About 41 light-years away from our sun -- a relatively small distance in our vast galaxy -- the star 55 Cancri spins in the constellation Cancer. A NASA press release on November 6, 2007 revealed the discovery of a fifth planet orbiting the star. Debra Fischer, an astronomer at San Francisco State University and one of the scientists in a team that found the fifth planet stated that the discovery represented the first quintuple-planet solar system. Even more notably, all five planets in the 55 Cancri solar system exhibit nearly circular orbits, with eccentricities all below 0.1.
Eccentricity is a term mathematicians and astronomers use to denote how circular a planet's orbit is around its parent star. A perfectly circular orbit has an eccentricity of zero, while a highly stretched orbit has an eccentricity closer to one. Most planets orbit in the higher end of this range, forming a shape known as an ellipse. However, a handful of planets have orbit eccentricity low enough to be considered nearly circular. In fact, Earth is one of these planets, as its eccentricity equals only about 0.02.
Without analyzing other factors, a circular orbit implies that a planet is fairly stable over the course of its orbit. The 55 Cancri planets, labeled 55 Cancri "b" through "f," sequentially contribute to a growing list of potentially habitable planets in the nearby regions of our Milky Way galaxy. Another factor important to determining the habitability of an extrasolar planet (planets outside our home solar system) is the distance from its parent sun.
An astronomical unit (AU) equals the distance between the sun and earth. The star 55 Cancri has a mass and age similar to our own sun, which means that planets distanced fairly close to 1 AU from it could support life based on temperature. The four original planets in the system left a large empty gap between 0.25 and 6 AU. However, the fifth planet is approximated to be 72.5 million miles from the star -- only slightly closer than Earth is to the sun. Further analysis and breakthroughs in technology will determine if it can support the stringent requirements for life.