The Pluto Files: The Rise and Fall of America's Favorite Planet Read Online Free Page A
Author: Neil deGrasse Tyson
the rescue, streamlining the task. Blink comparators exploit the remarkable ability of the human eye to detect change or motion amid an otherwise unchanging field: Place two photographic images of the same section of the sky, but taken at different times, side by side in precise alignment. Next, flash the two images back and forth in rapid succession. Against the background star field, any speck on the two photographs that brightens, dims, or shifts position from one image to the other becomes immediately apparent.
Percival Lowell died in 1916, but Clyde W. Tombaugh would later be hired by the observatory to carry on this arduous search, which led to the discovery of Planet X in 1930. The young fellow had been looking at a pair of photographic plates he took on January 23 and 29 of the region around Delta Geminorum, the eighth brightest star in the constellation Gemini. Tombaugh became the third and last person ever to discover a planet in our very own solar system.
In any well-designed, well-conducted survey, you don’t stop just because you’ve discovered something. By completing the survey, you might discover something else. So for the next thirteen years Tombaugh scoured more than 30,000 square degrees of sky (out of a total of 41,253 square degrees). He didn’t find any objects as bright as or brighter than Pluto. But the time wasn’t wasted. The survey discovered six new star clusters, hundreds of asteroids, and a comet and would stand for decades as the most thorough search of the outer solar system.
But was newly discovered Pluto the Planet X of everybody’s suspicions? Pluto was first presumed to be of commensurate rank in size and mass with Neptune, itself about 18 times Earth’s mass. If Pluto were to perturb Neptune with its gravity, as people suspected it was doing, then Pluto must be at least that size. But Pluto’s distance was far beyond the power of available telescopes to see anything other than an unresolved point of light. In fact, Pluto’s size and mass could only be guessed at based on Pluto’s brightness after you make an assumption about how reflective its surface is.
Figure 2.2. Clyde Tombaugh, age 22, poses proudly next to his homemade reflecting telescope. Two years later he would discover Pluto.
One clever method to estimate Pluto’s size, which gets you a little closer to its mass, is to time your observation for when Pluto moves against a background star, temporarily blocking the star’s light. When you combine the distance and orbital speed of Pluto with how long the star has dimmed, you can get a good estimate of Pluto’s width on the sky. As more and more stars passed nearer and nearer to Pluto, but without any dimming, astronomers were forced to continually downsize previous guesses for how large Pluto really is.
Figure 2.3. The original plot from Dressler and Russell (1980) in which they show the run of estimates for Pluto’s mass, dating back from when it was Planet X. The mathematical equation for M P (the mass of Pluto) is the best-fit model to the data, which carries the distressing news that if the trend continues, Pluto will disappear from the solar system by 1984. (From A. J. Dressler and C. T. Russell, “The Pending Disappearance of Pluto,” EOS 61, no. 44 [1980]: 690.)
In 1978, Pluto was discovered to have a relatively large, close-orbiting moon named Charon, allowing a quality estimate for Pluto’s mass. Thanks to a simple application of Isaac Newton’s laws of gravity, Pluto dropped precipitously from about Neptune’s mass to less than 1 percent the mass of Earth. A 1980 tongue-in-cheek article published in the geology newsletter EOS by A. J. Dressler, of Rice University, and C.T. Russell, of UCLA, plotted the mass estimates for Pluto, from its days as Planet X through the 1970s, and predicted that at the rate Pluto’s mass was dropping, it would disappear completely from the solar system by 1984 (Figure 2.3). 10
At this level, Pluto’s mass was far
Percival Lowell died in 1916, but Clyde W. Tombaugh would later be hired by the observatory to carry on this arduous search, which led to the discovery of Planet X in 1930. The young fellow had been looking at a pair of photographic plates he took on January 23 and 29 of the region around Delta Geminorum, the eighth brightest star in the constellation Gemini. Tombaugh became the third and last person ever to discover a planet in our very own solar system.
In any well-designed, well-conducted survey, you don’t stop just because you’ve discovered something. By completing the survey, you might discover something else. So for the next thirteen years Tombaugh scoured more than 30,000 square degrees of sky (out of a total of 41,253 square degrees). He didn’t find any objects as bright as or brighter than Pluto. But the time wasn’t wasted. The survey discovered six new star clusters, hundreds of asteroids, and a comet and would stand for decades as the most thorough search of the outer solar system.
But was newly discovered Pluto the Planet X of everybody’s suspicions? Pluto was first presumed to be of commensurate rank in size and mass with Neptune, itself about 18 times Earth’s mass. If Pluto were to perturb Neptune with its gravity, as people suspected it was doing, then Pluto must be at least that size. But Pluto’s distance was far beyond the power of available telescopes to see anything other than an unresolved point of light. In fact, Pluto’s size and mass could only be guessed at based on Pluto’s brightness after you make an assumption about how reflective its surface is.
Figure 2.2. Clyde Tombaugh, age 22, poses proudly next to his homemade reflecting telescope. Two years later he would discover Pluto.
One clever method to estimate Pluto’s size, which gets you a little closer to its mass, is to time your observation for when Pluto moves against a background star, temporarily blocking the star’s light. When you combine the distance and orbital speed of Pluto with how long the star has dimmed, you can get a good estimate of Pluto’s width on the sky. As more and more stars passed nearer and nearer to Pluto, but without any dimming, astronomers were forced to continually downsize previous guesses for how large Pluto really is.
Figure 2.3. The original plot from Dressler and Russell (1980) in which they show the run of estimates for Pluto’s mass, dating back from when it was Planet X. The mathematical equation for M P (the mass of Pluto) is the best-fit model to the data, which carries the distressing news that if the trend continues, Pluto will disappear from the solar system by 1984. (From A. J. Dressler and C. T. Russell, “The Pending Disappearance of Pluto,” EOS 61, no. 44 [1980]: 690.)
In 1978, Pluto was discovered to have a relatively large, close-orbiting moon named Charon, allowing a quality estimate for Pluto’s mass. Thanks to a simple application of Isaac Newton’s laws of gravity, Pluto dropped precipitously from about Neptune’s mass to less than 1 percent the mass of Earth. A 1980 tongue-in-cheek article published in the geology newsletter EOS by A. J. Dressler, of Rice University, and C.T. Russell, of UCLA, plotted the mass estimates for Pluto, from its days as Planet X through the 1970s, and predicted that at the rate Pluto’s mass was dropping, it would disappear completely from the solar system by 1984 (Figure 2.3). 10
At this level, Pluto’s mass was far
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