A planet does not have to shine to be detected. It does not even have to orbit a star. Free-floating planets, sometimes called rogue planets, travel through the Milky Way without being gravitationally bound to a host star. That makes them extraordinarily difficult to find. There is no parent star for the planet to transit, … Read more

In gravitational microlensing, astronomers often begin with a convenient simplification: they treat the distant source star as a single point of light. Usually, that approximation works remarkably well. But when the lens passes extremely close to the source, the star’s actual angular size can no longer be ignored. The resulting change in the observed light … Read more

Binary-lens microlensing events produce caustic crossings when a background star’s apparent path intersects a caustic created by two foreground masses. At an ordinary fold crossing, a pair of lensed images appears or disappears, producing a rapid change in the star’s observed brightness. The star does not physically pass through either lens. The crossing describes changing … Read more

Chromatic microlensing is a gravitational microlensing event in which the observed amount of magnification changes with wavelength, so the source appears to change color during the event. The important twist is that gravity itself is not acting like a prism. In ordinary gravitational lensing, the deflection of light is essentially independent of wavelength. The chromatic … Read more

Finding an Earth-like planet around another Sun-like star is partly a problem of seeing through dust. Even after a telescope suppresses the overwhelming glare of the host star, tiny grains produced by asteroids, comets, and other small bodies can scatter enough starlight to create a luminous haze around the very region astronomers want to examine. … Read more

A debris disk can look deceptively simple from a distance: a faint ring of dust surrounding a star, something like a scaled-up version of the Solar System’s asteroid belt or Kuiper Belt. But when astronomers observe these disks with instruments such as the Atacama Large Millimeter/submillimeter Array (ALMA), the Hubble Space Telescope, and the James … Read more

A Sun-like star may look perfectly ordinary in visible light while hiding a dusty secret. When astronomers observe some stars in infrared wavelengths, they detect more radiation than the stellar surface alone should produce. That extra infrared glow can come from microscopic grains orbiting the star. If the grains are relatively warm and are continually … Read more

A main-sequence star can look perfectly ordinary in visible light while hiding an entire architecture of dust around it. The star itself may dominate almost every photon reaching a telescope. Yet farther out, grains created by collisions among asteroids, comets, and larger planetesimals can form broad disks, narrow belts, halos, gaps, and rings. In some … Read more

Centaurs occupy one of the strangest neighborhoods in the Solar System. These small icy bodies generally orbit between Jupiter and Neptune, far beyond the region where ordinary comets become dramatically active. At such distances, sunlight is weak, temperatures are low, and water ice should remain almost completely frozen. Yet some Centaurs develop fuzzy comae, release … Read more

The outer solar system is not a quiet collection of planets moving through empty space. Between the orbit of Jupiter and the distant realm of Neptune lies a restless population of small icy bodies known as Centaurs. Centaurs are among the most intriguing objects in planetary science because they appear to occupy a transitional state … Read more