Multi-Planet Systems

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Exoplanetary Scratchpad

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Here is a list of systems that contain multiple sub-stellar companions. "Plus" systems also include either a circumstellar disk, a brown dwarf, a nearby companion star, or an unconfirmed planet.

Eight Planet Plus Systems

  • Solar System - Home system of Earth, the only confirmed habitable planet. Also has three other terrestrials, two Jovians, two Neptunians, several rocky and icy Dwarf Planets, a asteroid belt, a kuiper belt, and an Oort cloud.

Five Planet Systems

  • 55 Cancri System - Wide binary star consisting of a sun-like primary (A) and a red-dwarf secondary (B) separated by 1,100 AU, 41 light years away. Star A contains five exoplanets, the first system found with this many. It has three tightly packed eccentric planets close in to the star, including planet e (hot Super Earth/Neptunian), b (hot Jupiter), and c (hot Saturn). Planet e was the first Neptunian discovered. It was later found to be the shortest-period planet discovered (18 hours) and to transit. The planet has about half of Neptune's mass, but is Earth-like in size and density (2.17 Earth Radius). It is composed 70% of rock and the outer 30% is likely an ocean of super-critical water (between a gas and liquid state) that is 3000km thick. This is the hottest and densest super-Earth and the is the most watery planet found to date. It likely possesses a thick atmosphere of CO and CO2. The brightness of the star (also closest known to transit and only known naked eye star to do so) makes it more easily studied than other hot super Earths. Planet f is a very eccentric Saturnian in the habitable zone. Planet d is a super jovian at Jupiter-like distances, which was the first found at true Jupiter distances and still the exoplanet discovered with dopplar spectrometry with the largest known semi-major axis. The distant outer star causes planet d's axis to flip on its axis every million years. Planet d in turn causes the other planets to flip, including its star. The axis tilt of transiting planet e should be determined at some point. "Bode's law" predicts four undiscovered planets.

Four Planet Systems

  • Mu Arae System - At first believed to be a system dominated by orbit crossing eccentric Super Jupiters. Instead, three Jovians orbit in roughly circular orbits at Earth-like, Mars-like, and Jupiter-like distances in addition to a Hot Neptune (the first Neptunian announced).

Three Planet Plus Systems

  • PSR B1257+12 System - The first extrasolar system confirmed and one of the only planetary systems known around a pulsar. Has three rocky planets that orbit closer than Venus. The innermost has a Moon-like mass, while the next two out are Super Earths. May also have an outer "comet" sized body, the first sub-planetary object detected and possibly a representative of a mini "kuiper belt" at asteroid belt like distance.
  • HD 69830 System - First planetary system found that does not have a Jupiter-sized planet around a normal star (K0 spectrum). Contains 3 Neptunians and the first discovered asteroid belt that is like the size and age as the Sun's. The debris from this belt that was detected was from the breakup of an asteroid, is 20 times as massive as our own, and would cause zodiacal lights 1000 times brighter than we see from Earth. The smallest and outermost planet may be a 10 ME super Earth, is within the habitable zone, and is an inner shepherd for the asteroid belt. Halo 3 features a fictitious moon around this planet.

