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Scientists discover new way to weigh planets

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LONDON (PTI): For the first time, astronomers claim to have developed a new way to weigh the planets in our solar system -- using radio signals from the highly magnetised small spinning stars known as pulsars.

Observations of a set of four of these pulsars were used to calculate the masses of Mercury, Venus, Mars, Jupiter and Saturn, including their moons, says an international team led by Germany's Max Planck Institute for Radioastronomie.

"This is first time anyone has weighed entire planetary systems -- planets with their moons and rings. And, we have provided an independent check on previous results, which is great for planetary science," team leader Dr David Champion said.

Until now, planetary scientists 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 around it -- the orbit's size and how long it takes to complete.

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. And in the future combining pulsar timing with existing data sets will lead to even greater precision, the astronomers say.

In fact, the new method is based on corrections the 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 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 planets around the sun changes over time, the barycenter also changes relative to the sun.

To locate the barycenter, the 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," team member Dick Manchester of the Commonwealth Scientific and Industrial Research Organisation said.

If the mass of Jupiter and its moons is correctly calculated, the timing errors disappear from the data set, the astronomers say, adding that such measurements could provide data needed for future space missions.

The findings have been published in the latest edition of the 'Astrophysical Journal'.

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