Van de Kamp’s Planets
In the history of astronomy up to 1992 there were only two people who could cleanly claim to have discovered a planet: William Herschel found Uranus and Urbain Le Verrier can claim Neptune; if you’re feeling somewhat charitable, you can give half of Neptune to John Couch Adams. For almost eighty years Clyde Tombaugh was in this group, but Pluto was famously demoted in 2006. The discoverers of the first four asteroids (Giuseppe Piazzi who discovered Ceres; Pallas’ discovery by Heinrich Olbers; Karl Harding who claimed Juno; and Olbers again with Vesta) had a similar fate. For close to forty years they were planet-discoverers. All their “planets” were discovered between 1801 and 1807, and were considered important enough for the title because they were the only known planetoids until Astraea was discovered in 1845. But that discovery signaled a rash of new inhabitants for the Asteroid Belt—eighteen more by the end of 1852, and a total of 62 by 1860; it became clear that the previously lonely four were something quite different from planets and so they were downgraded.
Since the 1990s, though, a variety of new techniques has uncovered more than 500 more planets to date—the difference now being that increases in instrument sensitivity make it possible to see planets outside of the Sun’s system, in the systems of the much more distant stars. For a little while in the 1960s and early ’70s, though, one other astronomer made a plausible claim that he’d discovered a planet, and it too was outside the Solar System.
That astronomer was Piet “Peter” van de Kamp, from 1937 to 1972 the director of Sproul Observatory of Pennsylvania’s Swarthmore College. Van de Kamp’s claim needed to be taken seriously because his specialty was the tiny motions of stars in the sky and the announcement depended on just that.
It seems that while studying the proper motion of 18,000 stars he started considering the possibility that he could find a planet or planets around Barnard’s Star. Barnard’s Star is notable for two main reasons: it is the second closest star system to the Earth (third, if you count the Sun), and it has the largest proper motion of any star. In other words, it changes its position in the sky faster than any other, taking “only” 173 years to cover a width equal to the full Moon’s. Van de Kamp’s insight was that while planets themselves were invisble to the technology of the time, no star with planets would move in a straight line. Instead, the planets would tug it this way and that as they orbited the star, causing it to make tiny loops in the sky. As Barnard’s Star was so close, the loops would be relatively large and easy to see.
Of course, “relatively” is the key word here. Barnard’s Star was already making little loops because of the Earth’s own motion around the Sun, and they would be about 100 times larger than the ones caused by any planet orbiting the distant star. Depending on exactly how big the hypothetical planet was, and how far away it was from the star, the displacement it caused would be on the order of a micrometer (one one-millionth of a meter) on Sproul Observatory’s photographic plates. Nevertheless, van de Kamp thought he could pull it off.
He began his observations shortly after moving to Sproul in the spring of 1937, and kept them up for 26 years before announcing that he had in fact discovered a planet around Barnard’s Star. By his calculation it was about 60% bigger than the planet Jupiter, and it orbited the star at a distance of 4.4 AU (a bit shy of Jupiter’s distance from our own Sun). His discovery made quite a splash, as being the first to see an extra-solar planet (even indirectly) was a major coup. Other scientists had a hard time duplicating his results, but this was no great surprise: it relied on the Sproul Observatory’s 24-inch refractor, a kind of photographic telescope that was being mothballed in other observatories in favour of spectroscopic ones; furthermore van de Kamp had needed more than two decades of observations to be sure. It was going to take time for anyone else to check his results.
The first sign of trouble came after van de Kamp announced planets around other stars too: Epsilon Eridani, 61 Cygni, and one he’d mooted back in 1951, Lalande 21185. Another astronomer, Bob Harrington, noticed that the shape of the planetary wobbles was the same for all three, and for Barnard’s Star too—as if it were the photographic plates that were moving, not the stars. That turned out to be the case. When it was first made the Sproul Observatory telescope van de Kamp was using had had one of its lenses inserted the wrong way, and while the effect on its operation was very small, in 1949 it had been removed and reset the proper way. The slight change in the lens had made a slight change in the way light focused on photographic plates taken with the telescope, and by bad luck the change was about the same size as what van de Kamp had been expecting to see from his planets. He agreed that all of his data prior to 1950 was now suspect, but still argued that everything taken since then still supported his discovery.
With the idea of instrument error now in the open, though, another astronomer by the name of George Gatewood published a paper in 1973 which demolished van de Kamp’s planets. The consensus is now that there was a cycle causing the image of the stars to move, but that it was down here on Earth. The telescope underwent regular maintenance, and every time it did its focus shifted ever so slightly and made any star it observed appear to have moved. Ironically, Gatewood eventually changed his mind about one of van de Kamp’s claims, Lalande 21185, but this too has turned out to be instrument error
After retiring, van de Kamp returned to his native Netherlands, where he died on May 18th, 1995. To the end he believed he had found at least one planet around Barnard’s Star, and maybe two. One real set of planets, orbiting the pulsar PSR 1257+12, had been discovered in 1992 but they were a peculiar case having probably formed after a supernova and not giving any real insight into planets in the universe as a whole. The Golden Age of Extra-Solar Planets began when Michel Mayor and Didier Queloz announced they had discovered 51 Pegasi b (AKA Bellerophon) just under five months after van de Kamp passed away. Its existence, as well as that of hundreds of others of new planets since then, has been demonstrated conclusively using two new techniques called the Doppler Method and the Transit Method. Van de Kamp’s photographic method is now considered a dead end.