26 October 2019 | New Scientist | 23O
K, WHAT I’m about
to say may cause some
distress. You remember
how it was with Pluto? In 2006, the
International Astronomical Union
(IAU) voted to downgrade the
much-loved ninth planet to a
mere dwarf planet. More than a
decade later, it seems some people
still haven’t gotten over it. But
I’m still going to say it: things
are getting a little out of hand
with the solar system’s moons.
On 7 October, astronomers
announced the discovery of
20 moons around Saturn. This
took the total number of moons in
the solar system to 214. The thing
is, these new satellites are all about
5 kilometres in diameter. It would
take me less than 4 hours to walk
the entire circumference of one of
them. Compared with some of the
objects we currently call moons,
which would qualify as planets
if they weren’t orbiting another
planet (like Earth’s moon), these
are really just large rocks.
For all the controversy about
its redefinition of a planet, the IAU
has, as yet, no official definition
of a moon. Generally, planetary
scientists take a moon to be any
natural object orbiting a planet or
dwarf planet. Some add that other,
smaller objects can have moons as
well, including moons themselves,
whose theoretical satellites have
been dubbed “moonmoons”.
Some use the term “moonlet” for
small objects, for example those
orbiting within Saturn’s rings.
It is time for us to find a means
JOSIE FORDof distinguishing between real
Comment
Leah Crane is a reporter for
New Scientist based in Boston
@DownHereOnEarthViews
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That isn’t just because we are
discovering smaller and smaller
orbiting objects, but also because
the importance of moons is
growing. Three moons in our
solar system – Enceladus, Titan
and Europa – are widely agreed to
be the best places to look for signs
of life beyond Earth. What is
possible on these larger objectsjust isn’t feasible on a 5-kilometre
boulder with simple chemistry
and little surface diversity.
So where to draw the boundary?
The simplest way would be to set
a minimum moon size. Planetary
scientists often shy away from that
kind of thing, however, because
the sizes of objects can be difficult
to determine from afar. If we ever
detect an exomoon – a moonorbiting a planet outside our
solar system – it is likely that
we will be able to determine
its mass but not its radius.
Then perhaps we should
define a moon based on its mass.
One of the IAU’s requirements
for an object to be a planet is that
it has enough mass for its own
gravity to have pulled it into an
approximately spherical shape.
Icy objects become rounded
more easily than rocky objects,
so icy moons could be as small as
400 kilometres across, while rocky
ones would have to be closer to
600 kilometres in diameter.
That would exclude many of the
objects that we currently consider
moons, like Saturn’s ravioli-
shaped moons Pan and Atlas. We
would be left with only 19 known
moons in our solar system: four
orbiting Jupiter, seven around
Saturn, five of Uranus, one each
for Neptune and Pluto and, of
course, Earth’s moon. The rest
of the things we currently call
moons would be moonlets, or
perhaps dwarf moons.
This number of moons would
be a lot more manageable, and
would also mean that the group
of objects called moons would
have much more in common
with one another than they do
now. Job done. Now I will just sit
back and wait for the hate mail
saying I just demoted someone’s
favourite moon. ❚Shoot for the moons
New discoveries are degrading the concept of a moon. It is time
to do a Pluto and tidy up the solar system, says Leah Crane