Matese, Whitmire and their colleague Patrick Whitman first proposed the existence of this planet in 1999,[9] based on observations of the orbits of long-period comets. Most astronomers agree that long-period comets (those with orbits of thousands to millions of years) have a roughly isotropic distribution; that is, they arrive at random from every point in the sky.[10] Because comets arevolatile and dissipate over time, astronomers suspect that they must be held in a spherical cloud tens of thousands of AU distant (known as theOort cloud) for most of their existence.[10] However, Matese and Whitmire claimed that rather than arriving from random points across the sky as is commonly thought, comet orbits were in fact clustered in a band inclined to theorbital plane of the planets. Such clustering could be explained if they were disturbed by an unseen object at least as large asJupiter, possibly abrown dwarf, located in the outer part of theOort cloud.[11][12] They also suggested that such an object might explain thetrans-Neptunian objectSedna's peculiar orbit.[13] However, the sample size of Oort comets was small and the results were inconclusive.[14]
Whitmire and Matese speculated that Tyche's orbit would lie at approximately 500 timesNeptune's distance, some 15,000 AU (2.2×1012 km) from theSun, a little less than one quarter of alight year. This is well within theOort cloud, whose boundary is estimated to be beyond 50,000 AU. It would have an orbital period of roughly 1.8 million years.[15] A failed search of olderIRAS data suggests that an object of5MJ would need to have a distance greater than 10,000 AU.[7] Such a planet would orbit in a plane different from the ecliptic,[16] and would probably have been in a wide-binary orbit at the time of its formation.[7] Wide binaries may form through capture during the dissolution of a star'sbirth cluster.[7]
In 2011, Whitmire and Matese speculated that the hypothesized planet could be up to four times the mass ofJupiter and have a relatively high temperature of approximately 200 K (−73 °C; −100 °F),[7] due to residual heat from its formation andKelvin–Helmholtz heating.[citation needed] It would be insufficiently massive to undergonuclear fusion reactions in its interior, a process that occurs in objects above roughly 13Jupiter masses. Although more massive than Jupiter, Tyche would be about Jupiter's size sincedegenerate pressure causes massive gas giants to increase only in density, not in size, relative to their mass.[a] If Tyche was to be found, it was expected to be found by the end of 2013 and only be 1–2 Jupiter masses.[19]
Tyche (τύχη, meaning "fortune" or "luck" inGreek) was theGreek goddess of fortune and prosperity. The name was chosen to avoid confusion with an earlier similar hypothesis that the Sun has a dim companion namedNemesis, whose gravity triggers influxes of comets into the inner Solar System, leading tomass-extinctions onEarth.Tyche was the name of the "good sister" ofNemesis.[7] This name was first used for an outer Oort cloud object byJ. Davy Kirkpatrick at the Infrared Processing and Analysis Center of the California Institute of Technology.[20]
TheWide-field Infrared Survey Explorer (WISE)space telescope has completed an all-sky infrared survey that includes areas where Whitmire and Matese anticipate that Tyche may be found.[7] On March 14, 2012, the first-pass all-sky survey catalog of the WISE mission was released.[21] The co-added (AllWISE) post-cryo second survey of the sky was released at the end of 2013.[22] On March 7, 2014, NASA reported that the WISE telescope had ruled out the possibility of a Saturn-sized object out to 10,000—28,000 AU, and a Jupiter-sized or larger object out to 26,000—82,000 AU (0.4light-years).[6][23]
^A cold hydrogen-rich gas giant slightly more massive than Jupiter (below about1.6MJ) would be larger in volume than Jupiter,[17] but for higher masses,degenerate pressure will cause the planet to shrink,[17] and added mass is compensated for by increasing degeneracy pressure in the planetary core. For example,HD 17156 b is3MJ with a radius of 96% of Jupiter andHD 80606 b is4MJ with a radius of 92% of Jupiter. Even the brown dwarfCOROT-3b (22MJ) is estimated to be about the volume of Jupiter. After 10billion years of cooling, large gas giants are all very nearly Jupiter's radius, but more massive objects are slightly smaller.[18]
