Mars, Jupiter, and all the other planets and asteroids in the night sky that are visible to us can be seen because they reflect sunlight. The "albedo" is the quantity that quantifies a body's ability to reflect light; a value of one indicates that all of the incident light on the body is reflected. Each body has a unique surface composition and atmosphere that reflect light in their own ways. For example, the presence of clouds in a planet's atmosphere can make it highly reflective. All these and other important physical properties contribute to the albedo, making the albedo one of the key properties of a planet.
There are 307 extrasolar planets (also called "exoplanets") known to date, but until now none of them has a known albedo. Astronomers can determine the radii and masses of exoplanets from precise observations of their orbital motions, but because the reflected starlight is only about one hundredth of one percent of the star's direct flux, even for a Jupiter-sized planet, the task of measuring the albedo is daunting.
Four CfA astronomers, Matt Holman, Jose Fernandez, David Latham, and David Charbonneau, together with four of their colleagues, have just reported the first precisely measured upper limit to the albedo of an extrasolar planet: TrES-3, a 1.9-Jupiter-mass body orbiting a sun-like star at a distance less than the distance between Mercury and the sun. TrES-3 has an orbital inclination to our line-of-sight that happens to make it cross the face of its star (an occultation). The team watched six occultations of the planet using only small, ground-based telescopes. By precisely measuring the flux changes at three wavelengths, they were able to set useful upper limits to the planet's albedo at these three wavelengths -- and the data show that clouds probably do not exist in the planet's upper atmosphere. The results not only provide the first meaningful measurement of an exoplanet's albedo, they demonstrate that relatively small telescopes can, with patience and care, infer many key properties of distant planets around other stars.