Extrasolar Planets: A Bit of History
Created | Updated Mar 24, 2003
We know that the Supreme Ruler cannot have a seat so narrow, so miserable a throne...so that indeed with a puff of air it were brimful and with a single gulp it were emptied. On the contrary we recognize a noble image…a spectacle worthy of the excellence and supremacy of Him who transcendeth understanding, comprehension or grasp. 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.
(On the Infinite Universe and Worlds)
Written over four hundred years ago, these words today seem passionate, visionary and reverent. Given that a monk wrote them during the height of the powers of the Spanish Inquisition, they also seem foolhardy. Their author, Giordano Bruno, was a highly intelligent man whose vision vastly exceeded his circumspection and who, having an unerring habit of shooting his mouth off to his own detriment, could have written the definitive text on How To Lose Friends and Alienate People. In short, he was a smartass.
The Catholic Church then regarded any cosmological theory that varied from the Aristotelian geocentric model as heresy: Copernicus had the good fortune to die before his ideas became more widely known. However, Bruno was in a different league of heretic altogether. To suggest, as he did, that not only was the Earth not the centre of the Universe, but that every star had planets orbiting it was to multiply the heresy of Copernicus a hundred billion-fold. Predictably, this was seen as waving a bright ‘red rag’ in front of the Papal bull, and Bruno was thrown into prison where he was interrogated haphazardly for eight years, before recanting and then recanting his recantation. Bruno perished on February 17th, 1600, burned at the stake for daring to imagine a Universe whose majesty and scope matched that of its Creator.
Bruno is venerated today as a martyr for truth in the face of religious bigotry, in much the same way as Galileo or Scopes of the infamous Monkey Trial. In fact he contributed little to scientific posterity of direct value, as he had no evidence to back up his assertions. His main contribution was as a natural philosopher for asking a question whose existence predated him by at least a thousand years, but which has only now been answered with any degree of confidence. To date, there are over 100 confirmed extrasolar planets. They have been detected by a variety of ingenious methods, and provide a glimpse of a Universe far richer and stranger than that of the Church and Aristotle. Many of the observations challenged contemporary models of how the stars and planets form, their discoverers following the trail blazed (all too literally) by Bruno.
The ease with which a planet is detected depends entirely upon the technology used. Back in the days of the Romans, the naked eye could, on a clear night, make out four wandering points of light. The brightest, Venus, had a smaller orbit than the Earth and so appeared as an evening or morning star. Mars, Jupiter and Saturn orbited within the plane of the ecliptic, but were distinguished as planets by their motion against the stellar background. The Astronomer Royal, William Herschel, observed Uranus in 1781, originally naming it, in a fit of sycophancy, Georgum Sidus (George’s Star) after the then King. Johann Hieronymus Schroeter, who lived from 1745 to 1818, discovered Mercury. Neptune’s existence was inferred indirectly in 1846 by its effect upon the orbit of Uranus. In contrast Pluto was observed directly by Clyde Tombaugh in 1930. Quaoar and Varuna are two recent additions to the Solar System, each about the size of Pluto’s moon, Charon. Two other possible planets may exist: one might be an Earth-sized body orbiting just beyond Pluto, another a huge gas giant, up to six times the mass of Jupiter and half a light-year distant. Both these shadowy entities have betrayed their presence only recently, through their effect upon and long period comets.
If any themes emerge from the discovery of planets in our stellar backyard, they are that with increasing distance at which a planet orbits the Sun, it progressively becomes more difficult to observe directly and that indirect methods of inference become increasingly necessary. Detecting a planet orbiting a star many light-years distant is therefore a demanding and difficult process that requires patience, guile, rigorous attention to detail and very effective technology.
The easiest way of detecting a distant planet is to look for any effects upon its parent star. As forces always exist in opposite pairs, the gravitational force between two objects is felt equally by both of them. A planet being whirled around a star by its gravity also causes the star to orbit a point inside its own volume, rather like an Olympic hammer thrower gyrating in her circle as she winds the hammer up for the big throw. The bigger the planet, the more pronounced the effect. The closer the planet, the shorter its year, and hence the faster the star ‘wobbles’ in the sky. Large planets orbiting their stars close in cause larger and more frequent wobbles than smaller planets orbiting further out.
