Word of the Month: Trans-Neptunian

Each month, Scott Levine looks at a single word that can tell us about the workings of the Universe, with a quick sketch to outline his thoughts.

This time, the end of the Cassini mission around Saturn gets Scott thinking about the outer reaches of our Solar System.

"It’s enough to make those empty patches of sky seem a little bit less empty."
Credit: Scott Levine


As I sat outside last night, looking toward Saturn and feeling nostalgic about the discoveries Cassini has given us over the years, my mind wandered toward the deep end of the Solar System.

Sliding though what looks to be a mostly empty patch not far from Saturn, the New Horizons probe, which sped past Pluto in 2015, is on its way toward a far-off object called 2014 MU69. 

Pluto and MU69 are part of a large group of bodies called Trans-Neptunian Objects (TNOs), which sound dark and mysterious; like something from a spy thriller with trains, suitcases and silent nods.

It’s actually a blanket term for several sub-classes of things that orbit the Sun past the orbit of Neptune, where the Kuiper Belt begins; more than 30 astronomical units from the Sun.

As an astronomical unit is the average Sun-Earth distance, about 150 million km, these objects are all at least 5 billion km away. 


An artist’s impression showing humanity's exploration of the Kuiper Belt so far.
Credit: NASA


I thought about when I was first falling in love with astronomy. Then, Pluto was still considered a planet, and we didn’t know of much beyond it.

Since 1930, when Pluto became the first TNO discovered, more than a thousand others have been found.

Thanks to some of these, for instance Eris, we’ve clarified what it means to be a planet, and classified new types of objects.

Astronomers have even subdivided TNOs into exotic-sounding things like plutoids and cubewanos. With each new discovery, we learn a little more about our Solar System and a little more about ourselves – about how we fit into it all and where humans can go.

While I stared at that empty patch of sky, I thought about the end of Cassini, about New Horizons, and about the Voyager probes, which are tiptoeing into interstellar space.

It’s exciting to think about what discovery will be next, no matter how small; indeed, it’s enough to make those empty patches of sky seem a little bit less empty.


About the author

Scott Levine is a dad and astronomy lover who stares at the sky over his family’s home north of New York City. You can read more of his light-hearted look at astronomy at Scott’s Sky Watch.


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