The phenomenon known as planetshine occurs when reflected sunlight from a planet illuminates the dark side of one of its moons. Typically, this results in the moon's dark side being bathed in a soft, faint light. The best known example of planetshine is Earthshine, which can be seen from Earth when the Moon is a thin crescent. Planetshine has been observed elsewhere in the solar system: in particular, it has recently been used by the Cassini space probe to image portions of the moons of Saturn even when they are not lit by the Sun.
Diagram of planetshine
Earthshine is most readily observable shortly before and after a New Moon, during the waxing or waning crescent phase. When the Moon is new as viewed from Earth, the Earth is nearly fully lit up as viewed from the Moon. Sunlight is reflected from the Earth to the night side of the Moon. The night side appears to glow faintly and the entire orb of the Moon is dimly visible.
Leonardo da Vinci explained the phenomenon in the early 1500s when he realized that both Earth and the Moon reflect sunlight. Light is reflected from the Earth to the Moon and back to the Earth as Earthshine.
Earthshine is used to help determine the current albedo of the Earth. The data are used to analyze global cloud cover - a climate factor. Oceans reflect the least amount of light, roughly 10%. Land reflects anywhere from 10-25% of the Sun's light, and clouds reflect around 50%. So, the part of the Earth where it is daytime and from which the Moon is visible determines how bright the Moon's Earthshine appears at any given time.
Studies of Earthshine can be used to show how the Earth's cloud cover varies over time. Preliminary results show a 6.5% dip in cloud cover between 1985 and 1997 and a corresponding increase between 1997 and 2003. This has implications for climate research, especially with regards to global warming. Some clouds have a net warming effect because they trap heat, while others have a net cooling effect because they increase albedo, so the overall effect on global temperature remains unclear. 
The rings of Saturn light up the back side of Saturn, as shown in this long exposure Cassini Orbiter photo.
Search for terrestrial planetsEdit
Scientists at NASA's Navigator Program, which specializes in the detection of terrestrial planets, have backed the launch of a Terrestrial Planet Finder (TPF) mission.  TPF would detect planetshine from planets orbiting stars to investigate whether they could harbor life. It would use advanced telescope technologies to look for life-marks in the light reflected from the planets, including water, oxygen and methane.
Unlike many traditional astronomical challenges, the most serious challenge for these missions is not gathering enough photons from the faint planet, but rather detecting a faint planet that is extremely close to a very bright star. For a terrestrial planet, the contrast ratio of planet to its host stars is approximately ~10−6-10−7 in the thermal infrared or ~10−9-10−10 in the optical/near infrared. For this reason, Darwin and Terrestrial Planet Finder-I will work in the thermal infrared. 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. Therefore, NASA is also pursuing an Terrestrial Planet Finder-C mission that will search for and study terrestrial planets using the optical (and near infrared) wavelengths. While Terrestrial Planet Finder-C aims to study the planetshine of extrasolar planets, Darwin and Terrestrial Planet Finder-I will search for thermal infrared light that is reradiated (rather than scattered) by the planet, and most astronomers would not consider that be planetshine.
In preparation for these missions, astronomers have performed detailed Earthshine observations. Astronomers have paid particular attention to whether Earthshine measurement can detect the red edge, a spectral feature that is due to the plants. The detection of a similar spectral feature in light from an extrasolar planet would be particularly interesting, since it might be due to a light-harvesting organism. While the red edge is almost certainly the easiest way to directly detect life on earth via Earthshine observations, it could be extremely difficult to interpret a similar feature due to life on another planet, since the wavelength of the spectral feature is not known in advance (unlike most atomic or molecular spectral features).
- Ford, E. B., Turner, E.L. & Seager, S. (2001) ``Characterization of extrasolar terrestrial planets from diurnal photometric variability Nature, Volume 412, Issue 6850, pp. 885-887. link and preprint
- Seager, S., Turner, E. L., Schafer, J., & Ford, E. B. (2005) ``Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants Astrobiology, Volume 5, Issue 3, pp. 372-390. (link and preprint)
- Qiu J, Goode PR, Palle E, Yurchyshyn V, Hickey J, Rodriguez PM, Chu MC, Kolbe E, Brown CT, Koonin SE (2001). "Earthshine and the Earth's albedo: 1. Earthshine observations and measurements of the lunar phase function for accurate measurements of the Earth's Bond albedo". Journal of Geophysical Research-Atmospheres 108 (D22): 4709.
- Science@NASA: Earthshine
- NASA Astronomy Picture of the Day, 19 April 2002
- 'Earthshine' Linked to Solar Cycle, Climate Change, Space.com
- Scientists Watch Dark Side of the Moon to Monitor Earth's Climate, American Geophysical Unionca:Lluna cendrosa