George Dishman wrote:

At some point there needs to be a dividing line drawn
between a binary system and a star with a planetary
system. Based on your other posts, I would say the
latter is easy, it is one in which only one object
reaches the point of fusion. If more than one achieves
that then it is a binary star system, possibly also
including planets and other debris.

Thank you, George, for the clarification, and we're in
agreement here.


If none reaches fusion it could be harder for the
case of systems where no one body dominates. You
get free-floating double planetary systems I suppose.

This raises an interesting situation which I'm not sure
I've seen specifically mentioned in the papers I've read
so far on planetary definition.

However, it might be a nice kind of problem to illustrate
the difference between focusing solely or mostly on the
"characteristics" of an object (here two non-fusors, maybe
on the "superplanet" extreme of the spectrum, say 2-13
Jupiter masses), and also considering "circumstances" (here
that neither body is orbiting a fusor).

If we follow Basri, each would be a "planemo" or "planetary
mass object," but under a circumstantial definition of a
planet as a planemo _orbiting a fusor_, neither would qualify
as a "planet."

A more purely "characteristics-oriented approach" would say
that they're both non-fusors with sufficient mass to result
in near-sphericity or rounding -- and therefore planemos --
and that any planemo should be regarded as a planet, too.

Thus we indeed have a "free-floating binary planemo system";
and also also a "double planetary system," as you have
suggested, if we do not require that a planet orbit a fusor.

Anyway, that's my first take on this.

Most appreciatively,

Margo Schulter