Stepping Beyond the Solar System

The astronomers said: "Give us matter, and a little motion, and we will construct the universe."

Ralph Waldo Emerson

We have been straying towards the fringes of the Solar System, and now we have just about reached the edge. From 1979 until 1999 Pluto was not the outermost planet, its eccentric orbit making Neptune the furthest from the Sun. In February 1999, Pluto again attained its status of the most distant.

That would only be a factual statement if there were no other major body yet awaiting discovery out beyond Pluto's orbit. Since 1992 astronomers have spotted some hundreds of minor planets in the region between about 30 and 60 astronomical units from the Sun, members of what is called the Edgeworth—Kuiper belt, recognizing the scientists who suggested their existence more than four decades before the first of them was discovered. Another collective name for them is the trans-Neptunian objects (TNOs). Pluto is about 1,410 miles in diameter, and is generally classed as being a major planet, the ninth in the Solar System. These numerous TNOs are mostly between 200 and 300 miles in size. In the year 2000 a new TNO was found that may be as much as 600 miles across, perhaps even larger than Ceres, the biggest minor planet (or asteroid—the terms have the same meaning) in the main belt. The nature of the TNOs seems to be quite different from the asteroids in the inner Solar System, however. Those appear to be rocky and metallic in composition, whereas TNOs are largely icy, like Pluto itself. In many ways it might be better to think of TNOs as being giant comets, thankfully keeping their distance from us, rather than classing them as minor planets.

Clyde Tombaugh discovered Pluto in 1930. For some years he had been diligently scouring photographs of the deep sky, at the Lowell Observatory in Arizona, before he eventually found a telltale moving point of light. At first Pluto was thought to be much larger than is actually the case, with a mass perhaps six times that of the Earth. Over the seven decades since our estimates of its dimensions have systematically downgraded it, and only recently have its mass and diameter been determined properly from eclipse observations. We start this chapter by considering Pluto's eclipses, and then see how the basic techniques employed can be extended beyond the Solar System.

Telescopes Mastery

Telescopes Mastery

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