单选题   Para. 1 ①It used to be that finding even one new planet was enough to make an astronomer's career. ②Uranus was discovered in 1781 by William Herschel, who these days has, among other things, space telescopes, asteroids, schools and a street in Paris named in his honour. ③Urbain le Verrier, who predicted the existence of Neptune in 1846, based on its gravitational influence on Uranus, has likewise given his name to craters, asteroids and bits of the French capital. ④It is one of 72 engraved into the sides of the Eiffel tower.
    Para. 2 ①These days, though, astronomers can do better. ②William Borucki has thousands to his name. ③He is the researcher who conceived of and ran Kepler, a planet-hunting space telescope. ④Kepler has discovered around 2,600 exoplanets—those that orbit stars other than the sun. ⑤Another 3,000 candidates await confirmation from ground-based telescopes. ⑥The result has been a revolution in astronomy. ⑦Its practitioners had long assumed that other stars were likely to have planets of their own. ⑧These days, they know that to be true.
    Para. 3 ①The first exoplanets were detected in 1992, thanks to the gravitational effect they had on the pulsar around which they orbited. (A pulsar is the dead, ultra-dense remnant left over after a supernova.) ②A trickle of subsequent discoveries followed.
    Para. 4 ①Kepler transformed that trickle into a flood. ②Yet it almost did not happen. ③Dr Borucki proposed the mission four times to NASA before it was accepted. ④Rather than look for stellar wobbles caused by a planet's gravity, he suggested monitoring a star's light itself—looking for tiny dips in brightness as planets, if any, crossed in front of the stellar disc. ⑤It is a simple idea. ⑥But it required the construction of a light detector 1,000 times more sensitive than anything that had been built before.
    Para. 5 ①The advantage of this 'transit' method is that it is well-suited to mass production. ②Kepler was designed to stare fixedly at a single patch of sky in the constellation of Cygnus, observing around 150,000 stars simultaneously. ③The consequential torrent of data can be used to draw statistical conclusions about the rest of the galaxy. ④It seems likely that every one of the Milky Way's hundreds of billions of stars sports at least one planet.
    Para. 6 ①Kepler's biggest quarry was Earthlike planets at just the right distance around their stars for liquid water to exist on their surfaces. ②Several have turned up, but the search became harder in last year, by which time two of the four gyroscopes that kept the telescope stable had broken. ③Kepler's engineers came up with an ingenious fix, relying on the radiation pressure exerted by sunlight to re-steady the craft. ④With its fuel depleted, no technological rescue is possible this time. ⑤But nothing succeeds like success. ⑥Exoplanets are now the hottest topic in astronomy.
    Para. 7 ①The Transiting Exoplanet Survey Satellite, launched in April, is a Kepler-like mission designed to look for exoplanets in Earth's immediate neighbourhood. ②In theory, it should be possible to sniff the air of such neighbours for signs of life, or even make rough maps of their surfaces. ③Exoplanetology is just getting started.