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Distant astrophysical beacons reveal masses of the solar system's planets


Pulsars used to weigh the solar systemElectromagnetic pulses from far-flung celestial objects can provide a sort of scale with which to gauge the mass of the planets, according to a new study.

The technique relies on the regularity of ultrashort blasts of radiation from pulsars, which result from the collapse of a massive star to an extremely dense and rapidly spinning magnetized remnant. (Imagine the mass of the sun crammed into an object that could rest within the boundaries of a midsize American city.)

Pulsars spin so fast that the beams of radiation they emit sweep across Earth with a predictable cadence that can be measured with sub-microsecond precision. But Earth is not stationary with respect to these distant beacons; our planet's orbital motion changes its distance to any given pulsar by hundreds of light-seconds over the course of a year. To correct for Earth's changing position, astronomers calibrate the arrival times of pulsar radio waves to a hypothetical observer at the solar system's center of mass, a point in space within or close to the sun, depending on the position of the planets.

What David Champion of the Max Planck Institute for Radio Astronomy in Bonn, Germany, and his colleagues have done is to work backward, essentially, using the arrival times of pulsar signals as a metronome to refine the estimated position of the solar system's center of mass, which depends on the masses of the individual planets. With incorrect masses for the planets, the center of mass used by astronomers will drift from its true position, causing the calibrated pulsar times to vary over the years.

In the September 10 Astrophysical Journal Letters, Champion and his co-authors derived masses for five of the solar system's planets using years of data on four pulsars located hundreds to thousands of light-years away. All of the group's measurements jibe with existing—and currently more precise—masses estimated by other means, such as tracking the gravitational pull felt by passing spacecraft, but the researchers note that their method provides an independent check on those methods. The researchers also propose that, with a sizable data set, the pulsar method could be used to discover unknown objects beyond the orbit of Neptune; the unaccounted-for mass in the distant solar system would disrupt the calibrated pulsar times over long timescales.

Image credit: David Champion

The views expressed are those of the author and are not necessarily those of Scientific American.

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