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Planets for Humanity

Extrasolar Planets Overview

Known Planets Beyond the Solar System
Nearly all of the extrasolar planets discovered so far have masses comparable to that of Jupiter. More than likely, all are gas giants. Even so, there is some variety among them. A few of these massive worlds are frighteningly close to their suns — far closer than Mercury is to Sol. Several systems have more than one such planet, in fact, up to four worlds per system so far. Several reside in multiple star systems, that is, systems with more than one sun.

Below are descriptions of the details to be covered in this section. Or go directly to list of extra-solar planets.

Comparison of Orbits
Split images show the difference between the planetary orbits in an alien system and those in our own Solar system. Though some orbits, like those of Mercury, Mars, and Pluto, are markedly elliptical, these diagrams show only the average orbital distances for the Solar planets. This makes it easier to see the elliptical nature of some of the extrasolar planetary orbits.

Hover mouse cursor over picture to show sample split image comparison of orbits. (Browser must allow scripts or be Javascript enabled to view this detail)

To add meaning to the extrasolar system diagrams, we have included the location in a system where habitable worlds would be found based on temperature alone. The inner edge of this zone of ecologically friendly temperatures is naturally hot. Likewise, the outside edge is cold. Between the two temperature limits, we have marked the location where Earth-normal temperatures would exist.

Though not to scale, this diagram brings home the notion that the hotter the star, the farther away a planet has to be to have a comfortable average temperature. Sample spectral types illustrate a wide range of energy output.

The warm and cold extremes in the "to-scale" illustrations to come are conservative estimates. Certain conditions might make it possible to have habitable worlds beyond these limits, but these would be increasingly rare.

Tidal Braking
All significant gravitational bodies possess one nasty attribute. The closer a planet or moon gets to its parent body, the greater the tidal forces that are generated within the lesser body. These forces tend to slow the rotation of the planet or moon. In our own system, there are several extreme examples of this "tidal braking." Mercury, for instance, has a very slow rotation (58.65 days compared to its year of 87.97 days). Earth's moon has suffered a similar fate, always showing only one side to the citizens of our home planet. In the extrasolar diagrams, the zone of significant tidal braking is indicated by radial purple lines as shown in this illustration.

Eco-zone and Tidal Braking
Combining these two elements, it becomes easy to see how light-weight stars, called "red dwarfs," emit too little heat and light. Planets hoping to have enough warmth to remain habitable would have to orbit too closely to their parent sun ("primary"), and would find themselves imbedded in the tidal braking zone.

These diagrams are to scale and show how more massive stars have eco-zones far from their tidal braking zones. At the K0 V spectral type, the Earth-normal orbital radius is right at the edge of dangerous tidal braking levels. The G0 V type star is only slightly brighter and more massive than our own sun (G2 V).

Move your mouse cursor over the different star types to see the various scenarios for eco-zone versus tidal braking.

Investigate the Table of Extrasolar Planets, and see just how wild our universe can be.