Tyche (hypothetical planet)
Tyche (/ˈtaɪki/) is the nickname given to a hypothetical gas giant planet located in the Solar System‘s Oort cloud, first proposed in 1999 by astronomer John Matese of the University of Louisiana at Lafayette. Matese and his colleague Daniel Whitmire argue that evidence of Tyche’s existence can be seen in a supposed bias in the points of origin for long-period comets. They noted that Tyche, if it exists, should be detectable in the archive of data that was collected by NASA‘s Wide-field Infrared Survey Explorer (WISE) telescope. However, several astronomers have voiced skepticism of this object’s existence. Analysis over the next couple of years will be needed to determine if WISE has actually detected such a world or not.
Matese first proposed the existence of this planet in 1999, based on his observations of the orbits of long-period comets. Most astronomers agree that long period comets (those with orbits of thousands of years) have an isotropic distribution; that is, they arrive at random for every point in the sky. Because comets are volatile and dissipate over time, astronomers suspect that they must be held in a spherical cloud tens of thousands of AU distant (known as the Oort cloud) for most of their existence. However, Matese 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 the orbital plane of the planets. Such clustering could be explained if they were disturbed by an unseen object at least as large as Jupiter, possibly a brown dwarf, located in the outer part of the Oort cloud. He also suggested that such an object might also explain the trans-Neptunian object Sedna‘s peculiar orbit. However, his sample size was small and the results were inconclusive.
Whitmire and Matese speculate that Tyche’s orbit would lie at approximately 500 times Neptune‘s distance; equivalent to 15,000 AU from the Sun, a little less than one quarter of a light year. This is still well within the Oort cloud, whose boundary is estimated to be beyond 50,000 AU. It would have an orbital period of roughly 1.8 million years. A failed search of older IRAS data suggests that an object of 5 MJwould need to have a distance greater than 10,000 AU. Such a planet would orbit in a different plane in orientation to our current planet orbits, and probably formed in a wide-binary orbit. Wide binaries may form through capture during the dissolution of a star’s birth cluster.
Whitmire and Matese speculate that the hypothesized planet could be up to four times the mass of Jupiter and have a relatively high temperature of approximately 200 K (−73°C), due to residual heat from its formation and Kelvin–Helmholtz heating. It would be insufficiently massive to undergonuclear fusion reactions in its interior, a process which occurs in objects above roughly 13 Jupiter masses. Although more massive than Jupiter, Tyche would be about Jupiter’s size since degenerate pressure causes massive gas giants to only increase in density, not in size, relative to their mass.[a]
Origin of name
Tyche (Τύχη, meaning “fortune” or “luck” in Greek) was the Greek goddess of fortune and prosperity. The name was chosen to avoid confusion with an earlier similar hypothesis that the Sun has a dim companion named Nemesis, whose gravity triggers influxes of comets into the inner Solar System, leading to mass-extinctions on Earth. Tyche was the name of the “good sister” of Nemesis. This name was first used for an outer Oort cloud object by Davy Kirpatrick at the Infrared Processing and Analysis Center of the California Institute of Technology.
The Wide-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. However the vast amount of survey data will not be fully analyzed until March 2012 at the earliest.