When Pluto was discovered in 1930, it was entirely reasonable to call it a planet. It seemed, after all, to correspond to the so-called "Planet X" that was wrongly thought to be perturbing the orbits of Uranus and Neptune. Pluto's own orbit was odd—more elliptical than that of other planets, and set at a rakish tilt to the plane of the solar system—and the planet was very far away and rather small, barely two thirds the diameter of our moon. Nevertheless, it became accepted as the ninth planet, even if its discovery four years before Holst's death came too late for the composer to add a movement to his Planets suite.
Yet it was not long before the reasons for Pluto's eccentricities became plain. It is not actually a planet like the others, but merely the most visible of thousands of icy objects scattered in a disk called the Kuiper belt, beyond Neptune's orbit. If Pluto is classed as a planet, scientists wondered, might we have to start including hundreds of others once our telescopes become powerful enough?
That debate can be postponed no longer. In 2002 the Hubble space telescope showed that a Kuiper-belt object known as Quaoar is fully half the diameter of Pluto. And in November 2003, astronomers in California discovered Sedna, an object similar in size to Pluto but three times further away—possibly on the edge of an even more distant shell of icy bodies called the Oort cloud.
But 2003 UB313, a Kuiper-belt object discovered (despite its name) last year, now brings the issue to a head. The latest measurements show that it is larger than Pluto, making it the largest solar-system body to be discovered since Neptune in 1846. It is even further from the sun than Sedna, but there is no coherent case for retaining Pluto as an official planet while excluding UB313.
Its discoverers have no doubts—to them, UB313 is the "tenth planet." But a committee of the International Astronomical Union is now discussing how a planet should be defined. Clearly, such a body has to orbit the sun (rather than another planet). Beyond that, should it simply be a question of size, with some arbitrary cut-off such as a 1,000-km diameter (which would include Sedna and Quaoar)? Or any body big enough to become spherical under its own gravity—which would include Ceres in the asteroid belt, between Mars and Jupiter? Whichever way you slice it, unless we are prepared to lose Pluto as a planet, we are going to have to open the doors and admit that our solar system is far more populous than the textbooks say.
Has Bush gone green?
War makes strange bedfellows. But few could have anticipated that George W Bush's belief that his country is at war with the enemies of democracy would lead him to leap between the sheets with Greenpeace and the Sierra Club.
In his state of the union address in late January, Bush confessed that "America is addicted to oil," and stated that "we must… change how we power our automobiles." Cynics may suggest this is a little like a heroin addict admitting to being addicted to syringes; but the fact is that the president announced a series of initiatives—on biofuels, hydrogen and electric-powered cars and zero-emission coal plants—that might be expected to draw applause from Jonathon Porritt.
Lying behind this green agenda is a fear not of climate change but of dependency on oil from "unstable parts of the world." But are these grand ambitions matched by serious funding? It is hard to see how the extra $200m or so of Bush's Advanced Energy Initiative—a 22 per cent increase in the budget for clean energies—will mean anything other than slightly brisker business as usual. To put it in perspective, recall that two years ago Bush earmarked $12bn for creating a new manned mission to the moon (mostly snatched from other Nasa projects). Even Rick Wagoner, CEO of General Motors, has asked why the hydrogen fuel cell should not instead be "the moon shot of the next ten years."
It is not hard to understand why biofuels such as ethanol made from plant matter look attractive to Bush. Half of the cars manufactured in Brazil can run off ethanol made from sugar cane (most are "flex-fuel" vehicles, able to adapt to ethanol, petrol or a mixture of the two). The demand for such vehicles fluctuates with oil and sugar prices, but the technology is established.
But to run all US vehicles on corn-derived ethanol would require an area of cornfields 20 per cent larger than the total cultivated land in the country. Where would food, timber and cotton come from? The problem is that the power density even of intensively cultivated biomass is minuscule—particularly if the ethanol is produced by yeast-based fermentation, which can't get at the carbon bound up in cellulose fibres.
Hydrogen, meanwhile, only becomes an effective energy source if it can be extracted "free"—not, say, by using electricity for electrolysis, but by photocatalytic splitting of water using sunlight. This is possible in principle, but the process is not yet efficient enough to make it economically viable. It is still an act of faith that there exists, somewhere among the possible elemental combinations, a material that can efficiently absorb sunlight and use the energy to cleave hydrogen from water. The exotic mixtures tried so far suggest that this philosopher's stone of the hydrogen economy will take some seeking.
