"Twinkle little star, how I wonder what you are . . ."
Today, you might just as easily find astronomers humming this nursery rhyme as well as children. Rapid advances in telescope technology adaptive optics, space observatories, interferometry, image processing techniques are allowing astronomers to see ever fainter and smaller companions to normal stars. As telescopic capabilities sharpen, conventional definitions for planets and stars may seem to be getting blurry. In the search for other planetary systems, astronomers are turning up objects that straddle the dim twilight zone between planets and stars, and others that seem to contradict conventional wisdom, such as a planetary system accompanying a burned-out compacted star called a neutron star.
Stars are large gaseous bodies that generate energy through nuclear fusion processes at their cores where temperatures and pressures are high enough for hydrogen nuclei to collide and fuse into helium nuclei, converting matter to energy in the process. Stars are born out of clouds of hydrogen, that collapse under gravity to form dense knots of gas. This collapse continues until enough pressure builds up to heat the gas and trigger nuclear fusion. The energy released by this "fusion-engine" halts the collapse, and the star is in equilibrium.
A star's brightness, temperature, color and lifetime are all determined by its initial mass. Our Sun is a typical middle-aged star halfway through its ten billion-year life. Stars can be 100 times more massive than our Sun, or less that 1/10 its mass. A Hubble Space Telescope search for dim stars suggests that most stars in the galaxy are about 1/5 the mass of our Sun.
Following a fiery birth, stars lead tranquil lives as inhabitants of the galaxy. Late in a star's life, fireworks can begin anew as changes in the core heat the stars further, eject its outer layers, and cause it to pulsate. All stars eventually burn out. Most collapse to white dwarf stars dim planet-sized objects that are extraordinarily dense because they retain most of their initial mass. Extremely massive stars undergo catastrophic core collapse and explode as supernovae the most energetic events in the universe. Black holes and neutron stars ultra dense stellar remnants with intense gravitational fields can be created in supernova blasts.
At least half of the stars in the galaxy have companion stars. These binary star systems can undergo complicated evolutionary changes as one star ages more rapidly than the companion and dies out. If the two stars are close enough together, gas will flow between them and this can trigger nova outbursts. Supernovae and novae are key forces in a grand cycle of stellar rebirth and renewal. Heavier elements cooked up in the fusion furnaces of stars are ejected back into space, serving as raw material for building new generations of stars and planets.
Though the universe contains billions upon billions of stars, until recently only nine planets were known those of our solar system. The Solar System provides a fundamental model for what we might expect to find around other stars, but it's difficult to form generalities from just one example. It may turn out that nature is more varied and imaginative when it comes to building and distributing planets throughout the Galaxy.
In it simplest definition, a planet is a nonluminous body that orbits a star, and is typically a small fraction of the parent star's mass. Planets form out of a disk of dust and gas that encircles a newborn star. These embryonic disks have been observed around young stars, both in infrared and visible light. The planets' orbits in our solar system trace out the skeleton of just such a disk that encircled the newborn Sun.
Planets agglomerate from the collision of dust particles in the disk, and then snowball in size to solid bodies that continue gobbling up debris like cosmic Pac-Men. In the case of our solar system this led to eight major bodies, thousands to tens of thousands of miles across. (The ninth planet, Pluto, is probably a survivor of an early subclass of solar system inhabitants called icy dwarfs). A planet's mass and composition are determined by where it formed in the disk. In the case of our solar system the more massive planets are found far from the Sun, though not too far where material didn't have time to agglomerate (because orbital periods were so slow that chances for collisions were minimal).
Unlike asteroids which are cold chunks of solar system debris, a planet must be massive enough to have at least once had a molten core that differentiated the planet's interior. This is a process where heavier elements sank to the center and lighter elements float to the surface. According to this idea, planets should have dense rocky/metallic cores. Depending how far they formed from their parent star, they may retain a dense mantle of primordial hydrogen and helium. In the case of our solar system this establishes two families of planets: the inner rocky or terrestrial planets such as Earth and Mars, which have solid surfaces, and the outer gas giant planets Jupiter and Saturn that are mostly gaseous and liquid. Massive planet like Jupiter are still gravitationally contracting and shine in infrared light.
Ironically, the first bonafide planetary system ever detected beyond our Sun exists around a neutron star - a collapsed stellar core left over from the star's self-detonation as a supernova. Resembling our inner solar system in terms of size and distribution, these three planets orbiting the crushed star probably formed after the star exploded. Apparently a disk must have formed after the stellar death, from which the planets agglomerated. Other suspected extrasolar planets also seem to defy conventional wisdom. An object orbiting the star 51 Pegasus may have the mass of Jupiter, but is 20 times closer to the star than Earth is from the Sun.
Brown dwarfs are the galaxy's underachievers. They never quite made it as stars. Like stars, brown dwarfs collapse out of a cloud of hydrogen. Like a planet they are too small to shine by nuclear fusion, and radiate energy only through gravitational contraction. (More massive brown dwarfs might have initiated fusion, but could not sustain it.) Their predicted masses range from several times the mass of Jupiter to a few percent the mass of our Sun. Spectroscopically, the cool dwarfs may resemble gas giant planets in terms of chemical composition.
A Color-Guide to Dwarfs
The different type of so-called "dwarfs" in the Galaxy would even befuddle the storybook character, Snow White:
White dwarfs Burned-out stars that no longer shine through nuclear fusion, and have collapsed to Earth-sized objects. Ironically, their surface temperature rises as they collapse and so the star is white-hot.
Yellow dwarfs Normal stars with our Sun's temperature and mass.
Red dwarfs Stars that are small, cooler and hence, dimmer than our Sun. The cooler a star the redder it is, just as a dying ember fades from yellow-orange to cherry-red.
Brown dwarfs Substellar objects that have formed like a star, but are not massive enough to sustain nuclear fusion processes.
Black dwarfs White dwarfs that cool to nearly absolute zero. The universe isn't old enough yet for black dwarfs to exist.