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 ...
Dark matter -- something with the properties of (a) mass and (b) no interaction with light of any wavelength -- remains elusive. Astronomers infer the presence of lots of dark matter because the mass of merely visible matter doesn't account for observed galactic rotations. Alas, physicists have yet to identify anything as being dark matter.
A minority opinion among physicists and astronomers is that the problem isn't a failure to directly detect dark matter, but rather the inference that dark matter exists is mistaken. That is, perhaps we misunderstand how gravity behaves on a larger-than-solar-system scale. (These contrarian approaches are commonly lumped into the heading of Modified Newtonian Dynamics, aka MOND.)
|Putting a new spin on things|
Finally, self-described "Bad Astronomer" Phil Plait recently blogged about a truly amazing phenomenon. As he explains, there might be several explanations for a very odd observation. The one he finds most plausible is Seriously Keen. See "A 3 billion solar mass black hole rockets out of a galaxy at 8 million kilometers per hour. Yes, seriously." Wow.