round many of the stars in our Galaxy may be planets - solar systems, in their own right. The last decade has witnessed an explosion in the discoveries of these extrasolar planets, as they are called (extrasolar: outside of the domains of our solar system). The first confirmed planet orbiting a "Sun-type" star was discovered by astronomers Mayor and Queloz in 1995. Over 200 extrasolar planets have been identified, with the number ever increasing. A significant fraction of these planets are the so-called "Hot Jupiter" planets; these are planets whose masses are similar to that of Jupiter, but which have orbital periods of only a few days (in contrast, the planet Jupiter takes nearly 12 years to orbit our Sun). Typical distances may be only ten percent of Mercury's distance from the Sun; hence the name "Hot Jupiters."
One may well ask how such extrasolar planets can ever be detected, since they lie within the blinding light of their parent stars. The methodology is simple, but intriguing. As planets circle their parent stars, they induce a gravitational tug of war, and very small changes in velocity of the star can be measured, using a spectrograph. It was in this way that astronomers Udry and collaborators discovered a planet whose mass is only five times that of the Earth, in orbit about a star known as Gliese 581. The orbital period is just under thirteen days. The parent star, approximately twenty light years distant, has only one percent of the Sun's total luminosity, but the "Earth-sized" planet orbits the star at only seven percent of the Earth-Sun distance.
Another method is as follows: If such planets pass in front of their parent stars as seen from the vantage point of our Earth, a temporary signature will be a minute decrease in light intensity as a small fraction of the starlight is blocked. Such disk crossings may take place on a timescale of a few hours. An analogy in our solar system would be a Transit of Venus, when Planets Orbiting other Stars
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