PHASE PLOTS of Doppler spectroscopy data, reflecting changes in the velocity of a star caused by the gravitational pull of its companion planets, enable astronomers to determine the minimum mass, orbital period, and orbital path of each planet. Here is what they have found about the planets that embrace Upsilon Andromedae, after analyzing 11 years of spectoscopy data:

PLANET B, the first to be discovered orbiting Upsilon Andromedae, hugs its mother star so closely at a mere 0.06 AU (an Atronomical Unit is the distance from Earth to the Sun, or 93 million miles) that it is almost indiquishable from it and rockets around it every 4.6 Earth days. It's orbit is circular, like the planets in our solar system, but unlike our innermost planet, Mercury, b is a behemoth, weighing in at .72 the mass of Jupiter.

PLANET C flies an elliptical orbit that seems typical of the Jupiter type extrasolar planets identified so far. With a mass nearly twice that of Jupiter, it traverses its orbit--which, at .82 AU, would cross that of Earth were it in our system--every 242 days.

PLANET D, the heavyweight of the system at four times the mass of Jupiter, also tracks an oval orbit. The outermost planet, at 2.5 AU, it makes a stately transverse every 4.5 Earth years.

Images: San Francisco State University

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