Spinning Stars Help Scientists 'Weigh' Our Sun's Planets

A new method for "weighing" the planets in our solar system ? one that relies on the radio signals delivered from highly magnetized spinning stars ? has been developed by an international research team.

Observations of a set of four of these peculiar spinning stars, called pulsars, were used to calculate the masses of Mercury, Venus, Mars, Jupiter and Saturn, including ? for the first time ? their moons or rings.

The researchers reported that the masses calculated using pulsar data were consistent with the most-accurate previous measurements.

Previously, astronomers determined a planet's mass by relying on the readings of spacecraft that zoomed past ? such measurements are still considered the most accurate ? or by measuring the orbits of the planet's moons. Both the size of the orbit and the length of time needed to complete one are indicators of how massive a planet is, since orbits are determined by gravitational pull, and that pull is largely a product of the planet's mass.

In the future, the researchers said, combining pulsar timing with existing data sets will lead to even greater precision.

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

The new technique is quite precise ? sensitive to about 0.003 percent of Earth's mass, and a tenth of a millionth of Jupiter's mass. [Gallery: Planets of the Solar System]

Reading signals from pulsars

The method developed by Champion and his colleagues, is based on corrections that astronomers make to the regular radio wave "blips" that come from distant pulsars.

As the Earth travels around the sun, that movement affects the time it takes for pulsar signals to arrive at our planet. In order to get around this effect, astronomers instead calculate when the pulses would have arrived at the solar system's center of mass.

This point, called the barycenter, is the rotation center for all the planets. Since the arrangement of the planets around the sun changes over time, the barycenter also changes relative to the sun.

To locate the barycenter, astronomers use both a table with the positions of the planets in the sky (called an ephemeris) and the values for the planetary masses that have already been measured.

If these figures are wrong and the position of the barycenter is inaccurate, then a regular, repeating pattern of timing errors can be detected 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 study co-author Dick Manchester of the Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia's national science agency.

If the mass of Jupiter and its moons is correctly calculated, the timing errors disappear from the data set, the scientists said.

Getting it right

The researchers have been using this method to determine the masses of planets in our solar system. Such measurements could provide data needed for future space missions.

The pulsar observations were recorded by CSIRO's Parkes radio telescope in eastern Australia, with additional data contributed by the Effelsberg telescope in Germany and the Arecibo telescope in Puerto Rico.

The researchers said they got their best measurements from Jupiter and its system, as the pulsar technique is most accurate with more-massive planets.

"The more a planet can draw the barycenter away from the sun, the easier it is to measure the mass," Champion told SPACE.com. "We're less accurate with Mars and Mercury because they're far less massive. Mercury hardly moves the barycenter at all."

How it will help astronomers

Spacecraft will continue to provide the most accurate measurements for individual planets, but the new pulsar technique will be ideal for calculating the mass of planets not being visited by spacecraft, researchers said. It is also helpful for determining the mass of entire systems ? the combination of planets and their moons.

Additionally, repeating the measurements will help to refine the current values and make them more accurate.

?If astronomers observed a set of 20 pulsars over seven years, they would be able to weigh Jupiter more accurately than spacecraft have, the scientists said. The same process 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 Michael Kramer, who leads the Fundamental Physics in Radio Astronomy research group at the 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."

The research team consisted of scientists from Australia, Germany, the United States, the United Kingdom and Canada. The study is detailed in a paper that has been accepted for publication in the Astrophysical Journal.

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