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.