Discovery of the first solar system similar to our own.
 a prerequisite to life elsewhere in the Universe?
A hypothesis put forth by Peter D. Ward and Donald C. Brownlee - Rare Earth Hypothesis (2002), argues that humans might be alone, at least in the stellar neighborhood, and perhaps in the entire Universe. Their hypothesis is based on the belief that many of the events that favored the evolution of intelligent life were chance occurrences, unlikely to be repeated elsewhere in the Universe. A strong supporting point in their hypothesis was that we had not discovered planetary systems, similar to our Solar System, even after many years of searching. They suggested that hospitable alien planets might not be found. Some properties of our Solar System:
   Life giving planets would likely need an orbit that keeps the planet at exactly the right
        distance from its star to ensure that water remains liquid, not vapor or ice.

A large moon at just the right distance to minimize changes in a planet's tilt,
        ensuring climate stability. Enough carbon to aid the development of life but not
        so much to allow for runaway greenhouse conditions, as occur on superheated Venus.
   A rain of killer meteorites, unless prevented could lead to mass extinctions.
        Recall the idea that meteorites hit Earth 65 million years ago killing off many plants,
        animals, & the dinosaurs.
 Planetary studies suggest the rate of Earth impacts would
        be 10,000x higher, but for Jupiter, our solar system's largest planet, which may
        blocks many killer rocks.


        Ward & Brownlee's RARE EARTH Hypothesis suggests that the Earth may be in a unique protected Life Zone where because of the fortuitous presence of Jupiter we are in a planet system that may have favored the evolution of Intelligent Life.   If we could discover planetary systems similar to our own Solar System we might collect more evidence of life elsewhere in the Universe.

    After 15 years of observation and a lot of patience, the world's premier planet-hunting team has finally found a planetary system that reminds them of our own solar system. Dr. Geoffrey Marcy, astronomy professor at the UC Berkeley, and astronomer Dr. Paul Butler of the Carnegie Institution of Washington, on June 13, 2002 announced the discovery of a Jupiter-like planet orbiting a Sun-like star at nearly the same distance as the Jovian system orbits our own Sun.

    Discovery of a Jupiter-like planet in another Solar System is significant because only such a system could harbor a smaller, Earth-like planet that theoretically could support living creatures. A planet that is too big or too near its star or travels in an odd orbit probably could not be habitable.

    ''This discovery moves us one step closer to answering the question: Are we alone in the universe?'' said Anne Kinney, director of NASA's astronomy and physics division.




     "All other extra-solar planets discovered up to now [about 31 planet have been discovered since 1996] orbit closer to their parent star, and most of them have had elongated, eccentric orbits. This new Jupiter-like planet orbits as far from its star as our own Jupiter orbits our Sun,'' said Marcy. The newly reported planet is a giant ball of gas, four times more massive than Jupiter. It is located about 512 million miles from a star known as 55 Cancri, which is in the constellation Cancer, the Crab. In our own system, Jupiter is 484 million miles from our sun, five times further away from the sun than Earth. The unnamed planet takes 13 years to circle its mother star, slightly longer than Jupiter's 12-year orbit. On a dark night, 55 Cancri is visible to the naked eye in the northern sky, southeast of the Big Dipper.  It is 41 light years (about 240 trillion miles) from Earth, meaning that radio signals [SETI], if a civilization existed there, would take 82 years to make a round trip.

       55 Cancri was already known to have one planet (found in 1996 by Butler & Marcy). That planet is a gas giant slightly smaller than the mass of Jupiter and whips around the star in 14.6 days at a distance only one-tenth that from Earth to the Sun.  The star 55 Cancri is 41 light years from Earth and is about 5-billion years old. Further data are needed to determine whether yet another planet is orbiting it, because the two known planets do not explain all the observed Doppler wobbling. One possible explanation is a Saturn-mass planet orbiting about .24 AU from the star.






       Using as a yardstick the 93-million mile Earth-Sun distance, called an astronomical unit or AU, the newfound planet orbits at 5.5 AU, comparable to Jupiter's distance from our Sun of 5.2 AU (about 824 million kilometers or 512 million miles). Its slightly elongated orbit takes it around the star in about 13 years, comparable to Jupiter's orbital period of 11.86 years. It is 3.5 to 5 times the mass of Jupiter.

       "We haven't yet found an exact solar system analog, which would have a circular orbit and a mass closer to that of Jupiter. But this shows we are getting close, we are at the point of finding planets at distances greater than 4 AU from the host star," said Butler. "I think we will be finding more of them among the 1,200 stars we are now monitoring." 

        The existence of close analogs to our solar system adds great urgency to the search for Earth-sized planets elsewhere in the Universe.   Greg Laughlin, an astrophysicist at the UCSC, has run computer simulations of the 55 Cancri three-planet system. His calculations show that an Earth-sized planet could survive for billions of years in a stable orbit in the wide gap between the second and third gas giants. That would be long enough for possible life forms to arise.






    The Jupiter-like object around 55 Cancri greatly improves the chances that an Earth-like planet will eventually be found.

        Marcy, Butler, Debra A. Fischer and their team also announced a total of 13 new planets today, including the smallest ever detected: a planet circling the star HD49674 in the constellation Auriga at a distance of .05 AU, one-twentieth the distance from Earth to the Sun. Its mass is about 15 percent that of Jupiter and 40 times that of Earth. This brings the number of known planets outside our solar system to more than 90.

So far, no alien planets have been seen directly. Instead, their presence is deduced indirectly from the tiny wobble their gravity causes in the motion of their star as they circle around it. The stars' wobble is a movement of barely 50 yards per second, yet Earth-based instruments can detect it from 240 trillion miles away.

Planet Search Group