Planetshine is the dim illumination, by sunlight reflected from a planet, of all or part of the otherwise dark side of any moon orbiting the body.
The most observed and familiar example of planetshine is earthshine on the Moon, which is most visible from the night side of Earth when the lunar phase is crescent or nearly new,[1] without the atmospheric brightness of the daytime sky.
In particular, the Cassini space probe used Saturn's shine to image portions of the planet's moons, even when they do not reflect direct sunlight.
The New Horizons space probe similarly used Charon's shine to discover albedo variations on Pluto's dark side.
[2] Although using a geocentric model in 510 AD, Indian mathematician and astronomer Aryabhata was the first to correctly explain how planets and moons have no light of their own, but rather shine due to the reflection of sunlight in his Aryabhatiya.
Leonardo da Vinci explained the phenomenon in the early 16th century when he realized that both Earth and the Moon reflect sunlight at the same time.
So while the Earth's albedo is measurably increasing, the uncertainty over the amount of heat trapped means the overall effect on global temperature remains unclear.
They are thought to have been formed from drops of molten ejecta, produced by impact events, which cooled and solidified before falling back to the surface.
It would use advanced telescope technologies to look for life-marks in the light reflected from the planets, including water, oxygen and methane.
However, searching for terrestrial planets in the optical/near infrared has the advantage that the diffraction limit corresponds to a smaller angle for a given size telescope.
Astronomers have paid particular attention to whether earthshine measurement can detect the red edge, a spectral feature that is due to plants.