Can astronomers use eclipses to detect exoplanets?
Well…not exactly! An eclipse requires that the sun or star be fully covered by the disk of a planet as the planet passes between the star and an observer on Earth. From basic geometry, the amount of starlight dimming depends on the ratio of the circular area of the planet to the circular area of the star. For an eclipse, 100% of the star’s light is dimmed. This requires that the planet have the same diameter as the star, which is physically impossible. However, when the planet’s diameter is much smaller than the star, astronomers call this a transit, and this is one of many methods that are actually used to detect planets orbiting distant stars. For example, a Jupiter-sized planet has a diameter of 143,000 km while a sun-like star has a diameter of 1.4 million km, so the ratio of their areas is 1/100. As this exoplanet transits the disk of its star as viewed from Earth, the brightness of the star will dim by 1%. Since the 1990’s, astronomers have detected over 3,600 exoplanets orbiting 2,700 stars. Of these, NASA’s Kepler observatory has detected over 2,300 of the confirmed exoplanets using the transit method.
Another exciting aspect of exoplanet transit detections is that the star’s light passes through the atmosphere of the exoplanet on its way to Earth. By using an instrument called a spectroscope, astronomers can examine the way the atmosphere absorbs the star’s light to detect the composition of the atmosphere. Dozens of exoplanets have been studied in this way so far. Most reveal signs of carbon dioxide, water vapor and methane. If oxygen is ever discovered, this will be an important sign that the planet harbors a biosphere of some kind!