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ohwilleke
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- Aside from Mercury and Venus, what is the largest compact object closer to the Sun Than Earth that astronomers could still have failed to see?
Science involves not just knowing what you know, but accurately estimating how sure you are of what you know and having a decent understanding of what you don't know.
There are two known true planets closer to the Sun than Earth (Mercury and Venus, neither of which has a significant moon), both of which have been known since antiquity and can be seen with a sharp naked eye.
There are five true planets further from the Sun (on average) than Earth. There might be a Planet 9 too, but that would also have an average location further from the Sun than Earth.
All of the known or proposed dwarf planets are also further from the Sun than Earth. The illustration below is (by xkcd) is suggestive of the idea that we're probably not missing much in the way of compact solar system objects with solid surfaces.
There are certainly some rocky objects closer to the Sun than Earth, however.
That's bad, because our current astronomy instruments are very bad at seeing such an object on a collision course with Earth from the direction of the Sun but much better at seeing objects further out from the Sun than Earth.
To plan for the risk of a compact object closer to the Sun than Earth getting on a collision course with Earth, for which we may have little time to react once it is detected, it would be very helpful to know what the current observational bounds are on the largest possible compact object we could be missing that is closer to the Sun than Earth.
Is it 2000km in diameter? 1000km? 500km? 100km?
Background
Dwarf planets defined
To be a planet at all, a body must be approximately round due to gravitational effects which imposes a floor diameter of approximately 650 km to 800 km (with close cases resolved by inspection of whether it is round or not), in addition to not being a moon, or a star (the least massive of which is a brown dwarf with a minimum diameter of about 600,000 km). Thus, dwarf planets have diameters (by definition) from about 650 km to 4879 km, and in practice from 650 km to perhaps 2600 km (assuming that there might be a few Plutons in distant solar orbit a bit bigger than Pluto that are not yet discovered).
An inner dwarf planet and three near misses
One of them, Ceres, and three near asteroids that aren't quite round enough to qualify as planets are in the main asteroid belt and were discovered in the 19th century.
* Ceres 963x891 km
* Vestas 578x560x458 km,
* Pallas 570x525x500 km, and
* Hygiea 500x400x350 km
Ceres makes up about a third of the matter in the main asteroid belt between Mars and Jupiter. These four bodies combined make up about half of the matter in the main asteroid belt. About a third of all asteroids by number are part of one of 20 to 30 clusters of asteriods known as asteroid families.
Ceres was discovered in 1801 and was initially considered a planet. Pallas was the second asteroid discovered and was discovered in 1802 and was considered a planet until the discovery of more asteroids caused Ceres and Pallas to be downgraded to asteroids in 1845. Ceres was upgraded to dwarf planet status again in 2006.
Plutons
The remaining dwarf planet candidates (except Eris, 2007 OR10 and Sedna which are further out from the Sun) are Kuiper Belt Objects with orbits around the sun of 200 years or more (aka Plutons), the largest of which is Pluto, which was first discovered in 1930 before any of the rest. A classical Kuiper Belt object that aren't quite large enough to be classified as dwarf planets are sometimes called a cubewano (after the type specimen 1992QB1) or, depending upon its orbit, a plutino.
In addition to Ceres, the four other officially recognized dwarf planets, also known as Plutons, are (in order of largest dimension of size):
*Pluto 2370 +/- 10 km diameter (as of July 2015).
*Eris (aka Xena) 2326 +/- 12 km diameter (Eris is more massive than Pluto)
*Haumea (aka 2003 EL61 aka Santa) 1920 x 1540 x 990 km.
*Makemake (aka 2005 FY9 aka Easter Bunny) 1434 x 1422 +/- 14 km.
Eris is the most distant from the sun of these, about 60% further out than Pluto.
Another dozen strong dwarf planet candidates in order of size include:
* Sedna 1830-2320 km
* 2007 OR10 1280 +/- 210 km (the largest unnamed object in the solar system)
* Quaoar 1110 +/- 5 km
* Orcus 917 +/- 25 km
* Salacia 854 +/- 45 km
* 2002 MS4 934 +/- 47 km (the second largest unnamed object in the solar system)
* 2002 AW197 626-850 km (best fit 734 km)
* 2003 AZ84 661-789 (best fit 727 km)
* Varda 630-786 km (best fit 705 km)
* Varuna 574-812 km (best fit 658 km)
* Ixion 430-910 km (best fit 650 km)
* Chaos 470-740 km (best fit 600 km)
Beyond these there are probably about a dozen additional solar system bodies without names that are serious dwarf planet candidates.
