For most planets including Earth, it is reasonably accurate to assume that the nightside begins at a zenith angle of about 90°, which means approximately half the planet’s surface is in daylight while the other half experiences night. A zenith angle of 0° corresponds to the spot on the planet where its host star appears directly overhead (i.e. the sub-stellar point). However, for planets that orbit very close to their host stars, the large apparent size of the stellar disk means that an irradiance distribution which assumes the nightside begins at a zenith angle of 90° becomes inaccurate. On these planets, the nightside begins at a zenith angle significantly larger than 90°. This means the dayside covers a much greater extent than the nightside.
Planets that orbit close to their host stars are quite likely tidally-locked, where the same side of the planet always faces its host star, resulting in a permanent dayside and nightside. Extreme examples include COROT-7b, Kepler-10b and Kepler-78b. These planets orbit so close to their host stars that they each complete a year in a matter of hours. Temperatures at the sub-stellar point are expected to reach 2,500 K or more. Such temperatures are high enough to keep rock material molten, making it possible for lava oceans to exist on the hellish daysides of these planets.
On these intensely hot worlds, the host star looms so large in the sky that the nightside only begins at a zenith angle much larger than 90°, so that well over half the planet is always in daylight. In the case of Kepler-78b, more than two-thirds of the planet is always in daylight. These planets are too hot to hold onto any appreciable atmosphere than can effectively transport heat from the dayside to the nightside. As a result, even though the dayside can blaze as hot as the tungsten filament of an incandescent bulb, temperatures on the nightside can be as low as just a few tens of degrees above absolute zero.
COROT-7b, a rocky planet with 1.58 times Earth’s diameter, is in a 20.5-hour orbit at a distance of 0.0172 AU around a star measuring 0.82 times the Sun’s diameter. On COROT-7b, the nightside begins at a zenith angle of 102.6°, which means 61.2 percent of the planet’s surface is always in daylight. Kepler-10b is another rocky planet with 1.07 times Earth’s diameter. It is in a 20.1-hour orbit at a distance of 0.0168 AU around a Sun-like star measuring 1.07 times the Sun’s diameter. On Kepler-10b, the nightside begins at a zenith angle of 106.9°, which means 65.2 percent of the planet’s surface is always in daylight.
The most striking example is Kepler-78b, a rocky world with a girth that is 1.16 times the diameter of Earth. This planet is even crazier than COROT-7b and Kepler-10b when it comes to how close-in a planet can orbit its host star. Kepler-78b whizzes around its host star once every 8.5 hours. At a distance of 0.0089 AU from a star measuring 0.737 times the Sun’s diameter, Kepler-78b is only 1.6 stellar radii from the fiery surface of its host star. On Kepler-78b, the nightside begins at a zenith angle of 112.3°, which means a remarkable 70.5 percent of the planet’s surface is always in scorching daylight.