|
90377 Sedna (520 AU average) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.[122] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun.[123] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP113, discovered in 2012. Its aphelion is only half that of Sedna's, at 400–500 AU.[124][125] Oort cloud Main article: Oort cloud An artist's rendering of the Oort cloud, the Hills cloud, and the Kuiper belt (inset) The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is believed to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.[126][127] Boundaries See also: Vulcanoid asteroid, Planets beyond Neptune, Nemesis (hypothetical star) and Tyche (hypothetical planet) Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.[128] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.[129] Objects may yet be discovered in the Solar System's uncharted regions. Galactic context Position of the Solar System within the Milky Way Position of the Solar System within the Milky Way The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars.[130] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[131] The Sun lies between 25,000 and 28,000 light years from the Galactic Centre,[132] and its speed within the galaxy is about 220 kilometres per second (140 mi/s), so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year.[133] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.[134] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[g] The Solar System's location in the galaxy is a factor in the evolution of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms. Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.[136] The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.[136] Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun as radioactive dust grains and larger, comet-like bodies.[137] Neighbourhood Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. (see Local Interstellar Cloud) The Solar System is currently located in the Local Interstellar Cloud or Local Fluff. It is thought to be near the neighbouring G-Cloud, but it is unknown if the Solar System is embedded in the Local Interstellar Cloud, or if it is in the region where the Local Interstellar Cloud and G-Cloud are interacting.[138][139] The Local Interstellar Cloud is an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.[140] There are relatively few stars within ten light years (95 trillion km, or 60 trillion mi) of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 light years), Wolf 359 (7.8 light years), and Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright main-sequence star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The nearest brown dwarfs are the binary Luhman 16 system at 6.6 light years. The remaining systems within ten light years are the binary red-dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).[141] The Solar System's closest solitary Sun-like star is Tau Ceti, which lies 11.9 light years away. It has roughly 80% of the Sun's mass but only 60% of its luminosity.[142] The closest known extrasolar planet to the Sun lies around Alpha Centauri B. Its one confirmed planet, Alpha Centauri Bb, is at least 1.1 times Earth's mass and orbits its star every 3.236 days.[143] The closest known free-floating planet to the Sun is WISE 0855–0714,[144] an object of less than 10 Jupiter masses located roughly 7 light years away. A diagram of Earth's location in the observable Universe. (Click here for an alternate image.) Visual summary This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some omitted objects are larger than the ones included here, notably Pluto and Eris, because these have not been imaged in high quality. |
About us|Jobs|Help|Disclaimer|Advertising services|Contact us|Sign in|Website map|Search|
GMT+8, 2015-9-11 21:13 , Processed in 0.132354 second(s), 16 queries .
