Interstellar object
An interstellar comet is a comet that exists in the interstellar medium,[1] that is, it is not gravitationally bound to a star.[2] Although none has yet been conclusively identified, it is suspected that there must be a large population of these bodies.[1] An interstellar comet can only be detected if it passes through our solar system, and could be distinguished from an Oort Cloud comet by its strongly hyperbolic orbit (indicating that it is not gravitationally bound to the Sun).[2] Comets with weakly hyperbolic orbits have been observed but the trajectories of these are consistent with having been knocked loose from the Oort Cloud and do not indicate an origin in interstellar space.
Current models of Oort Cloud formation indicate that more comets are ejected into interstellar space than are retained in the Oort Cloud, by a factor of 3-100.[2] Other simulations suggest 90-99% of comets are ejected.[3] There is no reason to believe comets formed in other star systems would not be similarly scattered.[1]
If interstellar comets exist, they must occasionally pass through the inner solar system.[1] The fact that no comet with a speed greater than the Sun's escape velocity[4] has yet been seen places upper limits to their density in interstellar space. A paper by Torbett indicates that the density is no more than 1013 comets per cubic parsec.[5] Other analyses, of data from LINEAR, set the upper limit at 4.5 × 10^-4/AU3.[2]
An interstellar comet could, on rare occasions, be captured into a heliocentric orbit while passing through the Solar system. Computer simulations show that Jupiter is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years.[5] The comet Machholz 1 is a possible example of such a comet. It has an atypical chemical makeup for comets in our Solar system.[4]
Notes
- ^ a b c d
Valtonen, Mauri J. (March 1992). "Origin of oort cloud comets in the interstellar space". Celestial Mechanics and Dynamical Astronomy. 54 (1–3). Springer Netherlands: 37–48. doi:10.1007/BF00049542. Retrieved 2008-12-30.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b c d
Francis, Paul J. (2005-12-20). "The Demographics of Long-Period Comets" (PDF). The Astrophysical Journal. 635 (2): 1348–1361. Bibcode:2005ApJ...635.1348F. doi:10.1086/497684. Retrieved 2009-01-03.
{{cite journal}}: Check date values in:|date=(help); Unknown parameter|month=ignored (help)CS1 maint: date and year (link) - ^ Choi, Charles Q. (2007-12-24). "The Enduring Mysteries of Comets". Space.com. Retrieved 2008-12-30.
{{cite web}}: Check date values in:|accessdate=and|date=(help) - ^ a b MacRobert, Alan (2008-12-04). "The First Interstellar Comet". Australian Sky & Telescope. Retrieved 1008-12-30.
{{cite news}}: Check date values in:|accessdate=and|date=(help) - ^ a b Torbett, M. V. (July 1986). "Capture of 20 km/s approach velocity interstellar comets by three-body interactions in the planetary system". Astronomical Journal. 92: 171–175. doi:10.1086/114148. Retrieved 2009-01-01.