When an exoplanet passes in front of its star, the resulting dip in starlight often looks reassuringly neat. Brightness falls as the planet begins to cross the stellar disk, remains lower for a while, and then rises again as the planet leaves. In an idealized diagram, the first half of the transit almost mirrors the … Read more

A planet passing in front of its star may seem almost like a celestial clock. Every orbit, the planet crosses the stellar disk. The star becomes slightly dimmer, the planet continues along its orbit, and the light returns to normal. If the orbit were perfectly isolated and unchanging, these transits would repeat with remarkable consistency. … Read more

When astronomers discover an exoplanet passing in front of its star, they usually expect the event to behave like a reliable clock. If a planet takes exactly 10 days to orbit its star, for example, its transits should occur roughly every 10 days. After observing several transits, astronomers can predict when the next one should … Read more

A white dwarf is supposed to be simple. After a Sun-like star runs out of nuclear fuel, sheds its outer layers, and leaves behind its compact core, the remaining object gradually cools. In many cases, astronomers can predict fairly well how much light that cooling stellar remnant should emit at different wavelengths. Then some white … Read more

A planet can spend billions of years circling its star in apparent stability. Then the star begins to die. For planets around Sun-like stars, this does not usually mean an immediate explosion. Instead, the star gradually exhausts the hydrogen fuel in its core, expands enormously, brightens, and becomes a red giant. That transformation can completely … Read more

A white dwarf may look like the quiet endpoint of a star’s life, but its spectrum can contain evidence of a planetary system that once surrounded it. Among the most useful clues are absorption lines produced by calcium. Calcium should not remain visible for long in the atmospheres of many white dwarfs. The enormous surface … Read more

A white dwarf may look like the quiet ending of a planetary system. The star has exhausted its nuclear fuel, shed much of its outer material, and collapsed into a dense stellar remnant roughly comparable in size to Earth. Any planets, asteroids, and comets that once orbited the original star have already survived a period … Read more

A white dwarf should have an extraordinarily simple atmosphere. Its immense gravity pulls heavier elements downward, leaving the visible surface dominated mainly by hydrogen or helium. Elements such as calcium, magnesium, iron, and silicon should disappear beneath the photosphere relatively quickly. Yet astronomers repeatedly find white dwarfs whose spectra contain exactly these elements. They call … Read more