Eris is the most distant dwarf planet or dwarf planet candidate from the Sun other than Sedna. All of these except Orcus and Sedna are between Pluto and Eris in the Kuiper Belt, in their average distance from the Sun. Orcus is only about 1% closer to the Sun than Pluto. Almost all of the potential dwarf planets are no further from the Sun than Eris.
Sedna is the largest body not yet officially recognized as a dwarf planet, and is also about seven and a half times as far from the Sun as Eris.
Other than Sedna, there are only four known objects significantly further out than Eris in the solar system, and only one is further out than Sedna. The sizes and shapes of these four objects aren't known with great certainty. Current estimates putting them on the borderline of qualification as dwarf planets.
Pluto was discovered in 1930 and its moon, Charon, was discovered in 1978.
A host of new dwarf planets have been discovered in the solar system in the last fifteen years. Varuna was discovered in 2000, Ixion was discovered in 2001, Quaoar was discovered in 2002 (its moon, Weywot was discovered in 2007) as were 2002 MS4 and 2002 AW197, Sedna and 2003 AZ84 were discovered in 2003, Salacia, Orcus and Haumea were discovered in 2004, and Eris and Makemake were discovered in 2005. 2007 OR10, unsurprisingly, was discovered in 2007.
Many of the 21st century discoveries were made by Mike Brown. Another leading astronomer in this field is Gonzalo Tancredi.
Charon 1207 +/- 3 km diameter as of July 2015 is part of the same system of Pluto. Since Pluto is larger than Charon, Charon is currently officially classified by the IAU as a moon of Pluto under the existing definition, which disqualifies it from planet status. There are also four other moons in the Pluto-Charon system: Styx, Nix, Kerberos, and Hydra.
The IAU has proposed to classify Pluto and Charon as a double planet (with both Pluto and Charon having dwarf planet status) because the center of their rotation around each other is not within either planet. This would make it the only double planet system in the solar system. But, one analogy that discourages IAU members from adopting the double planet definition, is that the same definition would conclude that Jupiter is not a satellite of the Sun, but is instead as "double" object with it, since their center of gravity is no within either body due to Jupiter's great distance from the Sun.
Charon is not the only big planet-like moon in the solar system, sometimes called satellite planets:
Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not dwarf planets because they do not directly orbit the Sun. The seven that are more massive than Eris are the Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets. Alan Stern calls them "satellite planets".
There are two known true planets closer to the Sun than Earth (Mercury and Venus, neither of which has a significant moon), both of which have been known since antiquity and can be seen with a sharp naked eye.
There are five true planets further from the Sun (on average) than Earth. There might be a Planet 9 too, but that would also have an average location further from the Sun than Earth.
All of the known or proposed dwarf planets are also further from the Sun than Earth. The illustration below is (by xkcd) is suggestive of the idea that we're probably not missing much in the way of compact solar system objects with solid surfaces.
There are certainly some rocky objects closer to the Sun than Earth, however.
That's bad, because our current astronomy instruments are very bad at seeing such an object on a collision course with Earth from the direction of the Sun but much better at seeing objects further out from the Sun than Earth.
To plan for the risk of a compact object closer to the Sun than Earth getting on a collision course with Earth, for which we may have little time to react once it is detected, it would be very helpful to know what the current observational bounds are on the largest possible compact object we could be missing that is closer to the Sun than Earth.
Is it 2000km in diameter? 1000km? 500km? 100km?
Background
Dwarf planets defined
To be a planet at all, a body must be approximately round due to gravitational effects which imposes a floor diameter of approximately 650 km to 800 km (with close cases resolved by inspection of whether it is round or not), in addition to not being a moon, or a star (the least massive of which is a brown dwarf with a minimum diameter of about 600,000 km). Thus, dwarf planets have diameters (by definition) from about 650 km to 4879 km, and in practice from 650 km to perhaps 2600 km (assuming that there might be a few Plutons in distant solar orbit a bit bigger than Pluto that are not yet discovered).
An inner dwarf planet and three near misses
One of them, Ceres, and three near asteroids that aren't quite round enough to qualify as planets are in the main asteroid belt and were discovered in the 19th century.
* Ceres 963x891 km
* Vestas 578x560x458 km,
* Pallas 570x525x500 km, and
* Hygiea 500x400x350 km
Ceres makes up about a third of the matter in the main asteroid belt between Mars and Jupiter. These four bodies combined make up about half of the matter in the main asteroid belt. About a third of all asteroids by number are part of one of 20 to 30 clusters of asteriods known as asteroid families.
