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The astronomical unit (AU) is used to measure distances between planets and their host star, and the unit is based on the Earth-Sun distance. This varies because the Earth's orbit is elliptical, so the AU is defined to have a particular value. It is approximately 92.955 million miles, or 149.60 million kilometers. In our solar system, about 5 AU is the dividing line between the inner, rocky terrestrial planets (Mercury, Venus, Earth, and Mars), and the outer giant planets (the gas giants Jupiter and Saturn, and the ice giants, Uranus and Neptune). Neptune's orbit extends to approximately 30 AU from the Sun, and Pluto's orbit varies by a large percentage, going out as far as 49 AU, and approaching the Sun at closer distance then Neptune in places.
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Jupiter and Neptune are often used as “benchmarks” to liken exoplanets to, either in size or mass or both. Jupiter is approximately 318 times more massive than Earth, and nearly 11 times larger in diameter. Neptune is about is about 17 times more massive than Earth, and about 4 times larger in size than Earth. Thus, a “hot Jupiter” exoplanet would be something that is more massive than about half the mass of Jupiter, larger than about half the size of Jupiter, but closer than 1 AU to its host star. The latter is the criterion that earns the adjective “hot” since such an exoplanet is closer to its host star than our Jupiter is to the Sun. The terms are very loosely applied as there is no exact definition for an exoplanet to be called a hot Jupiter. A “cold Neptune,” on the other hand, would be an exoplanet that is, loosely speaking, comparable to the size and mass of our Neptune. The adjective “cold” refers to the fact that the exoplanet is further away from the host-stars so-called “ice line” (or “snow line”), the boundary for liquid water to exist or not exist (a few AU generally speaking). Such an exoplanet would be referred to as an ice giant.
Beyond Pluto lies a murky region called the “Kuiper Belt” that is essentially a region containing space junk, or rocky/icy debris. The other objects in the Kuiper Belt are known as Kuiper Belt Objects, or KBOs (don't fall off you chair in amazement!). They are also known as Trans-Neptunian Objects, or TNOs. More than 1,300 KBOs have been identified, but it is extremely difficult to measure their sizes because they are so far and faint. The New Horizons mission will go on to study the Kuiper Belt (which is thought to extend from about 30 AU to 50 AU) after it has left Pluto. The Kuiper Belt is thought to have a definite edge at about 50 AU, an inference that has been made from a measured drop-off in KBOs from surveys.1
The large KBO known as Quaoar was discovered in 2002 and has a diameter of about 800 miles (1,287 kilometers), which is more than half of the diameter of Pluto. 2 The dwarf planets Eris and Haumea are comparable in size to Pluto, and the dwarf planet Makemake is thought to be comparable in size to Pluto. The Kuiper Belt is also a reservoir for short-period comets (generally speaking, comets that take less than about 200 years to complete an orbit around the Sun). Quaoar also has a moon called Weywot (see references in note 2).
This brings us to another “exotic” member of our solar system, Sedna. This is an icy body, discovered in 2004, that merits the title of the most distant object ever seen in our solar system. Sedna's size is very difficult to measure but a diameter of about 800 to 1,100 miles (1,287 to 1,770 kilometers) has been estimated (i.e., about the same size as Quaoar). 3 Sedna is not a KBO for some very important reasons. For one thing, Sedna never gets closer to the Sun than about 76 AU and therefore never enters the Kuiper Belt. In fact, the Sun is not enclosed in Sedna's orbit so it cannot even be defined as a dwarf planet (the discovery paper called it a planetoid). Sedna's orbit is so eccentric that the planetoid travels as far as 1000 AU or so from the Sun. What is extremely puzzling is the mechanism that could have placed Sedna into such an orbit. If Sedna had been gravitationally scattered by Neptune, some part of the orbit should again approach Neptune, but Sedna's orbit passes nowhere near Neptune. The Sedna discovery paper states, “Such an orbit is unexpected in our current understanding of the solar system.” 4 Since the discovery, searches for similar objects have been unsuccessful. A satisfactory explanation of Sedna's orbit has not been found. Speculative ideas include scattering by an undiscovered planet, a “close” encounter with another star, and formation of the solar system in a star cluster. However, more information is required in order to make any robust deductions. The implications of the very existence of Sedna are of course profound, because it represents a dynamical relic of the early solar system and very likely holds clues pertaining to the true scenario of the formation of the solar system and probably planet formation in general.
Sedna is regarded as a member of the inner Oort cloud. The Oort cloud is a postulated region beyond the Kuiper Belt that was invoked (in 1950, well before the discovery of Sedna) as a reservoir of long-period comets. Since the Oort cloud is not an observed entity, only a hypothesized region, estimates of the locations of its inner and outer edges vary wildly in the literature. If Sedna is a member of the Oort cloud then its distance from the Sun of approximately 76 AU to 1000 AU obviously locates the innermost estimate of the inner edge of the Oort cloud. As for the outer edge, you can find up to 200,000 AU for its value in the literature. This upper end actually corresponds to more than 3 light years, so at such vast distances objects are only loosely bound to the host star and are subject to potential influence from other nearby stars.
There is one more “exotic” object I will mention. You may have heard of a giant planet in the outer solar system (in the Oort cloud) called Tyche. This is something that has not actually been observed, but rather it has been deduced to exist by means of the analysis and interpretation of the orbital elements of long-period comets. 5 A paper published in 2010 that presented the case for Tyche, suggested that the mass of the hidden object is between 1 and 4 times the mass of Jupiter. The paper is very careful not to call the object a planet, and simply refers to the object as a “Jovian-mass solar companion.” There are assumptions that go into the theory and analysis, and it is too early yet for the methodology and interpretation to have been decisively scrutinized by other researchers.
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1 Papers on the outer edge of the Kuiper Belt:
D. Jewitt, J. Luu, and C. A. Trujillo,
“Large Kuiper Belt Objects: The Mauna Kea 8K CCD Survey,”
The Astrophysical Journal 115 (1998), 2125-2135;
R. L. Allen, G. M. Bernstein, and R. Malhotra,
“The Edge of the Solar System,”
The Astrophysical Journal 549 (2001), L241-L244;
J.-M. Petit, et al.,
“The Kuiper Belt Luminosity Function from
mR= 22 to 25,”
Monthly Notices of the Astronomical Society 365 (2006), 429-438.
2M. E. Brown, et al., “2002 AW197,” Minor Planet Electronic Circ. 2002-O30 (2002). See also, M. E. Brown and C. A. Trujillo, “Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar,” The Astronomical Journal 127 (2004), 2413-2417; W. C. Fraser and M. E. Brown, “Quaoar: A Rock in the Kuiper Belt,” The Astrophysical Journal 714 (2010), 1547-1550.
3M. E. Brown, C. A. Trujillo, and D. Rabinowitz, “Discovery of a Candidate Inner Oort Cloud Planetoid,” The Astrophysical Journal 617 (2004), 645-649.
4M. Schwamb, M. E. Brown, and D. Rabinowitz, “A Search for Distant Solar System Bodies in the Region of Sedna,” The Astrophysical Journal 694 (2009), L45-48.
5J. J. Matese and D. P. Whitmire, “Persistent Evidence of a Jovian Mass Solar Companion in the Oort Cloud,” Icarus 211 (2011), 926-938, and references therein.
File under: Exoplanet types; Kuiper Belt; Oort Cloud; Quaoar and Weywot; Sedna; Tyche and the 10th planet.
© Tahir Yaqoob 2011.