Academic Reading — Test 198

Reading passage

The Kepler space telescope and the search for exoplanets For most of human history, the question of whether planets exist beyond our own solar system remained pure speculation. That changed dramatically with the launch of NASA's Kepler space telescope in March 2009, a mission specifically designed to determine how common Earth-sized planets are around other stars. Before Kepler, fewer than 400 planets outside our solar system, known as exoplanets, had been confirmed, most of them detected indirectly through the gravitational wobble they induced in their parent stars. Kepler took a fundamentally different approach, one that would eventually push the total count of known exoplanets past several thousand. Kepler's method relied on a phenomenon called the transit technique. As a planet orbits its star, it periodically passes directly between the star and an observer's line of sight, blocking a tiny fraction of the star's light. By staring continuously at a single patch of sky containing more than 150,000 stars, Kepler's photometer could detect these minuscule, repeating dips in brightness, often less than one hundredth of one percent. A regular, periodic dimming pattern strongly suggested the presence of an orbiting planet, while the depth of the dip revealed the planet's approximate size relative to its star. Crucially, this approach worked best for systems oriented so that the planet's orbital plane lay edge-on to Kepler's view; planets whose orbits were tilted away from this alignment would never be detected, meaning the telescope sampled only a fraction of the planets actually present in its field of view. The spacecraft itself was an elegant piece of engineering. It carried a 0.95-metre aperture telescope and a digital camera composed of 42 charge-coupled device chips, giving it an extraordinarily wide field of view for an instrument of its precision. Rather than orbiting Earth, Kepler followed an Earth-trailing orbit around the Sun, slowly drifting away from our planet over the course of the mission. This positioning kept the spacecraft free of the light and gravitational interference that a near-Earth orbit would have introduced, allowing for the stable, uninterrupted observations that the transit method demanded. For four years, Kepler monitored its target stars with remarkable consistency, accumulating a dataset that would occupy astronomers for decades to come. In 2013, a mechanical failure threatened to end the mission prematurely. Two of the four reaction wheels that kept the telescope precisely pointed had failed, leaving Kepler unable to maintain the stability required for its original survey. Rather than abandoning the spacecraft, engineers devised an ingenious workaround: by using the gentle pressure of sunlight itself as a kind of virtual third reaction wheel, they balanced the telescope sufficiently to resume limited observations. This second phase of the mission, named K2, shifted Kepler's gaze to a series of different fields along the ecliptic plane, broadening its search and yielding further discoveries until the telescope finally exhausted its fuel reserves in 2018. Among Kepler's most significant findings was the discovery that small, rocky planets are far more numerous in the galaxy than the gas giants that earlier surveys had favoured, largely because those giants were easier to detect. The mission also identified numerous planets orbiting within the so-called habitable zone, the range of distances from a star at which liquid water could plausibly exist on a planet's surface. Perhaps most striking was the revelation that multi-planet systems, where several planets orbit a single star in relatively compact configurations, appear to be a standard outcome of planetary formation rather than a rarity. By the end of its operational life, Kepler had confirmed the existence of well over 2,600 exoplanets, with thousands more candidates awaiting verification by other instruments. Kepler's legacy extends beyond the raw numbers it produced. It established the statistical groundwork for estimating how many stars in the Milky Way might host potentially habitable worlds, transforming a once-philosophical question into a calculation grounded in observational data. Subsequent missions, including the Transiting Exoplanet Survey Satellite, have built directly on the techniques Kepler pioneered, applying the transit method to stars far closer to Earth and thus easier to study in detail. In this sense, Kepler did not merely catalogue distant worlds; it redefined the scale of the search and supplied the methodological foundation on which the next generation of planet hunters continues to build.