The Planet That Wasn’t There
- The Hunt for Vulcan: How Albert Einstein Destroyed a Planet and Deciphered the Universe by Thomas Levenson
Head of Zeus, 229 pp, £7.99, August 2016, ISBN 978 1 78497 398 8
Last January, a pair of scientists at the California Institute of Technology, Konstantin Batygin and Michael Brown, announced that they had discovered compelling evidence of an as-yet-unseen giant planet – Planet X – orbiting the Sun, seven times further out than Neptune. This isn’t the first time that astronomers have believed there may be nine planets in the solar system. From its discovery in 1930 until 2006, Pluto – smaller than our moon, and nearly forty times as far from the Sun as the Earth is – was considered the ninth. And between 1859 and 1915 it was widely believed, for perfectly sound reasons, that a small planet known as Vulcan lurked invisibly close to the Sun, inside the orbit of Mercury. How that belief came about, and how Einstein came to demolish it, is the subject of Thomas Levenson’s eye-opening book.
Pluto lost its planetary status ten years ago after a group of astronomers, Brown among them, discovered another object of roughly the same size orbiting the Sun beyond Neptune. If Pluto was a planet, then so was Eris (as it’s now known): either a tenth planet had been discovered, or Pluto wasn’t a planet after all. The International Astronomical Union made its decision public in August 2006. Since then, a planet has been defined as a ‘celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit’. It’s the last of these conditions that Pluto and Eris don’t satisfy, so the IAU invented a new category for them: dwarf planets. A full-blown planet has enough mass to exert gravitational control over anything in its path; a dwarf planet doesn’t. (There are five known dwarf planets in the solar system: along with Pluto and Eris, there are two more beyond Neptune – Haumea and Makemake – as well as Ceres, the largest body in the asteroid belt between Mars and Jupiter. It has a diameter of nearly six hundred miles, roughly the length of Britain from Dunnet Head to Lizard Point. Discovered in 1801, and at first thought to be a comet, it had a brief life as a planet before sightings of other nearby objects led it to be classified as just one among many asteroids, until Pluto’s relegation gave it a place among the newly defined dwarf planets.)
Clyde Tombaugh, a 24-year-old researcher at the Lowell Observatory in Arizona, spotted Pluto on 18 February 1930. The observatory had been established in 1894 by Percival Lowell. As well as doing much to popularise the idea that there might be life on Mars (his many books include Mars and Its Canals and Mars as the Abode of Life), Lowell initiated a systematic search for a new planet beyond the orbit of Neptune, which he called – you’ve guessed it – Planet X. The body that Tombaugh discovered wasn’t exactly what Lowell had been expecting to find, but never mind. The name Pluto was suggested by Venetia Burney, an 11-year-old girl from Oxford whose grandfather passed the idea on to the Lowell Observatory.
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 Batygin and Brown published their findings in the Astronomical Journal on 20 January 2016.
 My Journey at the Nuclear Brink by William J. Perry (Stanford, 234 pp., $24.95, November 2015, 978 0 8047 9681 1).