C R O W D E D O R B I T S
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Inner system architectures of five nearby stars of sub-Solar mass. Star and planet icons express relative mass; star icons use a reduced scale because stars are much heavier, as well as much larger in radius, than planets. Distance is expressed in fractions of an astronomical unit (AU), using a discontinuous scale (on account of crowding) for the smallest orbital radii. Blue bubbles mark the approximate center of each system's habitable or liquid-water zone. Around K- and G-type stars, this zone extends for a few tenths of an AU in either direction. Around M dwarfs, it extends for a few percent of an AU in either direction. |
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This diagram represents the orbital space within 1 AU of five nearby stars that are dimmer and less massive than our Sun. All five systems are notable for their crowded orbits, such that most of them harbor three planets within an astrocentric radius smaller than the perihelion of Mercury. Each system's liquid water zone is centered on the radius marked by the blue bubbles. The equivalent region of the Solar System is centered on the orbit of the Earth, at 1 AU, while our two inner planets, Venus and Mercury, orbit at semimajor axes of 0.72 and 0.39 AU, respectively. Because of the eccentricity of Mercury's orbit, the planet's separation from the Sun varies from 0.31 to 0.47 AU over the course of a single period. Most of the planets in the five systems illustrated here have circular orbits, but a few have eccentricities comparable to Mercury's. These five systems contain all but one of the eight Super Earth candidates (mass range 2-10 M EA) announced to date. The remaining Super Earth orbits HD 181433, a K-type dwarf that may be similar in mass to HD 40307 and HD 69830.Note that 55 Cancri also harbors a fifth planet that is about four times as massive as Jupiter, orbiting at a semimajor axis of 5.84 AU (not shown in the diagram). For basic physical and orbital data on each system, see the Multi-Planet Table. For a side-by-side comparison of all known multi-planet systems, including our own, see the Diagram of Multi-Planet Systems. For a fuller discussion of each system, including the possible location of their habitable zones, visit the individual pages: GJ 581 GJ 876 HD 40307 HD 69830 55 Cancri July 2008 |
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All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption. |