Ceres was discovered in 1801 and was initially considered a planet. Pallas was the second asteroid discovered and was discovered in 1802 and was considered a planet until the discovery of more asteroids caused Ceres and Pallas to be downgraded to asteroids in 1845. Ceres was upgraded to dwarf planet status again in 2006.
Plutons
The remaining dwarf planet candidates (except Eris, 2007 OR10 and Sedna which are further out from the Sun) are Kuiper Belt Objects with orbits around the sun of 200 years or more (aka Plutons), the largest of which is Pluto, which was first discovered in 1930 before any of the rest. A classical Kuiper Belt object that aren't quite large enough to be classified as dwarf planets are sometimes called a cubewano (after the type specimen 1992QB1) or, depending upon its orbit, a plutino.
In addition to Ceres, the four other officially recognized dwarf planets, also known as Plutons, are (in order of largest dimension of size):
*Pluto 2370 +/- 10 km diameter (as of July 2015).
*Eris (aka Xena) 2326 +/- 12 km diameter (Eris is more massive than Pluto)
*Haumea (aka 2003 EL61 aka Santa) 1920 x 1540 x 990 km.
*Makemake (aka 2005 FY9 aka Easter Bunny) 1434 x 1422 +/- 14 km.
Eris is the most distant from the sun of these, about 60% further out than Pluto.
Another dozen strong dwarf planet candidates in order of size include:
* Sedna 1830-2320 km
* 2007 OR10 1280 +/- 210 km (the largest unnamed object in the solar system)
* Quaoar 1110 +/- 5 km
* Orcus 917 +/- 25 km
* Salacia 854 +/- 45 km
* 2002 MS4 934 +/- 47 km (the second largest unnamed object in the solar system)
* 2002 AW197 626-850 km (best fit 734 km)
* 2003 AZ84 661-789 (best fit 727 km)
* Varda 630-786 km (best fit 705 km)
* Varuna 574-812 km (best fit 658 km)
* Ixion 430-910 km (best fit 650 km)
* Chaos 470-740 km (best fit 600 km)
Beyond these there are probably about a dozen additional solar system bodies without names that are serious dwarf planet candidates.
Eris is the most distant dwarf planet or dwarf planet candidate from the Sun other than Sedna. All of these except Orcus and Sedna are between Pluto and Eris in the Kuiper Belt, in their average distance from the Sun. Orcus is only about 1% closer to the Sun than Pluto. Almost all of the potential dwarf planets are no further from the Sun than Eris.
Sedna is the largest body not yet officially recognized as a dwarf planet, and is also about seven and a half times as far from the Sun as Eris.
Other than Sedna, there are only four known objects significantly further out than Eris in the solar system, and only one is further out than Sedna. The sizes and shapes of these four objects aren't known with great certainty. Current estimates putting them on the borderline of qualification as dwarf planets.
Pluto was discovered in 1930 and its moon, Charon, was discovered in 1978.
A host of new dwarf planets have been discovered in the solar system in the last fifteen years. Varuna was discovered in 2000, Ixion was discovered in 2001, Quaoar was discovered in 2002 (its moon, Weywot was discovered in 2007) as were 2002 MS4 and 2002 AW197, Sedna and 2003 AZ84 were discovered in 2003, Salacia, Orcus and Haumea were discovered in 2004, and Eris and Makemake were discovered in 2005. 2007 OR10, unsurprisingly, was discovered in 2007.
Many of the 21st century discoveries were made by Mike Brown. Another leading astronomer in this field is Gonzalo Tancredi.
Charon 1207 +/- 3 km diameter as of July 2015 is part of the same system of Pluto. Since Pluto is larger than Charon, Charon is currently officially classified by the IAU as a moon of Pluto under the existing definition, which disqualifies it from planet status. There are also four other moons in the Pluto-Charon system: Styx, Nix, Kerberos, and Hydra.
The IAU has proposed to classify Pluto and Charon as a double planet (with both Pluto and Charon having dwarf planet status) because the center of their rotation around each other is not within either planet. This would make it the only double planet system in the solar system. But, one analogy that discourages IAU members from adopting the double planet definition, is that the same definition would conclude that Jupiter is not a satellite of the Sun, but is instead as "double" object with it, since their center of gravity is no within either body due to Jupiter's great distance from the Sun.
Charon is not the only big planet-like moon in the solar system, sometimes called satellite planets:
Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not dwarf planets because they do not directly orbit the Sun. The seven that are more massive than Eris are the Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). The term planemo ("planetary-mass object") covers both dwarf planets and such moons, as well as planets. Alan Stern calls them "satellite planets".
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