Excerpt from PLANET QUEST

Copyright by Ken Croswell. All rights reserved.


To believe that [suns illuminate other worlds], it would be necessary to have the mind of Giordano Bruno, who was burned by the judgment of the Holy Inquisition for having maintained such an impertinence.
--French cleric Marc Antoine Guigues, 1700

It was Thursday morning, February 17, 1600, and the crowd in Rome was jeering. A fifty-one-year-old former priest, condemned as a heretic by the Inquisition, was being led, in chains, to his death.

Giordano Bruno had spent the last eight years of his life in the dark dungeons of the Inquisition, where he was repeatedly interrogated and probably tortured. He refused to recant, however, hoping instead to convince the Inquisitors, and even the Pope himself, that his beliefs were correct. Of these beliefs, one of the most radical was that the stars were other suns, circled by planets like the Earth.

Bruno was born in 1548, just five years after Copernicus published his revolutionary claim that the Sun, not the Earth, constituted the center of the universe around which all else revolved. The idea was so daring that the publisher prefaced Copernicus's work with a statement that the model was only a mathematical device to calculate planetary positions and did not reflect reality. Bruno, though, gladly adopted Copernicus's heliocentric model and took it even further. Copernicus thought the stars were merely part of the firmament, a finite sphere that encased the solar system, but Bruno proclaimed that they were actually distant suns scattered throughout a universe infinite in size. Around these suns circled planets. Bruno also predicted that additional planets, too distant to be seen, revolved around the Sun beyond the orbit of Saturn, the farthest planet then known.

Because Bruno advanced these ideas, he might seem to have been a link between Copernicus and Galileo, the great Italian astronomer whose telescopic observations in the early 1600s would later support Copernicus's conception of the solar system. But Copernicus and Galileo were scientists, whereas Bruno was a philosopher steeped in medieval mysticism and magic; he eschewed observations and disdained mathematics. Bruno believed the Earth circled the Sun not because observations said so but because he thought the Earth was literally alive, and like all other living creatures, it must move.

Similarly, Bruno's belief in an infinite universe, filled with innumerable planets circling other stars, stemmed not from science but from religion. God is infinite, said Bruno, so his universe must be, too. "Thus is the excellence of God magnified and the greatness of his kingdom made manifest; he is glorified not in one, but in countless suns; not in a single earth, a single world, but in a thousand thousand, I say in an infinity of worlds," wrote Bruno in his 1584 work, On the Infinite Universe and Worlds. Elsewhere Bruno explained that these planets could not be seen because they were fainter than their stars. He imagined seeing a large ship at the harbor, surrounded by small boats. If we then see a large ship in the distance, said Bruno, it should also be surrounded by small boats, even though we cannot see them: the near and the far obey the same rule. "We are a celestial body for the Moon and for every other celestial body," wrote Bruno, "and we are the firmament just as much as they are for us." He thus contradicted Aristotle, whom the Church esteemed. Aristotle had held that the Earth, flawed and imperfect, was separate from the heavens, which were perfect and changeless.

Neither Copernicus nor Bruno was entirely original in his beliefs. Long before Copernicus, Greek astronomer Aristarchus had proposed that the Earth circled the Sun. And long before Bruno, Greek philosopher Epicurus had suggested that other "worlds"--by which he meant other solar systems with an Earthlike planet, rather than a star, at their centers--existed elsewhere. Both ideas challenged the teachings of the Church, but Bruno's were far more radical. Copernicus had yanked the Earth away from the privileged center of the universe; now Bruno was saying that the Earth was not even unique. If there were other planets with intelligent beings, then where did that leave Jesus Christ? Did those other planets need their own Jesus? Might they even have religions different from the Catholic faith?

Back in the 1570s, while Bruno was only in his twenties, Church authorities began to accuse him of heretical thinking. He was then a monk in Italy, and he fled his native land to wander for over ten years through Switzerland, France, England, and Germany. In 1591, however, he made the fatal mistake of accepting an invitation from a professed admirer who offered him free lodging in Venice. The next year, this person betrayed him to the Inquisition, and Bruno's eight-year-long ordeal began.

It is not clear today exactly why the Church condemned Bruno. In the end, it found him guilty of eight heresies, but precisely what those heresies were is unknown, because the documents relating to Bruno's final trial perished in the 1800s, before any historians examined them. However, older documents do survive, and they accuse Bruno of both religious and scientific errors. Two of his alleged scientific errors were his belief in an infinite universe and in planets circling other stars.

The final sentence, handed down by the Inquisition in early 1600, mentioned Bruno's eight heresies and then said: "We hereby, in these documents, publish, announce, pronounce, sentence, and declare thee the aforesaid Brother Giordano Bruno to be an impenitent and pertinacious heretic, and therefore to have incurred all the ecclesiastical censures and pains of the Holy Canon, the laws and the constitutions, both general and particular, imposed on such confessed impenitent pertinacious and obstinate heretics....We ordain and command that thou must be delivered to the Secular Court...that thou mayest be punished with the punishment deserved....Furthermore, we condemn, we reprobate, and we prohibit all thine aforesaid and thy other books and writings as heretical and erroneous, containing many heresies and errors, and we ordain that all of them which have come or may in future come into the hands of the Holy Office shall be publicly destroyed and burned in the square of St. Peter before the steps and that they shall be placed upon the Index of Forbidden Books, and as we have commanded, so shall it be done....Thus pronounce we, the undermentioned Cardinal General Inquisitors."

Bruno's response was eloquent: "Perchance you who pronounce my sentence are in greater fear than I who receive it."

At the time, most heretics condemned to the stake were not actually burned. They were executed in prison and burned only in effigy, and for centuries afterward certain Catholic apologists claimed that Bruno met such a fate. Some even claimed there never was a Bruno.

But eyewitness accounts recorded what actually happened on that Thursday morning, four hundred years ago: Giordano Bruno, poet, philosopher, and proponent of alien worlds, was bound to a stake in the ironically named Field of Flowers, a fire was lit, and he was burned alive.

Planets are vital for life: without them, intelligent life would never have arisen on Earth, for we owe our lives to the planet beneath our feet. But planets are so difficult to see that only in the 1990s, four centuries after Bruno's death, did astronomers succeed in fulfilling his dream by detecting genuine planets around other stars.

This book is the story of these recent discoveries and the people who made them. Planet Quest opens with an investigation of the one solar system scientists know best, our own. The narrative then shifts back in time, to an era when Saturn marked the known edge of the solar system, and joins astronomers as they look for and discover distant worlds orbiting the Sun. Then the book sets off for other stars, culminating with the stunning discoveries of the 1990s: the astonishing 1991 find of planets orbiting a dead star known as a pulsar, and the rich harvest of planet discoveries around Sunlike stars that began in 1995. These discoveries took astronomers closer than they had ever been to their goal of finding life among the stars, and this book's penultimate chapter explores new techniques that promise to divulge smaller planets, like Earth, circling stars like the Sun, the very planets that can harbor life. The last chapter of the book explores the challenging and controversial possibility that starships might someday journey to the planets of other stars.

As we explore the solar system and the Galaxy, studying old planets and searching for new ones, we will see just how hostile all other known planets are--from scorched Mercury to frigid Pluto, and even to the new worlds that astronomers have found around other stars. These forbidding worlds should make us appreciate more fully the special nature of the Earth, a warm, wet world that still remains amazingly unique and whose ultimate fate is in our own hands.

From Ken Croswell's Planet Quest. Copyright by Ken Croswell. All rights reserved.