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Scientists develop a new way to weigh planets

The method is based on corrections astronomers make to signals from pulsars.
Provided by CSIRO, Sydney, Australia
Jupiter mass
The Sun, Earth and Jupiter orbit a common barycenter
MPIfR-D. Champion
August 23, 2010
Astronomers from Australia, Germany, the United Kingdom, Canada, and the United States have come up with a new way to weigh the planets in our solar system, using radio signals from pulsars.

"This is (the) first time anyone has weighed entire planetary systems — planets with their moons and rings," said David Champion from the Max-Planck Institute for Radioastronomy in Bonn, Germany. "And we've provided an independent check on previous results, which is great for planetary science."

Measurements of planet masses made this new way could feed into data needed for future space missions. Until now, astronomers have weighed planets by measuring the orbits of their moons or of spacecraft flying past them. That's because mass creates gravity, and a planet's gravitational pull determines the orbit of anything that goes around it — both the size of the orbit and how long it takes to complete. The new method is based on corrections astronomers make to signals from pulsars — small spinning stars that deliver regular "blips" of radio waves.

Earth is traveling around the Sun, and this movement affects exactly when pulsar signals arrive here. To remove this effect, astronomers calculate when the pulses would have arrived at the solar system's center of mass, or barycenter, around which all the planets orbit. Because the arrangement of the planets around the Sun changes all the time, the barycenter moves around, too. To work out its position, astronomers use both a table (called an ephemeris) of where all the planets are at a given time and the values for their masses that have already been measured. If these figures and the position of the barycenter are slightly wrong, then a regular, repeating pattern of timing errors appears in the pulsar data.

"For instance, if the mass of Jupiter and its moons is wrong, we see a pattern of timing errors that repeats over 12 years, the time Jupiter takes to orbit the Sun," said Dick Manchester of Australia's Commonwealth Scientific and Industrial Research Organization's (CSIRO) Astronomy and Space Science. But if the mass of Jupiter and its moons is corrected, the timing errors disappear. This is the feedback process that the astronomers have used to determine the planets' masses.

Data from a set of four pulsars have been used to weigh Mercury, Venus, Mars, Jupiter, and Saturn with their moons and rings. Most of these data were recorded with CSIRO's Parkes radio telescope in eastern Australia, with some contributed by the Arecibo telescope in Puerto Rico and the Effelsberg telescope in Germany. The masses were consistent with those measured by spacecraft. The mass of the Jovian system, 0.0009547921(2) times the mass of the Sun, is significantly more accurate than the mass determined from the Pioneer and Voyager spacecraft, and consistent with, but less accurate than, the value from the Galileo spacecraft.

The new measurement technique is sensitive to a mass difference of two hundred thousand million million tons — just 0.003 percent of Earth's mass, and 1 ten-millionth of Jupiter's mass. "In the short term, spacecraft will continue to make the most accurate measurements for individual planets, but the pulsar technique will be the best for planets not being visited by spacecraft, and for measuring the combined masses of planets and their moons," said George Hobbs from CSIRO. Repeating the measurements would improve the values even more. If astronomers observed a set of 20 pulsars over 7 years, they'd weigh Jupiter more accurately than spacecraft have. Doing the same for Saturn would take 13 years.

"Astronomers need this accurate timing because they're using pulsars to hunt for gravitational waves predicted by Einstein's general theory of relativity", said Michael Kramer from the Max-Planck-Institute for Radioastronomy. "Finding these waves depends on spotting minute changes in the timing of pulsar signals, and so all other sources of timing error must be accounted for, including the traces of solar system planets."
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