No one's ever seen the planet known as HD 209458b. It's so close to its parent star that it's impossible to see through the star's glare. Even so, we know a lot about it. It's about two-thirds as massive as Jupiter, the giant of our solar system. Its atmosphere contains oxygen, carbon, and other elements, and winds at the top of the atmosphere blow at about 4,000 miles an hour. And the atmosphere is being blown into space by the nearby star.
We know all of this because the planet "transits" its parent star -- it passes across the face of the star as seen from Earth.
Astronomers have already discovered dozens of planets with the transit technique, and they expect to find many more. A NASA space telescope known as Kepler is looking for transits in more than 150,000 stars, and its observations may already hold the discoveries of hundreds of new worlds. Eventually, Kepler should also find Earth-like planets in Earth-like orbits.
When the transits are combined with other observations, they can reveal a planet's size, its mass, and its distance from its star. And by measuring changes in the properties of the star's light during a transit, astronomers can determine whether the planet has an atmosphere, and even what the atmosphere contains.
And future transit observations may reveal even more -- from the presence of moons and rings to details about a planet's shape and its interior -- all without ever seeing the planet at all.
Script by Damond Benningfield, Copyright 2010
Today's program was made possible in part by a grant from NASA.