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As Table 1 shows, there are a few ``Sol-Like'' systems that have been discovered, according to a simple rule: at least one planet with both a semi-major axis larger than 0.5 AU (i.e. outside the orbit of Mercury), and an eccentricity lower than 0.12 (higher than Mars (0.093) but lower than Mercury (0.206)). But these cases are marginal and exceptional: only 3 of 58 confirmed systems, and so-far, none so widely spaced as ours.
|No Detected Planets||21||—|
|Hot Giants||a < 0.5AU||—|
|Eccentric Giants||a ≥ 0.5AU||e > 0.12|
|Circular Giants||a ≥ 0.5AU||e ≤ 0.12|
55 Cnc is not classified because the eccentricity of the second planet is not known. ``Mult'' indicates the number of systems in each class which contain multiple planets. To be classed as a Hot Giant system, all planets must be closer than 0.5 AU, otherwise the outer planet will determine the class.
Table 2 shows a rough classification of the systems we do find. Oddly, there are a number of systems with multiple eccentric gas giants - a situation that was once thought unstable. The number of systems in which more than one planet is detected are indicated in the third column. Our Sol system would be classified as a ``Circular Giant'' system, but is not included here.
The fact that 55 Cnc, with a very heavy planet beyond 4 AU, is not better characterized may be an indicator that Sol-like systems are still too close to our observing limits (i.e. that we are looking at selection effects). But that's not the whole answer, becAUse if systems like ours were among those surveyed, observers have claimed that they would have detected them. Certainly this does not explain the preponderance of the hot giant systems.
Does this mean that low-mass, dust-rich disks, though rare, are the
only abodes for life? Does it increase the likelihood that we are alone
(or at least more isolated) in the Universe? Or does it merely mean that
most habitable planets are actually Earthlike moons around heavy gas-giants,
like the heavy planet around the G5 star HD28185? Given the difference
in mass from Jupiter, a protoplanetary cloud capable of forming Mars-
or Earth-mass moons does indeed seem plAUsible. Or are there terrestrial
planets to be found exterior to the short-period giants? These answers
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On 21 May 2001, 20:25.
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