Free Billions & Billions by Carl Sagan

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 Earth-like planets far above the boiling point of water. At 1,300 light-years away we will not be traveling to this system soon. It is a current 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 Sun-like 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 for a while toward us and then away from us, as determined by the changes in wavelength of its spectral lines, the Doppler Effect—akin to the changes in frequency of a car’s horn as it drives toward or away from us. Some invisible body is tugging at the star. Again, an unseen world is discovered by a quantitative agreement—between the observed slight periodic motions of the star and what you would expect if the star had a nearby planet.
    The planets responsible go around the stars 51 Pegasi, 70 Virginis, and 47 Ursae Majoris, respectively in the constellationsPegasus, Virgo, and Ursa Major, the Big Dipper. In 1996, such planets were also found orbiting the star 55 Cancri in the constellation Cancer, the Crab: Tau Bootis and Upsilon Andromedae. 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 more 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 Ursae Majoris. These systems may also contain smaller Earth-like planets, not yet discovered, but their layout is not like ours.
    In our Solar System, we have the small Earth-like planets on the inside and the large Jupiter-like 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 Jupiter-like planets, with immense atmospheres of hydrogen and helium, metallic hydrogen down deep and an Earth-like core still deeper. But we do know that the atmospheres of Jupiter-like 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, 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 became large enough to gravitationally attract the prevalent hydrogenand 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 is 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 Sun-like stars, it follows that our kind of solar system may hardly be