Cerca nel blog


Il primo cacciatore di pianeti extrasolare aveva toppato: la storia della Stella di Barnard

The Barnard's Star Blunder :: Astrobiology Magazine :: Search for Life in the Universe: "The extrasolar planets field started in many ways with Peter Van de Kamp. Van de Kamp had been a professor at the University of Virginia for several years. In 1937 he went to Swarthmore College and became director of the Sproul Observatory there. 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 program was a star called Barnard's star. Barnard's star is the second closest star system to our own... Van de Kamp started taking 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 results... He and his colleagues were looking for variations of plus or minus 1 micron in the position of the star on a photographic plate. They were trying to measure the photo center of these little blurry dots on the photographic emulsions to 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 photo center of the positions."

"...he found evidence that there was a bit of a wobble in Barnard's star, which fit with the curve that would result if Barnard's star was being orbited by a planet about 1.6 times the mass of Jupiter out at a distance of 4.4 AU. The odd thing about it was, though, that it didn't fit with a nice sine curve, which would indicate a roughly circular orbit, like Jupiter's, but it had a little bit of a cusp to it. So it was a somewhat eccentric orbit, but people thought, Well, maybe that's not so bad. This basically became the textbook example of an extrasolar planet...."

But then about 10 years later, in 1973, along came George Gatewood. He had been doing his Ph.D. on astrometry at the University of Pittsburgh... He did his own measurements, using different telescopes, the Allegheny Observatory's Thaw Refractor, as well as some plates taken from the Van Vleck Observatory... Instead of having them reduced by individuals sitting at a plate-measuring machine, though, they were reduced by a newfangled plate-measuring machine that the U.S. Naval Observatory had come up with... They found that some of the points in which they had the most confidence did not fit the curve produced by Van de Kamp at all. So they very politely and gently said that they had found no evidence for the traditional planet that Peter van de Kamp thought he had evidence for."

"Things got even worse for Barnard's star's planet that same year, because there was another paper published in Astronomical Journal by John Hershey, who was also working at the Swarthmore College Observatory. He had studied another star called Gliese 793, another low-mass M dwarf star, and he 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 other direction, implying that both of them either had exactly the same planet going around them, or else there was something else going on, namely some systematic errors."

"...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 makes a big difference when you're trying to measure things to 1/100th of the size of a blur. And in 1957, they made a lens adjustment. So van de Kamp had tried to correct for all these things, but clearly the corrections were not sufficient."