Sciency Words: Detached Object

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


The defining characteristic of detached objects is that they are, in a sense, “detached” from the rest of the Solar System.

They do orbit the Sun. They are part of the Solar System, but they keep their distance. Their orbits are so far away that they have no significant gravitational interactions with the eight known planets. As far as the detached objects are concerned, there may as well be no planets in the Solar System at all.

Fb03 Confused Sednoids

Hmm… Sedna and 2012 VP113 bring up a good point. While researching last week’s edition of Sciency Words, I initially thought “sednoid” and “detached object” were synonyms. But the list of known detached objects is a bit longer than the list of known sednoids.

The definition of sednoid is clear and specific: sednoids never come closer to the Sun than 75 AU. So if an object’s perihelion (point of closest approach to the Sun) were at 74.999999999 AU, that object would be disqualified from the sednoid club.

Detached objects don’t have to live with this kind of arbitrary restriction. If they stray within about 40 AU, they run the risk of gravitationally interacting with the planets. But it is that gravitational interaction that would change a detached object into some other kind of object; not the crossing of an imaginary 40 AU line in space.

Sednoids and detached objects are so new to our knowledge of the Solar System that the terminology is still evolving. You could choose to think of sednoids and detached objects as distinctly different groups, or you could think of sednoids as a subset within the larger population of detached objects.

It’s also possible that as we learn more about the Oort cloud (the existence of which has not yet been observationally confirmed) and the Ninth Planet (the existence of which has not yet been observationally confirmed), we may abandon this terminology in favor of new names that make more sense.

So in a distant Sci-Fi future when humanity ventures out beyond the orbit of Neptune to explore these strange objects and harvest their resources, perhaps we’ll invent some new sciency words that better describe them.

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