On Tue, Jan 22, 2019 at 06:41:36PM -0500, Peter L. Berghold wrote:
> Some of these:
>
> * Pluto is no longer a planet but a planetesimal -- curious as to how
> they came to that conclusion...

It's not a conclusion, but clarification of the meaning of the word
"planet" into something more objectively decidable.  Pluto meets two
of the three newer criteria.

Pluto is now the largest known Trans-Neptunian Object, with no
especially clear boundary between it and some of the other objects.
Its official classification is now a dwarf planet.  Ceres (in the
asteroid belt), and Haumea, Makemake, Eris (all TNOs) are also
recognised by the International Astronomical Union as dwarf planets.
There are numerous others that meet the criteria but have not yet been
officially designated as such.

If you accept Pluto as a planet by dropping the "gravitational
clearing" criterion, then our system has at least 20 planets.

- Tim