The biggest news last week had nothing to do with presidential elections, overseas financial crises, or fiddly football coaches. The really big news, the only news that will be popularly remembered 500 years hence, was that a human artifact was, for the first time, on the verge of departing our solar system, en-route to who-knows-what in the interstellar infinitude. That was the Voyager 1 probe, launched from Cape Canaveral in 1977 to tour the outer planets and collect data on the “interstellar medium” — the thin soup of particles, trace gasses, and other teensy cosmic detritus in the appalling vastness between the stars. Voyager completed the former mission decades ago, and is now on the verge of assuming the latter, powered on its way by radiation emitted from our ever-more distant sun. The news of Voyager’s imminent departure from our system was carried everywhere. Unfortunately, almost none of the coverage addressed two central questions: What do we mean when we say “solar system”? And: How do we know we’re out of it?
Say the words “solar system,” and nearly every American with an elementary school education will think of the following image: An orange ball surrounded by eight (or nine!) somewhat smaller balls, each fixed in a circular orbit. The image looks like this (see right):
As a pre-adolescent, I constructed more than one model solar system along those lines. I assume you did the same. But that solar system is imaginary.
As a matter of fact, an accurate model of the solar system is literally impossible to visualize. Most of its constituent parts are so far from each other, and so tiny relative to the distances involved, that the human eye cannot see them at once. Even if you were to reduce the sun to the size of a tennis ball, and reduce the sizes of and distances between other solar objects by a commensurate amount, even a keen-eyed human standing beside the sun would be unable to locate our system’s planets without a telescope.
Let’s say the sun really was the size of a tennis ball. If you like, picture it sitting in the turf, smack in the middle of the 50-yard-line at a football stadium. The nearest planet is Mercury, and you can’t see it. It’s less than half the diameter of the period ending this sentence, and it’s lost in the grass, ten feet from our tennis ball sun.
The next nearest planet is Venus, slightly smaller than a period. It’s 20 feet away. Earth is next — roughly period-sized, and orbiting the sun at a distance of 27 feet, brushing the sidelines twice per year. Then there’s Mars, smaller than a period, orbiting at 41 feet, which means it spends much of the Martian year floating amongst rich season ticket holders.
These are the inner planets, separated from their larger, outer siblings by an asteroid field and enormous distance. Mighty Jupiter, by far the largest and closest of the outer planets, is the size of a pea, orbiting at a distance of 141 feet. It spends nearly all of the long Jovian year in the stands, gliding through the end zones twice per orbit. Saturn, the size of a very sickly pea, never nears the field at all: it circles through the crowd at a distance of 247 feet. Uranus, the size of a BB, endures eternally in the cheap seats, at an average distance of 515 feet. Neptune, similar in size to Uranus and the most distant of the planets, seldom passes closer than 807 feet, which means it sometimes departs the stadium altogether.
(If Pluto was still a planet, it wouldn’t enter the stadium at all. It’s one sixth the size of a period, and subsists 1,060 feet in the icy distance, along with its many planetoid cousins.)
One might think that you leave the solar system by escaping the orbit of Neptune. Not so. The most commonly understood boundary of the solar system has nothing to do with the orbit of its farthest-flung planet, and everything to do with “solar wind” — the gale of energetic protons and electrons continually expelled from the sun’s corona at a velocity of 250 miles per second, and a rate of 4 billion tons per hour. At a distance of 12 billion miles from the sun, the wind has been so slowed by pressure from the interstellar medium that its speed passes below the speed of cosmic sound. Voyager 1 passed this point in 2004. Shortly beyond it, pressure and gravity cause the solar wind to begin moving sideways relative to the sun, as opposed to away from it. Then the wind disappears, and there is little but the interstellar medium between us and the stars.
This is the boundary that Voyager 1 seemed to approach early this month, 14 billion miles from the sun and receding at a rate of 38,000 miles per hour. (Which is to say: 1,500 feet from the tennis ball, and crawling through the stadium parking lot at a speed of one foot every two weeks.) Traveling at light speed, transmissions from the spacecraft reach scientists in a little over 16 hours.
But some scientists disagree about the solar system’s boundary. One popular notion places it not at the terminus of the solar wind, but rather at the Oort Cloud — an imponderably vast, roughly spheroid field of small, icy objects surrounding the sun, floating at a more-or-less static distance of 5,577,360,000,000 miles. That’s a light year; more than 60,000 times the distance between the sun and Earth. If the stadium housing our tennis ball is Miami’s Sun Life stadium, then the boundary of the Oort Cloud is somewhere in New Hampshire, a 1,400-mile distance. If we accept the Oort Cloud as the solar system’s boundary, Voyager 1 won’t leave our solar system for another 14,000 years.