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Selected Articles from the
April 2001 Odyssey

Editor: Terry Hancock

The Rarity of Sol-Type Systems

Terry Hancock

Science often presents us with philosophical challenges when we try to interpret the results of data - especially when regarding a single case.

We have such a problem in understanding the case of life on Earth. How do we know what factors lead to a habitable planet? If we find something remarkable about our place in the universe, we might assume that this characteristic is special because it is necessary for life on Earth - i.e. that the only reasonable explanation for something far from normal, is that, without it, we wouldn't be here to make the observation. This is the ``anthropomorphic principal'' in astronomy. But, we must also be prepared for the other statistical effect: given enough improbabilities, it is highly probable that one of them will turn out to be true! So far, most of the characteristics of our place in the universe have been fairly commonplace: we're not in the center of the universe (there isn't one), nor of the galaxy (there is one, but you wouldn't really want to live there), nor is our star unusual. Our planet is a little unusual in fairly obvious ways that are related to life: a thick atmosphere, lots of liquid water, and temperature amenable to the carbon-based chemistry on which we are based, but those peculiarities have a known relationship to our presence here, and are not themselves viewed as terribly exceptional.

There is one odd thing about Sol System, though, which recent extrasolar planet research has revealed: our system of nine neat little planets in almost perfectly circular orbits is just plain weird! We expected to find lots of systems like it - but we don't. Our planetary system is puny - many systems exist with a great deal more mass invested in the planets. And high eccentricities are the norm, not the exception. So are systems with gas giant planets orbiting close to their stars (much closer than Mercury orbits Sol). Based on the theories which have recently been proposed to explain these results, it appears that these other systems (i.e. most planetary systems) are formed from heavy, gas-rich protoplanetary disks. Our system, by contrast, is the result of accretion in a relatively lightweight, dust-rich disk. That might be important - after all, it's easier to understand terrestrial planets (like ours) forming in a dust-rich disk. In gas-rich disks, terrestrial planets might be relatively rare - or perhaps they tend to form as moons, instead.

Name Mass
Min   Mean   Max
HD27442′ 1.43 1.16 1.18 1.20
47 UMa′ 2.41 1.90 2.10 2.30
HD28185′ 5.6 0.94 1.0 1.06
HD10697′ 6.59 1.76 2.0 2.24
Jupiter 1.0 4.953 5.203 5.453

Table 1: ``Sol-Like'' Systems
(Principle Gas-Giant)

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.

System Types # Mult
No Detected Planets 21
Hot Giants 28 2
Eccentric Giants 25 4
Circular Giants 4 0
Unclassified 1 ?
Total Surveyed 79 6

Table 2: Classes of extrasolar
planetary systems detected.

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 remain elusive.

File translated from TEX by TTH, version 2.25.
On 21 May 2001, 20:25.