The way Kepler has discovered all those "new" planets and their characteristics is by staring at stars.
When a planet orbits and passes in front of its star (in what's known as a "transit"), it naturally blocks some of the light being emitted by that star. The brightness of that star, then, drops -- and, under certain conditions, Kepler's instruments can register that drop. As NASA puts it:
"By measuring the depth of the dip in brightness and knowing the size of the star, scientists can determine the size or radius of the planet. The orbital period of the planet can be determined by measuring the elapsed time between transits. Once the orbital period is known, [Johannes] Kepler's Third Law of Planetary Motion can be applied to determine the average distance of the planet from its star" -- and this, along with the probable temperature of the star, can be used to determine the likely temperature on the planet.
Earth-based instruments have used a similar technique -- involving a planet's gravitational pull on its star, as opposed to changes in the star's brightness -- to spot new planets. In fact, in 2010, astronomers working with a spectrometer and this "wobble method" at Hawaii's Keck Observatory announced they'd discovered the first real example of a potentially life-friendly exoplanet.
But the brightness-based "transit method" of planet-finding provides information the wobble method doesn't -- perhaps most importantly, a planet's size. And Earthbound tools can't use the transit method -- Earth's orbit and the changing night sky prevent constant monitoring of the same stars, and atmospheric conditions interfere. Since it sits comfortably out in space, Kepler avoids these issues (and it has special characteristics that a space telescope like Hubble doesn't). Its data can be combined with info gleaned by Earthbound and other instruments to create profiles of planets.
May 19, 2013 12:01 AM PDT
Photo by: NASA Ames
| Caption by: Edward Moyer
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