A new way to weigh planets

Diagram showing the Sun, Earth and Jupiter orbiting a common barycentre

Measurements of signals from pulsars (purple lines) are affected by Earth's movement in its orbit around the Solar System's centre of mass, the barycentre. The barycentre moves too, due to the gravitational influence of the other planets. By working backwards from the pulsar signals, scientists can work out the gravitational pull of the planets, and from that deduce their masses.

  • Radio waves from pulsars affected by Solar System’s gravity
  • Adjusting the pulsar wave measurements gives gravity readings
  • From the gravity readings, the planet’s masses can be found

Astronomers from Australia, Germany, the UK, Canada and the USA have come up with a new way to weigh the planets in our Solar System, using radio signals from pulsars.

“This is first time anyone has weighed entire planetary systems—planets with their moons and rings,” said team leader Dr. David Champion of the Max-Planck-Institut fuer Radioastronomie 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 the trajectories of spacecraft flying past them. That’s because mass produces 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 adjustments astronomers have to 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 exact 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.

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,” said Dr Dick Manchester of CSIRO Astronomy and Space Science.

The CSIRO's Parkes radio telescope

The CSIRO's Parkes radio telescope made most of the pulsar signal measurements.

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.

Better measurements of planet 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 (Jupiter and its moons)—0.0009547921 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 200,000 million million tonnes—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,” said CSIRO’s Dr George Hobbs, another member of the research team.

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”, said Professor Michael Kramer, head of the ‘Fundamental Physics in Radio Astronomy’ research group at the Max-Planck-Institut fuer Radioastronomie.

“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 [influences] of Solar System planets.”

Adapted from information issued by CSIRO / D. Champion, MPIfR.

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