Academic Reading — Test 115

Reading passage

For most of human history, the planets that circle other stars remained entirely hypothetical. A planet shines only by reflecting the light of its parent star, and that faint glow is utterly overwhelmed by the brilliance of the star itself, much as the flame of a candle would vanish beside a searchlight. Because of this enormous difference in brightness, astronomers were unable to photograph such worlds directly. Instead, they devised indirect techniques that infer a planet's presence from its effect on the star. Among these methods, the most productive has proved to be the transit dimming technique, which has been responsible for the discovery of the majority of the thousands of confirmed exoplanets catalogued to date. The principle behind the method is disarmingly simple. When a planet passes directly between its star and an observer on Earth, it blocks a small fraction of the starlight, and the star appears momentarily fainter. This event is called a transit. By measuring the brightness of a star with great precision over long periods, astronomers can detect the tiny, regular dips that betray an orbiting body. The amount of light lost during a transit depends chiefly on the relative sizes of the planet and the star: a large planet crossing a small star blocks a greater proportion of the light than a small planet crossing a large one. For a giant planet similar to Jupiter passing in front of a star like our Sun, roughly one per cent of the light is blocked, whereas an Earth-sized planet dims its star by only about one part in ten thousand. Detecting such a minuscule change demands instruments of extraordinary sensitivity. Crucially, a single dip in brightness is not enough to confirm a planet. Many other phenomena can cause a star to fade temporarily, including dark patches on the star's own surface, known as starspots, or the presence of a faint companion star. To distinguish a genuine planet, astronomers look for repetition. A true transit recurs at strictly regular intervals, because the planet returns to the same position once every orbit. The time between successive dips therefore reveals the length of the planet's year, and from this orbital period the average distance between the planet and its star can be calculated. The shape of the dip carries further information: the depth indicates the planet's size, while the duration and the gentleness of the dimming at the beginning and end hint at the geometry of the crossing. The technique does have an important limitation. A transit can only be observed if the planet's orbit happens to be aligned almost edge-on as seen from Earth, so that the planet actually passes across the face of the star from our vantage point. Most planetary systems are tilted at other angles and never produce transits that we can see. As a result, the method detects only a modest fraction of the planets that genuinely exist, and the absence of a transit signal never proves that a star is without planets. To compensate for this low success rate, astronomers monitor enormous numbers of stars simultaneously, reasoning that even if only a small percentage of systems are favourably oriented, surveying hundreds of thousands of stars will still yield a substantial harvest of detections. Space-based observatories have transformed the practice of transit hunting. The Earth's atmosphere causes starlight to shimmer and twinkle, introducing fluctuations in brightness that can swamp the faint signal of a transiting planet. By placing telescopes above the atmosphere, agencies have been able to measure stellar brightness with a steadiness that is impossible from the ground. The Kepler mission, launched in 2009, stared continuously at a single patch of sky containing more than a hundred thousand stars and discovered thousands of candidate planets, demonstrating that small, rocky worlds are abundant throughout the galaxy. Its successors have extended the search across the whole sky. The transit method offers a further bonus that goes beyond merely counting planets. When a planet with an atmosphere transits its star, a small amount of starlight filters through the thin shell of gas surrounding the planet before reaching us. Different gases absorb light at characteristic wavelengths, leaving distinctive fingerprints imprinted on the spectrum of the filtered light. By analysing these fingerprints, astronomers can begin to determine the chemical composition of a distant atmosphere without ever leaving the Solar System. This technique, known as transmission spectroscopy, has already detected water vapour and other molecules in the atmospheres of certain exoplanets, and it represents one of the most promising avenues in the long search for worlds that might support life.