Three Planet Systems

  • Upsilon Andromedae System - A nearby (44 ly) multi-star system which is the first multiplanet system found around a main sequence star. The main star around which the planets orbit is a yellow-white star somewhat younger than the sun and its companion is a red dwarf in a wide orbit. It is one of the most well studied non-transiting star systems. Roaster Planet b (0.05 au, 1.4 MJ, e=0.013) is nicknamed the Fire and Ice Planet because it is hot on one side and cold on the other. The hottest parts of the planet are near the trailing side terminator at the equator, due to high velocity winds transporting heat to the night side. This is 80deg offset from the starward pole and a much greater offset than other observed hot Jupiters. This threw astronomers off and caused them to doubt the wind-theory, though later observations of other planets have shown that winds indeed can travel fast enough to cause this. The middle planets c (0.83 au, 14 MJ, may actually be a brown dwarf star, e=0.224) and d (2.5 au, 10 MJ, e=0.26) have had their inclinations and masses determined with astrometry. They are very eccentric and highly inclined to each other (30 deg). Planet scattering was thought to be a source until the outermost planet was discovered. This is planet e (5.2 au, 1.05 MJ, e = 0.005), which is the most Jupiter-like exoplanet known, and is in 3:1 resonance with planet d. The star appears to have no Kuiper-belt like disc, perhaps due to its companion star sweeping away this material.
  • Gliese 876 System - Very nearby quadruple planet system and the first Red Dwarf found to have planets. The innermost planet (d) was the first found rocky planet around a normal star (the first true Super-Earth, at epistellar distances). The outer three planets c (Saturnian), b (Jovian), and e (Neptunian) are in 1:2:4 (30d/60d/120d) resonance (the exoplanet resonance and first triple-resonant planets discovered). The outermost planet has a Mercury-like orbit however it is very much colder than Mercury. Gliese 876 e actually receives only slightly more warmth from Gliese 876 than Jupiter does from our Sun. Planet b is second discovered by ELODIE after 51 Peg b and the second to have its mass exactly measured and the first to have done so by astrometry.
  • HD 37124 System - The fourth triple planet system discovered around a normal star, the last one announced with 4 other multiplanet components in 2005. A Sunlike star with Jovian planets that received Venus-like, Mars-like, and Asteroid Belt-like radition from their stars. Simulations showed that no terrestrial planets could form between the Venus-like and Mars-like Jovian planets.
  • Gliese 581 System - Small nearby Red Dwarf with six planets in tight circular orbits and a distant Kuiper belt where many comets orbit. Gliese 581 e was, at the time of its discovery, the smallest known dopplar-detected exoplanet and a super-Mercury, b is a hot-Neptunian, c is a super-Venus and the first detected in the habitable zone (initially heralded as habitable, but later thought too hot due to the greenhouse effect), g is a super-Earth and the first detected in the middle of the HZ (and is highly controversial, having many doubters and defenders), d is a super-Earth on the outer edge of the HZ which could support liquid water (due to its presumably large atmospheric pressure and carbon dioxide), and f (its existence is also highly controversial) is a cold super-Earth. Much further out, from 25 ± 12 AU to more than 60 AU, there is a cold debris disk reminiscent of the Kuiper belt but with 10 times more comets than the one in our solar system. The star is not very active.

Two Planet Plus Systems

  • HD 82943 System - Yellow dwarf with two large orbit-crossing Jovians locked in 1:2 orbital resonance that would span the inner solar system, which were disccovered by the Swiss team by 2001. The planets have nearly identical mass (1.8 MJ). Shown to have a dust disk by Spitzer.
  • HD 128311 System - An orange dwarf star with two jovians (a Jupiter analog detected in 2005 and an eccentric giant in the outer habitable zone) possibly in 1:2 resonance and a dusk disk detected by Spitzer.

