a fellow dwarf planet. Eris has about 27% more mass than Pluto, despite being slightly smaller. This suggests that Eris is largely comprised of rocky materials, more so than Pluto. In addition, Eris appears mostly white compared to Pluto.

Concluding Statements

These are only four of the many TNOs discovered to date. Some of them have names; others are just numbers and letters. Observing TNOs seem to be the stepping stone to characterizing, and understanding what lies beyond our inner solar system. Not to mention, all the objects beyond our solar system. Objects that would take tens, to thousands, to ten-thousands of human lifetimes just to reach, even if travelling at the speed of light. It is surely humbling to think about.

On a side note, TNOs seem like they are great, novel settings to science fiction. I wonder what life on a Sedna colony would be like.


I do not own any of the graphics I used for this article.

Information and Graphics Sources:

The TNOs’ wikipedia pages,,, NASA


*This sentence has not aged well, unlike a grape used for winemaking.