In the 18th and 19th centuries, astronomers proposed that the planets accumulated out of a flat disk of gas and dust surrounding the early Sun, which also was newly formed from the disk. The idea was called "the nebular hypothesis" (nebula comes from the Greek word for cloud).
In the last 15 years, the nebular hypothesis has been confirmed, using orbiting observatories and other instruments. When we look at young Sunlike stars -- such as our Sun was 4 billion to 5 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 closer to the star seem to be empty of dust and gas, as if planets already had formed there, gobbling up the interplanetary matter. It is not definitive evidence, but it strongly suggests that stars like our own Sun 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. If we call the distance from the Earth to the Sun one Astronomical Unit (1 AU), the nearest star is almost a million AU distant -- and in visible light, planets 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, the largest planet in our solar system, 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, in which we are able to detect the planets of other stars. The first planetary system reliably discovered accompanies a most unlikely star:
B1257+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 paths such 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 B1257+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 neutron star itself? Over the years the pulses have been found to vary in precisely the way expected if there were planets going around B1257+12, tugging slightly, first this way and then that. The agreement is so exact that the conclusion is compelling: Wolszczan has discovered the first planets known beyond the Sun's.
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 distance from the Sun: 1 AU.
Might we expect life on these planets? Unfortunately, there is a gale of charged particles hurtling out of the neutron star, which will raise the temperature of its Earthlike planets far above the boiling point of water. At 1300 light-years away, we will not soon be travelling to this system. It is a mystery whether these planets survived the supernova explosion that made the pulsar or were formed from the debris of the supernova explosion.
Shortly after Wolszczan's epochal discovery, several more objects of planetary mass were discovered (mainly by Geoff Marcy and Paul Butler of San Francisco State University) going around other stars--in this case, ordinary Sunlike stars. The technique used was different and much more difficult to apply. These planets were found by conventional optical telescopes monitoring the periodic changes in the spectra of nearby stars. Sometimes a star may be moving toward us for a while, then away from us. We know this by observing the changes in the wavelength of its spectral lines -- known as the Doppler effect -- akin to the changes in frequency of a car's horn as it drives toward or away from us. An invisible body is tugging at the star.
The planets responsible go around the stars 51 Pegasi, 70 Virginis and 47 Ursae Majoris, respectively, in the constellations Pegasus, Virgo and Ursa Major, the Big Dipper. Just this year, such a planet was found orbiting the star 55 Cancri in the constellation Cancer the Crab. Both 47 Ursae Majoris and 70 Virginis can be seen with the naked eye in the spring evening sky. They are very near as stars go.
The masses of these planets seem to range from a little less than Jupiter to several times greater than Jupiter. What is most surprising about them is how close to their stars they are, from 0.05 AU for 51 Pegasi to a little more than 2 AU for 47 Ursae Majoris. [AU = astronomical unit]
There may of course be smaller, Earthlike planets in these systems not yet discovered, but the layout is not like ours. In our solar system, we have the small Earthlike planets on the inside and the large Jupiterlike planets on the outside. For these four stars, the Jupiter-mass planets seem to be on the inside. How that could be, no one now understands.
We do not even know that these are truly Jupiterlike planets, with immense atmospheres of hydrogen and helium, metallic hydrogen down deep and an Earthlike core still deeper. But we do know that the atmospheres of Jupiterlike planets at such close distances to their stars will not evaporate away. It seems implausible that they formed in the periphery of their solar systems and somehow wandered much closer to their stars. But maybe some early massive planets could have been slowed by the nebular gas and spiraled in. Most experts hold that a Jupiter could not be formed so close to the star.
Why not? Our standard understanding of the origin of Jupiter is something like this: In the outer parts of the nebular disk, where the temperatures were very low, worldlets of ice and rock condensed out of the gas and dust, something like the comets and icy moons in the outer parts of our solar system. These frigid worldlets collided at low speeds, stuck together and gradually formed a "core" large enough to gravitationally attract hydrogen and helium gases from the nebula -- forming a Jupiter from the inside out.
In contrast, nearer to the star, it is thought, the nebular temperatures were too high for ice to condense in the first place, and the whole process was short-circuited. But I wonder if some nebular disks were below the freezing point of water even very close to the local star.
In any case, with Earth-mass planets around a pulsar and four new Jupiter-mass planets about Sunlike stars, it follows that our kind of solar system may hardly be typical. This is key if we have any hope of constructing a general theory of the origin of planetary systems: It now must encompass a diversity of planetary systems.
As for life on these new worlds, it is no more likely than on our own Jupiter. But what is probable is that these other Jupiters have moons, like the 16 that circle our Jupiter. Because these moons, like the giant worlds they orbit, are close to the local star, their temperatures, especially in the 70 Virginis system, might be clement enough for life. At 35 to 40 light-years away, these worlds are close enough for us to begin to dream of one day sending fast spacecraft to visit them, the data to be received by our descendants.
To me, it appears likely that in the coming decades we will have information on at least hundreds of other planetary systems close to us in the vast Milky Way Galaxy--and perhaps even a few small blue worlds graced with water oceans, oxygen atmospheres and the telltale signs of wondrous life.
Original file name: .CNI - Sagan.new planets 6.17
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