A New Way to Weigh Planets
Weighing the planets, from Mercury to Saturn
The new measurement technique is sensitive to just 0.003% of the mass of the Earth, and one ten-millionth of Jupiter’s mass (corresponding to a mass difference of two hundred thousand million million tons). The results are described in an article for the Astrophysical Journal, which is publicly accessible via preprint-server.
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. Measurements of planet masses made this new way could feed into data needed for future space missions.
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 center of mass, or barycenter, the rotation center for all the planets. Because the arrangement of the planets around the Sun changes with time, the barycenter moves around too (relative to the sun).
To work out its position, astronomers use both a table with the positions of the planets in the sky (called an ephemeris), and the values for their masses that have already been measured. If these figures are slightly wrong, and the position of the barycenter 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,” says Dr. Dick Manchester of 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.
The new measurement technique is sensitive to a mass difference of two hundred thousand million million tons -- just 0.003% of the mass of the Earth, and one 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. Repeating the measurements would improve the values even more. If astronomers observed a set of 20 pulsars over seven 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”, says Prof. 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.”