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Authors: Carl Sagan
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discovered. A brief rendition of the Kant-Laplace hypothesis goes as follows: Imagine an irregular, slowly rotating cloud of gas and dust sitting between the stars. There are many such clouds. If its density is sufficiently high, the gravitational attraction of the various parts of the cloud for each other will overwhelm the internal random motion, and the cloud will start contracting. As it does so, it will spin faster, like a pirouetting ice skater bringing in her arms. The spin won’t retard the collapse of the cloud along the axis of rotation, but it will slow the contraction down in the plane of rotation. The initially irregular cloud converts itself into a flat disk. So planets that accrete or condense out of this disk will all be orbiting pretty much in the same plane. The laws of physics suffice, without supernatural intervention.
But predicting that such a disklike cloud existed before the planets formed is one thing; confirming the prediction by actually seeing such disks around other stars is quite another. When other spiral galaxies like the Milky Way were discovered, Kant thought that
these
were the predicted preplanetary disks, and that the “nebular hypothesis” of the origin of planets had been confirmed. (
Nebula
comes from the Greek word for cloud.) But these spiral forms proved to be distant star-studded galaxies and not nearby birthing grounds of stars and planets. Circumstellar disks proved hard to find.
It was not until more than a century later, using equipment including orbiting observatories, that the nebular hypothesis was confirmed. When we look at young Sun-like stars, like our Sun of four or five billion years ago, we find that more than half of them are surrounded by flat disks of dust and gas. In many cases the parts close to the star seem to be empty of dust and gas, as if planets had already formed there, gobbling up the interplanetary matter. It is not definitive evidence, but it strongly suggests that stars like our own frequently, if not invariably, are accompanied by planets. Such discoveries expand the likely number of planets in the Milky Way Galaxy at least into the billions.
But what about actually detecting other planets? Granted, the stars are very far away—the nearest almost a million AU distant—and in visible light they shine only in reflection. But our technology is improving by leaps and bounds. Shouldn’t we be able to detect at least large cousins of Jupiter around nearby stars, perhaps in infrared if not visible light?
In the last few years we have entered into a new era in human history, where we are able to detect the planets of other stars. The first planetary system reliably discovered accompanies a most unlikely star: B 1257 + 12 is a rapidly rotating neutron star, the remnant of a star once more massive than the Sun that blew itself up in a colossal supernova explosion. The magnetic field of this neutron star captures electrons and constrains them to move in such paths that, like a lighthouse, they shine a beam of radio light across interstellar space. By chance, the beam intercepts the Earth—once every 0.0062185319388187 seconds. This is why B 1257 + 12 is called a pulsar. The constancy of its period of rotation is astonishing. Because of the high precision of the measurements, Alex Wolszczan, now at Penn State University, was able to find “glitches”—irregularities in the last few decimal places. What causes them? Starquakes or other phenomena on the neutronstar itself? Over the years, they have varied in precisely the way expected were there planets going around B 1257 + 12, tugging slightly, first this way and then that. The quantitative agreement is so exact that the conclusion is compelling: Wolszczan has discovered the first planets known beyond the Sun. What’s more, they’re not big Jupiter-sized planets. Two of them are probably only a little more massive than the Earth, and orbit their star at distances not too different from the Earth’s
But predicting that such a disklike cloud existed before the planets formed is one thing; confirming the prediction by actually seeing such disks around other stars is quite another. When other spiral galaxies like the Milky Way were discovered, Kant thought that
these
were the predicted preplanetary disks, and that the “nebular hypothesis” of the origin of planets had been confirmed. (
Nebula
comes from the Greek word for cloud.) But these spiral forms proved to be distant star-studded galaxies and not nearby birthing grounds of stars and planets. Circumstellar disks proved hard to find.
It was not until more than a century later, using equipment including orbiting observatories, that the nebular hypothesis was confirmed. When we look at young Sun-like stars, like our Sun of four or five billion years ago, we find that more than half of them are surrounded by flat disks of dust and gas. In many cases the parts close to the star seem to be empty of dust and gas, as if planets had already formed there, gobbling up the interplanetary matter. It is not definitive evidence, but it strongly suggests that stars like our own frequently, if not invariably, are accompanied by planets. Such discoveries expand the likely number of planets in the Milky Way Galaxy at least into the billions.
But what about actually detecting other planets? Granted, the stars are very far away—the nearest almost a million AU distant—and in visible light they shine only in reflection. But our technology is improving by leaps and bounds. Shouldn’t we be able to detect at least large cousins of Jupiter around nearby stars, perhaps in infrared if not visible light?
In the last few years we have entered into a new era in human history, where we are able to detect the planets of other stars. The first planetary system reliably discovered accompanies a most unlikely star: B 1257 + 12 is a rapidly rotating neutron star, the remnant of a star once more massive than the Sun that blew itself up in a colossal supernova explosion. The magnetic field of this neutron star captures electrons and constrains them to move in such paths that, like a lighthouse, they shine a beam of radio light across interstellar space. By chance, the beam intercepts the Earth—once every 0.0062185319388187 seconds. This is why B 1257 + 12 is called a pulsar. The constancy of its period of rotation is astonishing. Because of the high precision of the measurements, Alex Wolszczan, now at Penn State University, was able to find “glitches”—irregularities in the last few decimal places. What causes them? Starquakes or other phenomena on the neutronstar itself? Over the years, they have varied in precisely the way expected were there planets going around B 1257 + 12, tugging slightly, first this way and then that. The quantitative agreement is so exact that the conclusion is compelling: Wolszczan has discovered the first planets known beyond the Sun. What’s more, they’re not big Jupiter-sized planets. Two of them are probably only a little more massive than the Earth, and orbit their star at distances not too different from the Earth’s
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