New Horizons

Supplementary material

Pluto was demoted to the status of "dwarf planet" in 2006, after the successive discovery of several bodies of comparable mass in the Kuiper belt, a discovery that forced us to either add more and more planets to the list of celestial bodies in our solar system or to define more precisely what we mean by "planet".

Although I am attached to the idea of Pluto being a planet, I can see the point: why should this icy ball with a diameter of about 2400 km deserve that lofty title while Eris, with its comparable 2300 km, be referred to as a "trans-neptunian object"? Among the criteria that help define a planet nowadays, Pluto misses one: it didn't clear its orbit of debris the way its bigger neighbours did.

Planet or no, Pluto will be visited next summer (July 2015) by the New Horizons space probe. That's extremely exciting, because Pluto is the last of the "traditional" planets yet to have been visited up close. It's very, very far away: although its orbit is very eccentric, on average it is almost 40 times as far away from the sun as Earth (5,913,520,000 km, give or take; Earth's orbit, defined as "one astronomical unit", is a meagre 149,600,000 km from the sun). The encounter between the dwarf planet and the very fast ship will be brief; New Horizons travels at 14,71 km/s relative to the sun. A timeline of the probe-Pluto encounter can be seen on the mission's website.

A good thing that came from Pluto's demotion is that Gustav Holst's orchestral suite The Planets, composed before Pluto was discovered, no longer lacks a ninth track. I wonder if Collin Matthew's Pluto, the renewer, will still be associated with Holst's work as it sometimes has been for the sake of astronomical completion.