Interstellar comet
Encyclopedia
An interstellar comet is a comet
that hypothetically exists in the interstellar medium
, that is, it is not gravitationally bound to a star. Although none has yet been conclusively identified, it is suspected that there must be a large population of these bodies. 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 trajectory (indicating that it is not gravitationally bound to the Sun). Comets with weakly hyperbolic trajectories have been observed, but the trajectories of these comets 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. Other simulations suggest 90–99% of comets are ejected. There is no reason to believe comets formed in other star systems would not be similarly scattered.
If interstellar comets exist, they must occasionally pass through the inner solar system
. They would approach the solar system with random velocities, mostly from the region of the constellation Hercules
because the solar system is moving in that direction. The fact that no comet with a speed greater than the Sun
's escape velocity 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
. Other analyses, of data from LINEAR
, set the upper limit at 4.5/AU
3.
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. The comet Machholz 1 is a possible example of such a comet. It has an atypical chemical makeup for comets in our Solar system.
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
that hypothetically exists in the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, that is, it is not gravitationally bound to a star. Although none has yet been conclusively identified, it is suspected that there must be a large population of these bodies. An interstellar comet can only be detected if it passes through our solar system, and could be distinguished from an Oort cloud
Oort cloud
The Oort cloud , or the Öpik–Oort cloud , is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun...
comet by its strongly hyperbolic trajectory (indicating that it is not gravitationally bound to the Sun). Comets with weakly hyperbolic trajectories have been observed, but the trajectories of these comets 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. Other simulations suggest 90–99% of comets are ejected. There is no reason to believe comets formed in other star systems would not be similarly scattered.
If interstellar comets exist, they must occasionally pass through the inner solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. They would approach the solar system with random velocities, mostly from the region of the constellation Hercules
Hercules (constellation)
Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
because the solar system is moving in that direction. The fact that no comet with a speed greater than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's escape velocity 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
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
. Other analyses, of data from LINEAR
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
, set the upper limit at 4.5/AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
3.
An interstellar comet could, on rare occasions, be captured into a heliocentric orbit while passing through the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. Computer simulations show that Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years. The comet Machholz 1 is a possible example of such a comet. It has an atypical chemical makeup for comets in our Solar system.