These days, it is accepted as a scientific fact that we live in a universe teeming with planets orbiting other stars. Indeed, as of 24 September, there are 490 worlds that we know of orbiting stars other than our Sun. Detecting these planets has become a routine voyage of discovery engaging well-tested and accepted methods.
The primary methods include radial velocity (Doppler displacement of spectral lines in the star's light due to the star 'wobbling' as it orbits the common centre of mass of the star and its planetary companion), and the transit method (a dip in starlight as an exoplanet moves across the disc of the star, thus reducing the amount of starlight). Other successful methods include astrometry, where there are minuscule changes over time in a star's precise co-ordinates in the night sky because of its orbit around the stellar system's centre of mass, and microlensing (a bending of light from a distant star due to the gravity of the foreground star and its associated planet – see below).
Finally, of course, there's the most spectacular method, one that will become more important as detectors improve -- that of direct imaging, as in the cases of Fomalhaut b and Beta Pictoris b and associated stellar debris discs, as observed by NASA's Hubble Space Telescope (HST).
However, it has always been this way. Speculation has always run wild about whether planets orbit other stars, and by implication about the possibility of extraterrestrial life. Indeed, the Roman Catholic monk Giordano Bruno was burnt at the stake in 1600 by the Inquisition for the trouble he caused by publicising the then heretical view that the universe was teeming with other worlds and with life.
Throughout the nineteenth century, theories ebbed and flowed about the formation of planets and the circumstellar discs from which they arise. One particularly popular anthropocentric theory suggested that our star was unique in the Milky Way in possessing a solar system. It had hypothesised that this had come about through a ridiculously improbable event -- a close pass of another star had ripped material out of our Sun, which eventually coalesced to form planets.
However, in order for the question to be resolved, the whole issue of exoplanets would have to await better technology and observations, not better theories. After all, other stars are at gargantuan distances from the Earth; the nearest, Proxima Centauri, is part of the Alpha Centauri triple system and is nearly 40 trillion kilometres distant. Whatever detection methods are employed, the effect of planetary companions will turn out to be almost infinitesimally small.
Proxima Centauri is a dim red dwarf -- its larger Sun-like siblings Alpha Centauri a and b are so bright that the pale reflected light from the parent star of any planetary companions would be completely emasculated by the star itself. Even a Jovian mass giant would be a billion times fainter than the central star. As an analogy to illustrate the immense technical difficulties involved with direct imaging, think of a dim candle placed a couple of metres from a floodlight viewed via a telescope from a vantage point a thousand kilometres distant. What would be your chances of viewing the dim candlelight?
|Peter van de Kamp|
The next year, he began a long-term search for very low-mass companions to stars. One of the first stars he put on the search programme was Barnard's Star. This is the second-closest star system to our own, at six light years distance. Unfortunately, it's an M-type dwarf, so it can't be seen by the naked eye, but it can easily be seen with a small telescope.
Van de Kamp started collecting data on Barnard's Star in 1938, and continued taking data for roughly 25 years. In 1963, he finally felt confident enough to present his first excruciatingly difficult astronometrical measurements. He and his colleagues were looking for variations of plus or minus 1 micron in the position of the star on a photographic plate! In other words, they were endeavouring to measure the photographic centre of these little blurry dots on the photographic emulsions to a staggering 1 part in 100. They would have 10 people measure the same plates independently, and then try to average over whatever individual systematic errors they would introduce, to find the true photographic centre of the positions.
After looking at some 2400 plates, van de Kamp found evidence that there was a small 'wobble' in Barnard's Star, which fitted with the curve that would result if it were being orbited by a planet about 1.6 times the mass of Jupiter at a distance of 4.4 AU. The peculiar attribute of the star movement, though, was that it didn't fit into a neat sine curve, which would indicate a roughly circular orbit like our own Jupiter's. Instead, it had a little bit of a cusp to it.
However, most astronomers could live with a planet with a quite elliptical orbit, and Barnard's Star's planetary companion soon became the textbook example of an extra-solar planet. However, all was not well with the data that lay behind van de Kamp's momentous discovery, and ten years later in 1973 the astronomer George Gatewood would reveal major flaws in van de Kamp's observations.
Gatewood had been undertaking his Ph.D. in astrometry at the University of Pittsburgh and, although he was reluctant at first, his professors were extremely keen that he study Barnard's Star. And so, unbeknown to him, Gatewood was to become reluctantly involved with a profound cosmic controversy. While studying Barnard's Star, he undertook his own observations and measurements, using different telescopes: the Allegheny Observatory's Thaw Refractor, and some plates taken from the Van Vleck Observatory.
