There are approximately 120 of these in our solar system, but this number may double or triple in the next 10 years or so. What are they?

Our solar system is home to all sorts of stuff, most all of it ancient, forming several billion years ago. A good deal of debris was swept up by the Sun soon after it coalesced into a mid-sized star, some of which condensed into the nine spherical lumps of rock and gas we refer to as planets. In addition to the planets, our Sun plays host to an assortment of particles and plasmas, molecules, asteroids, comets, and both artificial and natural satellites.

There has long been a debate as to what constitutes a planet. Pluto, the last of the nine planets to be discovered, was found 73 years ago by Clyde Tombaugh, based on orbital calculations by Percival Lowell. However, because of it's eccentric orbit, on occasion it slips inside of Neptune's orbit, and it's puny size, in some circles it's often regarded as merely a captured chunk of cold stone, non worthy of planetary status. However, if Pluto is blackballed from the planet realm, that old mnemonic for memorizing the planets "My Very Excellent Mother Just Served Us Nine Pizzas." would result in an incomplete sentence. This alone is reason for keeping poor Pluto a planet. Anyway, this question isn't about the planets, rather it's about their moons.

When Pluto was first sighted, there were approximately 2 dozen known moons in the solar system. Now, there are at least 120. Obviously, our "seeing" power has increased over the decades, and we're able to detect ever smaller bodies further out in the solar system. So far this year, 18 new moons of Jupiter have been discovered by astronomers at the University of Hawaii, bringing the total of the biggest planet to 58! These little fellas may be only about a mile (1.6 km) across, and they were likely captured by big ol' Jupiter, not too long after it formed. David Jewitt, who announced the discovery of Jupiter's newest moons, believes that this number will eventually reach 100. When the Cassini mission reaches Saturn next year, it'll surely find myriad new moons, which have been here-to-for too minuscule to permit detection with our best Earth-bound telescopes and cameras. Even asteroids have moons. Thus far, about 30 asteroids have been identified having debris in orbit about them.

But is any coupling of boulders, pieces of asteroids or other debris, regardless how small, a moon? Well, maybe. After all, Irish Wolfhounds, Boxers and Chihuahuas are all dogs. While there's a consensus among astronomers that molecules and dust grains don't count, no one has yet drawn a line in the sand as to whether size or mass matters.

Astronomers have at least designed a loose system classifying moons based on distance to their parent planet. For instance, inner moons, those nearest to a planet, are relatively young and unstable. Since they're often associated with planetary ring systems, they're also known as "ringed moons." Next out are the regular moons, such as ours and most all of the other ones we can observe from Earth. Furthest out are the irregular moons, odd-shaped conglomerations that were likely captured by the parent planet over the eons. The orbits of some of these "rocks" are extremely eccentric. This is the region where the overwhelming number of new moons have been found.

The asteroids themselves are sometimes lumped into the "moon" bin. Most are unattached, but a number orbit Jupiter and are referred to as Trojan satellites. Even the Earth and Mars have a asteroid or two that buzz around them. However, we should be able to exclude those asteroids that don't orbit anything other than the Sun from the moon pile.

Another possible means to assess whether or not something is a moon is to determine just where it orbits its parent body. Satellites closely attached to their planet orbit directly over its equator. Those that don't have likely been captured, Therefore, maybe we can say that if it doesn't orbit in the plane of the planet's equator, it's not really a moon. There, that should settle it. But, hold on. Using this definition, we're excluding a pretty important hunk of real estate from moon status; namely ours.

Our own huge moon has an orbit that's tilted a bit. It revolves around us in a plane that's more or less the same as that of the ecliptic -- the plane the planets move about the Sun. It's actually tipped from the plane of the ecliptic by about 5 degrees. This means that it's considerably offset from our equator. It turns out, our moon is different in a lot of ways from other moons; it's unusually large, it's far away, and it's attracted by the Sun more than are other moons.

I'm proposing that a satellite can only be a true moon if you can stand on it without having to worry about falling off the darn thing. So now the question becomes, how massive does a hunk of rock have to be to have sufficient gravity to keep us on-board? It turns out that, if the density for an asteroid is assumed to be half that of the Earth's density, it needs to be a minimum of about 3.5 miles in diameter (5.6 km). Anything smaller than this and we could end up in orbit while playing a simple game of hoops. It should be pointed out that it's believed many smaller asteroids aren't very dense at all, they're more airy than rock-like. Anyway, in order to meet my criteria, we would have to know something about the density of every speck in our solar system -- a daunting task to say the least. So for now, as long as it orbits one of the planets and it's bigger than a grain of sand, it's considered to be a moon. Technically, at least, our faithful lunar companion has lots of company -- they just don't have names yet. Come to thing of it, though, we've never really given our big moon an honest name.

The next question will be answered in early January.

Wishing you the best during the holiday season!

This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Last year Dr. Foster decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.