The essential difference between stars and planets is that when stars formed out of the giant molecular cloud which predated their stellar system, they were large enough that their gravitational pressure forced the hydrogen at their core to begin undergoing nuclear fusion, whereas planets were small enough that this never occurred. This basic difference in mass and size accounts for the other observable differences: stars are extremely hot and bright (but will eventually run out of hydrogen fuel, and either explode or collapse), whereas planets do not emit light (unless they are reflecting starlight), and may get colder, but will probably never destroy themselves.
- Stars -
Stellar systems, including our own solar system, form out of the collapse of giant molecular clouds - enormous clouds of mostly hydrogen and helium left by the supernovae and planetary nebulae of a previous generation of dead stars. Most of this mass - over 99% or so, in the case of our own Sun - is focused directly in the centre, where, within a few hundred million years, it becomes so concentrated that the heat and gravitational pressure begins to force hydrogen atoms to fuse together, forming helium. This process of nuclear fusion releases spectacular amounts of light, heat, and energy - essentially, it creates a star. This fusion process is something which occurs only within stars, not planets.
That stars are larger than planets, and therefore begin nuclear fusion (unlike planets), leads to a third difference: stars have a measurable, limited lifespan. Even within an object as massive as a star, there is a limited amount of hydrogen to be fused. How long it takes to burn through its fuel supply depends upon the size of the star. Extremely large stars are so powerful, in terms of gravity, that they force most of their fuel supply to fuse all at once, running out of hydrogen in as little as a few million years. These stars become so hot that they glow blue.
Much smaller stars, like our sun, can only force a smaller proportion of their hydrogen to fuse at a time. This means that they live much longer lives, typically measured in billions of years. Such stars are cooler, and glow yellow. The smallest stars, the so-called red dwarfs, can only fuse small amounts of hydrogen at a time. Because of this, they glow a dull red, but may live for hundreds of billions or even trillions of years.
The final separation between stars and planets follows from this process of nuclear fusion and exhaustion: stars, unlike planets, eventually die. How spectacular their death is, again, depends upon their size. To our knowledge, no red dwarfs have actually died yet in the history of the universe; when they do, they will likely simply become a heated ball of glowing helium remains, known as a white dwarf. Yellow stars, like our Sun, will rapidly puff up into a red giant as they run out of hydrogen in their cores; eventually, they, too, will become white dwarf remnants, though only after they have blown away much of their surface, forming a strikingly beautiful planetary nebula. The giant blue stars, however, are most destructive: no longer able to hold themselves together under pressure, they will explode as a massive supernova, or collapse under their own weight, forming (depending upon their size) either a neutron star or a black hole.
- Planets -
Planets - as the variety in our own solar system demonstrates - can be much more beautiful and varied than stars. In terms of what separates planets from stars, however, they are substantially less intriguing. Planets are simply those balls of rock (rocky planets, like Earth) or gas (gas giants, like Jupiter and Saturn) which formed out of the giant molecular cloud near a star and have since begun orbiting that star. Planets are only a tiny fraction of the size and mass of a star - in our case, all of the planets in our solar system put together weigh less than a percent of the mass of the Sun.
Like stars, what shape a planet takes does depend to an extent upon its size. Smaller planets, like Earth, are usually rocky: that is, they consist almost entirely of a mass of metals like iron and silicates, surrounded by a relatively thin atmosphere. Larger planets, like Jupiter and Saturn, form essentially in reverse: they have a small rocky core, surrounded by an enormous atmosphere of mostly hydrogen and helium, taking up many times the mass and volume of the Earth.
So far, scientists have been most effective in finding gas giants around other stars, because they are much larger and easier to detect. If, however, planets occur in more or less the same ratio in other systems as they do in our own (about half rocky planets, and half gas giants), then we can assume that the hundreds of gas giants we have discovered are accompanied by hundreds of rocky planets, as well. It is currently assumed that life has its best chance of evolving on rocky planets, like Earth.
Because they have not grown large enough to undergo nuclear fusion, planets do have the benefit of never dying, as stars do. That is, because they are not continually using up an internal fuel source to survive, they will essentially remain as they are indefinitely. This is not to say there will not be enormous changes: when the Earth formed, it was heated to the point that its surface was molten. Today, only its core remains molten; eventually, that, too, will cool and freeze.
Moreover, when a planet's sun catastrophically explodes or collapses, the planets themselves are often destroyed as well. In the Earth's case, it is likely that when the Sun begins to run out of hydrogen and puffs up into a red giant in several billion years, the Earth will fall into the outer layer of the Sun and be completely burned up.
- In Between: Brown Dwarfs -
Of course, if the difference comes down to size and mass, then this raises the question of what lies in between a star and a planet. After all, over the course of billions of years and across the vast expanse of the universe, there have been plenty of cases of stellar formation just at the boundary between star and planet.
One of these, CHXR 73B, was discovered about three years ago by the Hubble Space Telescope. At about a dozen times the mass of Jupiter, CHXR 73B is admittedly pretty large for a planet, but it would have to be several times larger again before it could force core hydrogen to begin fusing into helium. At the time, scientists tentatively identified it as a low-mass brown dwarf. Brown dwarfs are objects which have formed like stars, but have never grown large enough to force more than a small percentage of their heavier hydrogen atoms (a special isotope of hydrogen known as deuterium) to fuse together. Brown dwarfs are often referred to informally as "failed stars," since they have failed to begin the fusion process common to all stars.