Instead of determining the mass of a planet by measuring the orbits of moons or spacecraft around them, astronomers have come up with a new method using radio signals from pulsars.

A planet's mass creates gravity, and the gravitational pull of the planet determines the size and length of the orbit of anything that is travelling around it, making this the standard method for determining the weight of a planet. But a new technique turns to the radio signals given off by pulsars – the dense spinning cores of dead stars. As the Earth journeys around the Sun the arrival time of the pulsar signal varies, so astronomers calculate the expected time of arrival at the Solar System's centre of mass instead. Because the centre of this mass also changes with time due to the movement of all the planets around the Sun, astronomers compare the position of the planets in the sky with the value of the masses already obtained. If the values are different, and the position of this centre is 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,” explains Dick Manchester of CSIRO Astronomy and Space Science. If the mass of Jupiter and its moons is corrected, the timing errors disappear.

The new technique yields the weight of a planet to within just 0.003 percent of the mass of Earth or one ten-millionth of the mass of Jupiter (equivalent to a mass difference of two hundred thousand million million tons). To date, the team have used data from four pulsars – collected by the CSIRO Parkes radio telescope in eastern Australia and the Effelsberg telescope in Germany, and Arecibo in Puerto Rico – to weigh Mercury, Venus, Mars, Jupiter and Saturn, complete with moons and rings.

The mass of the Jovian system was determined as 0.0009547921 times the mass of the Sun, which is 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.

“This is first time anyone has weighed entire planetary systems-planets with their moons and rings,” says team leader David Champion of the Max Planck Institute for Radio Astronomy. “In addition, we can provide an independent check on previous results, which is great for planetary science.”

Although measurements provided by spacecraft will continue to offer the most accurate data in the short term, the pulsar technique will allow repeated measurements for planets not visited by spacecraft and for whole systems of moons and rings. The team add that if astronomers observed a set of 20 pulsars over seven years they would weigh Jupiter more accurately than achieved using spacecraft; for Saturn this 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”, says Michael Kramer, head of the Fundamental Physics in Radio Astronomy research group at Max Planck Institute for Radio Astronomy. “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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