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Science: Eccentric test for pulsar's planets

作者:耿听矫    发布时间:2019-03-03 03:03:06    

By KEN CROSWELL in BERKELEY There is a way of confirming that the two planets recently claimed to be orbiting a pulsar are real, say astronomers in the US. The gravity of one planet should disturb the other in a precise way, and this should be detectable from Earth, they say. The pair of planets (there could even be three) was announced late last year by Aleksander Wolszczan of Arecibo Radio Observatory in Puerto Rico and Dale Frail of the National Radio Astronomy Observatory in New Mexico (New Scientist, Science, 14 December 1991). The planets revealed themselves because they pulled periodically on the pulsar, known as PSR 1257+12. One planet takes 67 days to orbit the pulsar, while the other takes 98 days (the third, if it exists, takes about a year). The two inner planets are at about the same distance from the pulsar as Mercury is from the Sun, while the possible third planet orbits the pulsar at about the Earth’s distance from the Sun. The two inner planets are nearly in a 3:2 resonance. The planet with the 67-day orbital period completes about three orbits in the time it takes the planet with the 98-day period to complete two. In our Solar System, Neptune and Pluto are in such a 3:2 resonance. Frederic Rasio, Philip Nicholson, Stuart Shapiro and Saul Teukolsky of Cornell University in New York say that because of this resonance, the pulsar’s inner two planets are periodically in the same position relative to each other. The gravity of each planet can therefore perturb the other planet greatly. And the effect will be enhanced because the planets are massive. One planet has a mass of at least 3.4 times that of the Earth, while the other planet has a mass of at least 2.8 Earth masses. Rasio and his colleagues predict that over time this periodic tugging should alter the eccentricities of the planets’ orbits in a measurable way. The eccentricity of an orbit is a measure of how elliptical it is. A perfect circle has an eccentricity of zero, while an extremely elliptical orbit has an eccentricity of close to one. Wolszczan and Frail found that the pulsar’s planets have nearly circular orbits, with eccentricities of only 0.02. This is similar to the eccentricity of the Earth’s orbit around the Sun, which is 0.017. Rasio and his colleagues say that the resonance will cause the planets’ orbital eccentricities to change in just a few years. The changes will be small but detectable. If the eccentricities change in the way predicted, say the astronomers, this ‘would provide irrefutable confirmation’ that planets exist around the pulsar. The astronomers will be publishing their work in this week’s issue of Nature. Last summer, astronomers in Britain claimed to have discovered a single planet orbiting another pulsar. But their report was viewed with scepticism because the planet circled the pulsar once every six months, suggesting it may just have been an artefact of the Earth’s orbital motion around the Sun. Now one of the astronomers, Andrew Lyne of Jodrell Bank has found that the planet was indeed such an artefact. He discovered a flaw in the data analysis only two weeks ago. Announcing the error last week at a meeting of the American Astronomical Society in Atlanta, Georgia, Lyne told a packed auditorium: ‘Our embarrassment is unbounded,

 

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