In total, Gatewood produced 240 plates, and for his thesis project he set about reducing the data from them. Instead of their being reduced by individuals sitting at a plate-measuring machine, they were automatically reduced by a new, state-of-the-art plate-measuring machine produced by the U.S. Naval Observatory. Of equal importance, the data was reduced using a different technique from that used before. Gatewood's thesis adviser, Heinrich Eichhorn, was one of the fathers of analytical astrometry, and it was Eichhorn's technique that was invoked for the data analysis.
Their results on Barnard's Star were published in 1973, and were bad news for van de Kamp – some of the data points in which they had the most confidence did not fit van de Kamp's curve. Without confrontation, they quietly stated that they had found no evidence whatsoever for Peter van de Kamp's planet. And it got worse -- that same year, another paper was published in the Astronomical Journal by John Hershey, who was also working at the Swarthmore College Observatory.
Hershey had studied a star called Gliese 793, another low-mass M-type dwarf star, and found that, if he plotted the astrometric wobbles of Barnard's Star and Gliese 793 together, both of them took a jump in one direction in 1949, and in 1957 took another jump in the opposite direction. The implications of Heshey's data were devastating for van de Kamp's 'discovery': either both stars had exactly the same planet orbiting them, or else there were major systematic errors in the latter's observations.
It turned out that van de Kamp's observations were riddled with major systematic inaccuracies. In 1949, there had been a major change in the telescope; they put in a new cast-iron cell to hold the Swarthmore College refracting lens. They also changed the photographic emulsions they were using, which made an enormous difference when measuring objects in size down to one-hundredth the size of a star's blur. In 1957, they made another change – a lens adjustment.
And so it was, that after a lifetime's work, much of it studying Barnard's Star, van de Kamp discarded forty years' worth of data. Still continuing to believe that the star should possess a planet, he started anew with more observations. However, by the autumn of 1973, following his discredited observations, most astronomers were no longer prepared to give his work much credence and the field of exoplanet research fell into a deep sleep for two decades.
Roll the clock forward 22 years: and on 6 October 1995 Michel Mayor and Didier Queloz announced the verified discovery of the first genuine exoplanet, orbiting a Sun-like star located 50.9 light years (15.6 parsecs) away in the constellation of Pegasus. The discovery, via the radial velocity method, of 'Belleraphon' (as the planet became known) orbiting 51 Pegasi was made in France at the Observatoire de Haute-Provence, using the ELODIE spectrograph.
It turned out that this planet from hell was a gas giant, approximately half the mass of Jupiter, but with an orbital period of just over four days -- a fraction of that of Mercury around our star. In the intervening years, a host of such 'hot Jupiters' have been discovered, and they're unlike anything in our solar system. They're located so close to their stars that they have atmospheric temperatures nearing 1000 °C, they're tidally locked to their stars, and hence must have turbulence and winds to dwarf anything in our solar system. The smart money is placed on a theory that suggests that such planets have migrated from original positions in their solar systems similar to that of our own Jupiter.
And what of Barnard's Star? The HST's fine-guidance sensor team, led by Fritz Benedict of the University of Texas, has been following the star to ascertain whether it has planetary companions, but has so far drawn a blank. Instead, Barnard's Star is more useful for debugging mechanical problems on the HST, because when the star seems to wobble, it usually means that there's something wrong with the space telescope!
For now though, the jury's still out on whether our very closest stellar neighbours possess Earth-mass planets. Astronomers are fairly certain that these stars do not possess gas giants, and speculate that Earth-mass planets may be orbiting any of the stars in the Alpha Centauri triple star system, for example. The habitable zones of these stars lie at a distance similar to that of our Sun's, and are close enough to the stars to ensure that gravity from their stellar companions would not eject terrestrial-sized rocky worlds from the triple system. There may indeed even be Earth-mass planets orbiting Barnard's Star. And a final resolution to these questions won't be long in coming, either -- a Planetary Society project is underway called FINDS Exo-Earths (an acronym for Fibre-optic Improved Next generation Doppler Search for Exo-Earths). This new high-end optical system has been installed on the 3-metre telescope at the Lick Observatory, dramatically increasing discoveries of smaller exoplanets and playing a crucial role in verifying Earth-sized planet candidates from the Kepler planet-hunter mission. Peter van de Kamp believed in another world orbiting Barnard's Star, but his observations and data were not repeatable or verifiable -- a situation that ultimately is not worth much in science. With only the relatively primitive technology of the early twentieth century, he believed too much.
Ironically, however, the next generation of telescopes may yet prove his beliefs to be correct
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