(In 1992 berichtten Amerikaanse astrononomen dat zij planeten om een pulsar hadden aangetroffen. Het gaat om de pulsar PSR 1257 +12, op 1600 lichtjaar afstand van de aarde in het sterrenbeeld maagd. Fluctuaties in de frequentie van de pulsar wijzen op de aanwezigheid van vier planeten, met omlooptijden van 25,34 dagen, 66,54 dagen, 98,22 dagen en circa 170 jaar. De eerste planeet, die niet veel groter zou dan onze maan, lijkt een artefact te zijn. Volgens Duitse en Amerikaanse onderzoekers is deze fluctuatie in de pulsarfrequentie vermoedelijk te wijten aan de rotatie van onze eigen zon).

Max Planck Institute of Aeronomy

December 12, 1997

Does solar rotation simulate a planet around the Pulsar PSR 1257+12?

A German-American research team describes in this week's Science magazine a heliospheric radio-signal modulation that mimics a planet around the pulsar PSR 1257+12. The analysis is partly based on the radioscience experiment onboard Pioneer 10, one of four deep space sprobes in the heliosphere, i.e. the circumsolar region dominated by the solar wind plasma.

The Pioneer 10 data are obtained from a "two-way radio link" experiment (PI: John D. Anderson, JPL): a signal is transmitted from Earth (uplink), transponded by the spacecraft and hours later received back at Earth (downlink). With a local oscillator (a highly stable hydrogen maser) it is possible to determine the Doppler shift to an accuracy better than mHz. After removing systematic changes in the position and velocity of Pioneer 10 a main period of 25.3 days was found by the Max-Planck scientist K. Scherer, his colleagues H. Fichtner from the University of Bonn and J. Anderson and E. Lau from JPL. The data were recorded when the spacecraft was moving from a distance of 40 to 60 AU (6 to 9 billion km) from the Sun.

The modulation of the Pioneer 10 radio signal is caused by an electron density fluctuation of a particle stream (solar wind) from the Sun which is imprinting its (siderial) rotation period on the variations.

The pulsar is also observed via radio-signals at similar frequencies. A 25.3 day periodicity in this signal was originally interpreted as the reflex motion of the pulsar due to an orbiting (moon-sized) planet. According to that interpretation, the pulsar as well as the planet would describe ellipses around their common center of mass, because of the mutual gravitational attraction, The semimajor axis of the pulsar's ellipse would be less than 1 km. Therefore, the distance to the pulsar and, correspondingly, the time of arrival (TOA) of the radio waves would vary periodically. This way one could explain the microsecond variations in the TOA occuring with a period of 25.3 days.

However, the research team demonstrated, that, annalogous to the modulation of the Pioneer 10 signal, the 25.3 day variation in the electron density of the solar wind also affects the pulsar's radio waves and can cause their TOA variation in the range of microseconds. Therefore, the scientists concluded that this (innermost) planet around the pulsar PSR 1257+12 is simulated by solar rotation.

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