This simplified diagram of an exoplanet phase curve shows the change in total brightness of a star–planet system as the planet orbits the star. The system looks brighter when more of the lit side of the planet is facing the telescope (full phase). It looks dimmer when more of the dark side is facing the telescope (new phase), when the planet is blocking some of the starlight (transit), and when light from the planet is blocked by the star (secondary eclipse).
(Top) Diagram showing the change in a planet’s phase (the amount of the lit side facing the telescope) as it orbits its star.
(Bottom) Three-dimensional graph showing the change in the total brightness of the star–planet system as the planet orbits its star. In this graph, known as a light curve, the horizontal plane is orbital location and the vertical axis is brightness.
(Right) Scale bar. In both the orbital diagram and the light curve, color indicates the observed brightness of the star + planet: from dark purple (less amount of light detected) to white (more light detected).
Researchers use phase curves to study variations in reflectivity and temperature of a planet with longitude (from one side to another), which can provide insight into the surface composition and atmospheric conditions of the planet.
NASA, ESA, CSA, Dani Player (STScI), Andi James (STScI), Greg Bacon (STScI)