P A C K E D O R B I T S
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Inner system architectures of six 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) by using a continuous linear scale beyond 0.2 AU and discontinuous scales (on account of packing) for smaller orbital radii. The approximate semimajor axes of Mercury and Venus are indicated by M and V, respectively. 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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A key concept in the architecture of planetary systems is the Hill radius - the area dominated by the gravitational influence of a planet in orbit around its host star. More massive planets have larger Hill radii than less massive ones, and planets orbiting close to their stars have smaller Hill radii than those at greater distances. For orbital stability, the Hill radii of two planets cannot overlap. Thus planetary orbits can be more closely spaced at smaller semimajor axes and more widely spaced at larger semimajor axes, as we see in the Solar System. The diagram above represents the orbital space within 1 AU of six nearby stars that are dimmer and less massive than our Sun. All six systems are notable for their packed orbits, such that five of them harbor three to four planets within an astrocentric radius similar to the aphelion of Mercury. Each system's habitable or liquid water zone is centered on the radius marked by the blue bubbles. By comparison, 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, that 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 six systems illustrated here have circular orbits, but a few have eccentricities comparable to, or even greater than, the eccentricity of Mercury (e = 0.206). These six systems contain many of the Super Earth candidates (mass range 2-10 M EA) announced to date. Additional Super Earths orbit GJ 176, GJ 433, GJ 667C, and GJ 1214, which are M dwarfs; HD 7924, HD 181433, and HD 215497, which are K dwarfs similar in mass to HD 40307 and HD 69830; and CoRoT-7 and HD 1461, which are G dwarfs like our Sun.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 (and thus not shown in the diagram). All announced planets of the other systems appear above. 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, visit their individual pages: GJ 581 GJ 876 HD 40307 HD 69830 55 Cancri 61 Virginis Last update June 2010 |
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All text is copyright Raymond Harris 2006-2010. Image credits appear in the accompanying caption. |