Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson

data, you don’t get anywhere near a planet’s worth of mass. You get about 5 percent the mass of Earth’s moon. So the prediction from Titius-Bode, that a red-blooded planet lurks at 2.8 AU, was a bit exaggerated.
    Most asteroids are made entirely of rock, though some are entirely metal and some are a mixture of both; most inhabit what’s often called the main belt, a zone between Mars and Jupiter. Asteroids are usually described as being formed of material left over from the earliest days of the solar system—material that never got incorporated into a planet. But that explanation is incomplete at best and does not account for the fact that some asteroids are pure metal. To understand what’s going on, one should first consider how the larger objects in the solar system formed.
    The planets coalesced from a cloud of gas and dust enriched by the scattered remains of element-rich exploding stars. The collapsing cloud forms a protoplanet—a solid blob that gets hot as it accretes more and more material. Two things happen with the larger protoplanets. One, the blob tends to take on the shape of a sphere. Two, its inner heat keeps the protoplanet molten long enough for the heavy stuff—primarily iron, with some nickel and a splash of such metals as cobalt, gold, and uranium mixed in—to sink to the center of the growing mass. Meanwhile, the much more common, light stuff—hydrogen, carbon, oxygen, and silicon—floats upward toward the surface. Geologists (who are fearless of sesquipedalian words) call the process “differentiation.” Thus the core of a differentiated planet such as Earth, Mars, or Venus is metal; its mantle and crust are mostly rock, and occupy a far greater volume than the core.
    Once it has cooled, if such a planet is then destroyed—say, by smashing into one of its fellow planets—the fragments of both will continue orbiting the Sun in more or less the same trajectories that the original, intact objects had. Most of those fragments will be rocky, because they come from the thick, outer, rocky layers of the two differentiated objects, and a small fraction will be purely metallic. Indeed, that’s exactly what’s observed with real asteroids. Moreover, a hunk of iron could not have formed in the middle of interstellar space, because the individual iron atoms of which it’s made would have been scattered throughout the gas clouds that formed the planets, and gas clouds are mostly hydrogen and helium. To concentrate the iron atoms, a fluid body must first have differentiated.
     
     
    BUT HOW DO solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid’s ability to reflect light, its albedo. Asteroids don’t emit light of their own; they only absorb and reflect the Sun’s rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you’re thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid’s surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid’s size. Ultimately you’re trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can’t know the answer simply by looking at how bright it is.
    This method of spectral analysis led initially to a simplified three-way classification scheme, with carbon-rich C-type asteroids, silicate-rich S-type asteroids, and metal-rich M-type asteroids. But