sjb (06/19/82)
Scientists at the Ames Research Center are now talking more than ever about another body of some sort in our solar system. They say that Pluto is too small to be affecting the orbits of Uranus and Neptune as much as they had earlier expected. So, Pioneers 10 and 11 are now searching around to find the object. They hope that the two spacecraft, launched ten years ago and now on opposite sides of the sun, will be affected by the gravitational pull of the object and will yield the approximate position of it. Possibilities are a tenth planet (though it would have to be very dark to have evaded observation), a brown dwarf star (one that never ignited), a dark star that is just passing through the solar system, or even a black hole, and many others. A black hole is considered unlikely though, since the energy usually found around it has not been detected.
Lynn.ES@PARC-MAXC@sri-unix (06/28/82)
Some of the possibilities to explain outer planet perturbations are very speculative, to say the least. Tombaugh (discoverer of Pluto) and other astronomers knew in the 30's that Pluto was too small to explain the perturbations, if the estimated accuracies of positions for Uranus and Neptune were correct. That is why Tombaugh continued his search of all parts of the sky except the polar regions for more than an additional decade. He pretty well ruled out the possibility of a planet with any reasonable brightness within a reasonable distance of the sun (several times Pluto's distance). The alternative, of course, is that the accuracy of positional measurements of Uranus and Neptune was just not that good. Both the discoveries of Neptune and Pluto near the places predicted by perturbation analysis have been called lucky coincidences, since the perturbations being analyzed were nearly indistinguishable from errors in positional measurement. The orbits and masses of the predicted planets didn't agree very well, only the positions at discovery time did. One thing that has sparked new hope for another planet/object is the discovery in Galileo's notes of his sighting of an object that has to be Neptune. The position is somewhat off from predictions, but again is of questionable accuracy. The newly derived mass of Pluto from measurements of its satellite was claimed to have prompted excitement, but even its old erroneously large mass estimate was far too small to explain the perturbations. Jupiter has been called a near-star that never ignited, so I am not sure there is any difference between a large planet and a "brown dwarf". Anyway, don't hold your breath till an object is found. I think the odds are against it, but that does not mean we should not analyze the Pioneer spacecraft data for perturbations. It just might lay to rest the speculations on another planet. Then there is always a chance ... /Don Lynn