The travelling astronomer in Prague 5

Debating the definition of a planet

Debating the definition of a planet

Today saw the lunchtime discussion on what is the definition of a planet. Though the proposal put to the meeting had already been somewhat modified there was some strong opposition and heated debate – unusually heated for a group of astronomers. There was so much debate that the meeting reconvened at 5:30 pm. By that time the proposal had been changed further to try to achive concensus.

The new proposed definition of a planet states that it must a) orbit the Sun b) must be in hydrostatic equilibrium, ie it must be big enough for gravity to have pulled it into a near spherical shape c) that it must dominate its region of space, ie there are no other objects of comparable size crossing its orbit.

The last point is to satisfy the astronomers who study the motion and dynamics of the motion of the planets and who led the opposition to the previous proposal. The obvious consequence of the extra point is that there will be only eight planets in the solar system – all the current ones except Pluto, which will be in a new catergory of dwarf planet.

With some more slight modifications the new proposal should be accepted by the astronomers at Thursday’s vote. I feel much happier about voting in its favour.

The reason the meeting was held at lunchtime and then late in the afternoon because the planet definition is not the main business of the conference. For example, today I attended sessions on the latest results from space missions, on archeoastronomy, on astronomy from Antarctica and on binary (double) stars. And to finish I heard a brilliant invited lecture on the study of the black hole at the centre of the galaxy, the mass of which has now been established as 3.5 million times that of the Sun.

One response to “The travelling astronomer in Prague 5

  • Nick, Hi

    I’m intrigued concerning the third criterion of being a planet, namely “c) that it must dominate its region of space, ie there are no other objects of comparable size crossing its orbit.”. After all the Earth is significantly effected by the moon, the moon crosses the Earth’s orbit. The tides on Earth are primarily driven by the moon. Whilst obviously the Earth has the greater gravitational effect in the Earth moon system, was there a consideration of the Earth-Moon system when this decision was made, and how could this be compared to the Pluto-Charon system. Anyway, I think there are a few other path ways here – maybe a good topic for the President’s address.

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