A Ploonet is something that is something that has been hypothesized, but not actually seen. Ploonets are a shortened name for Tidally detached exomoons, or orphaned exomoons. These exoplanets were formerly exomoons of another planet, before being ejected from their orbits around their parent planets by tidal forces during planetary migration, and becoming planets in their own right.^[1] Currently, no tidally detached moons have yet been definitively detected, but they are believed to be likely to exist around other stars, and potentially detectable by photometric methods.

The name Ploonet looks to be a portmanteau of Planet and Exomoon, which is really interesting. The term “ploonet” was first seen when it was used in a paper published in the Monthly Notices of the Royal Astronomical Society,^[2] has received attention from mainstream media sources.^[2]

Researchers at Columbia University have suggested that a disrupting detached exomoon may be causing the unusual fluctuations in brightness exhibited by Tabby’s Star.^[3]

The name Ploonet does evoke goofiness, which is similar to another valid astronomical phenomena, a Blanet. Like a Ploonet, a Blanet is a funny astronomical term that is fun to think about.

Sources

[1] = Wada, K.; Tsukamoto, Y.; Kokubo, E. (31 July 2020). “Formation of “Blanets” from Dust Grains around the Supermassive Black Holes in Galaxies”: 1–16. arXiv:2007.15198
[2] = Starr, Michelle (10 July 2019). “Scientists Are Trying to Make ‘Ploonets’ a Thing, And We Are Here For It”. ScienceAlert. 
[3] = Greene, T. (2020-08-04). “Scientists: What if black holes had a safe zone where little planets could live? Let’s call them ‘blanets'”. The Next Web.

https://astronomy.com/news/2019/07/ploonets-when-a-planets-moon-goes-rogue