Two Planet Systems

  • 47 Ursa Majoris System - (aka Ursae Majoris) One of earliest systems discovered one. Planets b (2.5 MJ, 2.1 AU) and c (0.5 MJ, 3.6 AU) are in circular orbits at asteroid-belt like distances, while planet d (1.6 MJ, 11.6 AU) is in a distant more eccentric orbit. Planet b was the first found to have a circular orbit beyond the habitable zone. The discovery of planet c made the system the first multiplanet system whose planets have circular orbits. Planet d has not yet completed a full year yet since its discovery, but is the furthest out planet discovered with the dopplar spectrometry method. Studies have shown a terrestrial planet could only form in the innermost part of the habitable zone. Several transmissions have been sent to the star system.
  • HIP 14810 System - System with two eccentric planets discovered in 2006 by the Carnegie team. One is a moderately eccentric massive Hot Jupiter, while the other is a very eccentric half-Jupiter orbiting at Mercury-like distances.
  • HD 217107 System - Contains the first discovered moderately eccentric Hot Jupiter. Its outer planet was suspected when the inner one was discovered and confirmed with four other new multiplanet systems in 2005. Outer planet is highly eccentric and skirts the outer edge of the habitable zone.
  • Gliese 777 System - Outer planet of the primary star was initially believed to be a Jupiter analog, but later found to be eccentric (its apastron is at Jupiter-like distance). Its second planet was the fourth Neptunian discovered and was announced with four other "second" planets in 2005.
  • HD 74156 System - Sunlike star with two planets more massive than Jupiter, one in about Mercury's position, and one at an Asteroid Belt-like position. A planet was predicted in between these two at Earth-like distances and later found, the first vindicated prediction since Neptune. This supports the "Packed Planetary Systems" theory. Some have suggested that it's one Earth year orbit period may mean its detection is due to Earth based observation errors.
  • HD 169830 System - Bright star with two eccentric planets known in 2003. One is likely too hot to have any clouds, while the other one is likely to sport white water cloud bands.
  • HD 12661 System - System with two Jovians that could support water clouds discovered in 2000 and 2002. The innermost one may be an "Eccentric Sulfurous Cloud Giant".
  • HD 108874 System - System with two Jovians that could have bands of white water clouds. The inner planet is in a circular orbit and enjoy's Earth-like heat. Its second planet, which was announced with four other new multiplanet systems in 2005, is in an eccentric orbit that would span our solar system's inner asteroid belt.
  • HD 47536 System - Very large orange giant star with two large eccentric jovian discovered by the European-Brazilian team. The inner planet is at Earth-like distance, but is too hot for clouds to appear.
  • HD 187123 System - System containing one of the earlier discovered Hot Jupiters (similar to 51 Peg b) and that had early indications of an outer planet. The confirmation of this massive distant eccentric jovian was announced at the May 2007 AAS media briefing, along with 27 other exoplanets.
  • HD 11964 System - Sunlike star with two eccentric gas giants detected in 2005. The outermost one was retracted but then re-confirmed as one of the 28 planets announced at the May 2007 AAS media briefing. The inner most has half the mass of Saturn and is too hot for clouds. The outer one is cool and likely dominated by white water clouds. A middle planet may also exist.
  • HD 73526 System - System with two large orbit-crossing Jovians locked in 1:2 resonance that would span the inner solar system to the asteroid belt. The second planet was one of the 28 planets announced at the May 2007 AAS media briefing.
  • HD 155358 System - The extrasolar system with the lowest metalicity host star. Its two known Jovian planets were discovered recently and challenged planetary formation theorists. The two planets interact gravitationally with each other and are on opposite sides of the star's habitable zone.

One Planet Plus Systems

  • Errai System - Primary star also known as Gamma Cephei. It is the first close in binary star found to have a planet. May contain the nearest "Sulfurous Cloud Giant" planet.
  • HD 38529 System - Subgiant star with a cloudy Hot Jupiter, Brown Dwarf, and Red Dwarf companion. Simulations showed that Earth-sized planet could exist between the planet and brown dwarf, but that an asteroid belt can be expected there instead.
  • HD 117176 System - One of the six extrasolar systems known to have planets to be first shown to also have a dust disk by Spitzer.
  • HD 52265 System - One of the six extrasolar systems known to have planets to be first shown to also have a dust disk by Spitzer. Planet independently discovered by CORALIE and Carnegie teams and is roughly Jupiter-sized in a hot eccentric orbit.
  • HD 50554 System - One of the six extrasolar systems known to have planets to be first shown to also have a dust disk by Spitzer.
  • Epsilon Eridani System - Nearest single non-red dwarf star to the Sun and the second nearest system with a confirmed planet. The planet is a very elliptical Jupiter-like world. It also has two asteroid belts and a kuiper belt, with evidence of planets in between.
  • HD 202206 System - System with an eccentric brown-dwarf at Venus-like distances and an even further out eccentric Jovian found in 2004.

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