Planets, Moons, Stars, and Suns
What are they?
Stars and planets are always round. If not perfectly spherical, they are somewhat elliptical in shape. Moons, for the most part, are also round. Only the very smallest moons can maintain a shape other than spherical. Typically, moons vary in size from a few miles across to a few thousand miles in diameter. Planets range from a few thousand to tens of thousands of miles across. And, where planets end, stars begin. But, for planets, moons and stars, there's more to their differences than size.
Stars are large, massive, balls of gas. It's hard to imagine this when we typically think of "gas" as something light and insubstantial. The gas in our own sun, however, weighs more than all the mountains on Earth in fact, much more than Earth and all the other planets in the Solar system combined.Stars are born in large clouds of gas, many trillions (1012) of miles across. Gravity causes the gas to compress. And compressing a gas gives off heat. Gravity continues to pile tons of gas on top of more tons of gas. All of that bone crushing weight compresses the gasses at the core until an unimaginable quantity of heat is trapped deep within this future star.
The heat released by that compression, if great enough, forces hydrogen atoms to combine to form helium. That combination releases far more energy in fact, nuclear energy. When a collapsing cloud of gas is massive enough to build and spark a nuclear furnace, it qualifies being called a newborn star.
There may be no official consensus on this point, but our own system may act as a model for the discussion. Until recently, the only system known to have planets was our own. The generic term for the star that warms these planets is, of course, the "sun."We call it "the sun," but its formal name is "Sol." That name forms the basis of the name of our star system the "Solar" system. In our world's many languages, speaking of our sun carries with it feelings of warmth and life-giving light. The term "sun" carries with it a sentimental attachment a relationship similar to that between parent and child. At the very least, our sun is the "campfire" around which we huddle to keep warm in the dark of space.
Generally, we don't talk of stars that way. This familiar star, our sun, gives our home planet the ability to harbor life. Reciprocally, without planets, a star has nothing to warm and nurture. Arguably then, a star could be considered a "sun" when it is found to have planets.
Imagine then, there are hundreds of known suns beyond our own. Many of those are multiple suns planetary systems with more than one parent star.
Planets are non-luminous, but reflective bodies, typically orbiting one or more stars. They seem to come in two varieties. There may be other varieties, but we may never find out about them until we travel to other star systems. The most familiar variety is called "terrestrial." These are worlds with solid surfaces (or, as in the case of Earth, a combination of solid and liquid). Terrestrials do not necessarily have atmospheres. In the Solar system, these include Mercury, Venus, Earth, Mars, and Pluto (well,... at least Pluto used to belong on the list).The other variety of planet is called "gas giant." In the Solar system, these include Jupiter, Saturn, Uranus, and Neptune. Though these may have rock and iron at their cores, that material is buried underneath a thick atmosphere. At the bottom of such an atmosphere, the air may have been compressed to the point where it is a liquid. For Jupiter, this is many thousands of kilometers below the visible cloud cover.
The boundary between gas giants and stars is a bit hazy. There is a point in the formation of a large planet where additional mass makes it a star. A newborn planet can become a star when its mass compresses the gasses at its core sufficiently to spark a nuclear reaction. The exact mass is still under debate, and mass is likely to be only one of several factors. There may be, however, a critical mass where other factors cannot prevent the nuclear furnace from igniting. Beyond this mass, a body would no longer be considered a gas giant planet, but a red dwarf star. Below that mass, the heat of compression may be sufficient to make the planet glow. These "failed" stars are called "brown dwarfs."
Moons are natural satellites of planets. Held by gravity, they orbit their parent body, much as our own moon orbits about Earth. Moons, however, don't have to be airless rocks. Titan, for instance Saturn's largest moon has an atmosphere made largely of methane.The simple definition of "moon" is any natural body that orbits a planet. This is compared to artificial satellites like Sputnik, and more recently, the International Space Station. With that definition alone, there must be many billions or even trillions of moons in our planetary system. Each of the four gas giants have rings made of countless rocks and icy debris. For practical purposes, though, such ring structures are thought of as whole units and not typically discussed in terms of their individual components. Whew! We could easily run out of names for all those would be moons.
There could exist twin worlds, where both could be considered moons of each other or moons of neither. Another possible scenario might include a habitable, Earth-like planet orbiting a dim red dwarf star, but gaining the bulk of its light and warmth from a second star, possibly the system primary, perhaps a G‑type star. Such a binary star system would be unusual, but not impossible. The habitable world would be a "planet," in that it is orbiting the red dwarf. Also, it could arguably be called a "moon" because it is orbiting a body (typically the position of a planet) that is orbiting a star (the G‑type, sun-like star). Another fact reinforces this fanciful argument. The bulk of the Earth-like world's light and heat comes not from its nearby parent, but the system's main star. So, even the line between "moon" and "planet" becomes a bit blurred.And, of course, there could be Earth-like worlds that orbit gas giant planets. Such a world was depicted in the popular motion picture, Star Wars a jungle planet orbiting the fictional gas giant, Yavin, in a galaxy far, far away.
Another oddity to consider are planets that don't orbit stars. First, what about planets that orbit brown dwarfs? Would these more properly be called "moons?" And what about those brown dwarfs that don't orbit stars? Also, consider planets that don't orbit any other body rogue worlds hurtling through the emptiness of space between star systems. Are such bodies possible? Perhaps a few might exist because they once belonged to normal planetary systems, and unluckily became deflected from their home by a rare, near collision. There are a small number of star systems in the Solar vicinity that have intruding stars hurtling dangerously close. Collisions are not likely, but some gravitational disruption is inevitable.
Opening new frontiers can sometimes prove to be a bit awkward without the right terms. Some might go so far as to say that "Earth" is a poor name for our entire home planet. As a generic term, "earth" means "dirt, soil, or ground."
Conversely, it might be said that the perfectly good name for our planetary system is being used too loosely. In recent years, it has been popular to talk of other "Solar systems." The intended implication is that there are other planetary systems out there, similar to our own. The choice of the term, "Solar system," however, is arguably a troubled one. Some would point out that there is only one "Solar" system, because "Sol" is the name of our own sun, and not a name to be used with other stars.
Some have even argued that "sun" should only be used as the name of our own system's primary star. The point is well taken, but without another generic term to describe stars with planets, "sun," for now, will have to do.
Some names stick, however, no matter how inappropriate. Native Americans, for instance, have long been called "Indians." Christopher Columbus had wanted a shorter route to the Asian sub-continent, but ran into America, instead. If some day humans homestead worlds beyond our own star system, the term "Solar system" might no longer refer to home. And the home planet of their ancestors might be misremembered as the planet "Dirt."