Physics Question #9225

Brian Buydens, a 52 year old male from Weyburn asks on April 22, 2015,

When did scientists first realize that planets had mass? I was thinking that by the time of the search for Neptune physicists knew about planetary mass since they used calculus to predict where Neptune should be. I also think that the ancient Greeks did not view planets as bodies with mass but rather as lights controlled by spheres. So, the notion that planets had mass must have arisen sometime between these two events. It would seem that thinking of planets as having mass would require knowledge of Newton's laws of gravitation but I believe Galileo (who predated Newton) used gravitation effects of the tides to argue for a sun-centered solar system. So, when did scientists first realize that planets had mass?

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The answer

Jaymie Matthews answered on April 27, 2015

Galileo was the first to formulate concepts of inertial mass as opposed to weight. But for the planets, credit goes to Isaac Newton for demonstrating in 1684 that they have mass, with earlier help from astronomers Tycho Brahe and Johannes Kepler.

Tycho Brahe had made the most accurate measurements in the era before telescopes of the angular positions on the sky of stars and the "wandering stars" we now know to be planets. After Brahe died, Kepler spent two decades analysing the observations and formulating what we now call Kepler's Three Laws of Planetary Motion. Kepler, using Brahe's data, told us *how* planets move around the Sun, but he didn't know *why* they move that way.
In August 1684, Edmond Halley urged his friend Newton to tackle the problem of deriving Kepler's Laws from physical principles. Three months later, in November 1684, Newton presented Halley with a 9-page manuscript entitled "Of the motions of a body in orbit". Newton has derived Kepler’s Laws using his new theory of an inverse-square force between bodies - what we now know as Newton's Universal Law of Gravitation between two bodies with mass m_1 and m_2.
Galileo's telescopic observations of Venus and Jupiter had established in peoples' minds that they were not just pinpoints of light, but bodies, perhaps worlds like Earth. So they had substance. But the leap from planetary matter to planetary mass - as an inertial property - was made by Newton.

Dr. Jaymie Matthews is a Professor of Astronomy in the Department of Physics & Astronomy at the University of British Columbia.

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