Such as? A planet(s), perhaps?
|Pluto closeup (Thank you, New Horizons)|
Help's on the way -- a new definition of planet has been suggested, the least of its features being a return to planetary status for Pluto. Said trial balloon has, as of yet, no official status, but still ...
To summarize that proposal, if an object is sub-stellar (and exhibiting, or having undergone, fusion is a pretty unambiguous characteristic) and it's basically round: that's a planet. None of the "cleared out its orbital neighborhood" judgment call, the cause of Pluto's demotion. The proposed rule would apply nicely to bodies orbiting other stars, where we have no possibility (for the time being, anyway) of knowing what orbital neighborhoods have or have not been cleared. By this definition, dear old Sol has about a hundred known planets (with the familiar Moon becoming our closest planetary neighbor)! For more about this proposal, see "Behind the Push to Get Pluto Its Planetary Groove Back."
It's an interesting concept, but I'm not completely on board. I like that mass -- which can be ascertained across even many light-years of distance -- is the determining factor:
- Anything massive enough will collapse to be basically round.
- Anything too massive will sweep (or have swept) up enough hydrogen from its precursor nebula to initiate fusion ignition.
Speaking of orbiting things ...