How do scientists weigh planets?

Astronomers currently weigh planets by measuring the orbits of their moons or of spacecraft flying past them. They use this principle because mass creates gravity, and a planet's gravitational force determines the orbit of anything that goes around it, in terms of not only the size of the orbit but also how long it takes to complete.

This method could soon go into the history books because an international CSIRO-led team of astronomers has developed a new way to weigh the planets by using radio signals from the small spinning stars called pulsars.

This new method has not only enabled opportunity for astronomers to for the first time weigh entire planetary systems (planets with their moons and rings) but it also provided an an independent check on previous weighing results. Also, these new measurements will new much more reliability to the data needed for future space missions.

The working principle of this new weighing method is based on corrections astronomers make to signals from pulsars. The Earth is traveling around the Sun, and this movement affects exactly when pulsar signals arrive here. Astronomers first need to calculate when the pulses would have arrived at the Solar System's centre of mass, or barycentre, around which all the planets orbit. The barycentre moves around too, and so scientists need to use a table (called an ephemeris) of where all the planets are at a given time, and also the values for their masses that have already been measured.

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

Scientists also said that repeating the measurements would make the values even more accurate. If astronomers observed a set of 20 pulsars over seven years they'd weigh Jupiter more accurately than any spacecraft would. Doing the same for Saturn would take 13 years.