new method to weight planets

A new method to weight planets in our Solar System has been developed by an international CSIRO-led team of astronomers-using radio signals from the small spinning stars called pulsars.

"This is first time anyone has weighed entire planetary systems - planets with their moons and rings," said team leader Dr David Champion from Germany's Max-Planck-Institut fur Radioastronomie.

"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.

The new method is based on corrections astronomers make to signals from pulsars - small spinning stars that deliver regular 'blips' of radio waves.

The Earth is travelling 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 centre of mass, or barycentre, around which all the planets orbit.

Because the arrangement of the planets around the Sun changes all the time, the barycentre 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.

CSIRO Astronomy and Space Science (CASS) researcher, Dr Dick Manchester, says that if these figures are slightly wrong, and the position of the barycentre 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," Dr Manchester said.

"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."

The new measurement technique is sensitive to a mass difference of two hundred thousand million million tonnes - just 0.003 per cent of the mass of the Earth, and one ten-millionth of Jupiter's mass.