The Solar System in Close-Up
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Temperature and Seasons
The surface temperature on Pluto varies between about −235 °C and −210 °C (very cold). The warmer regions roughly correspond to the darker regions on the surface. Pluto’s surface appears darker when it is close to the Sun (as its atmosphere vaporises and exposes the dark surface), and brighter when furthest from the Sun (as its atmosphere condenses on the surface in a frozen state—which reflects light better).
Because the orbit of Pluto is so elliptical, the amount of solar radiation it receives varies markedly between its extreme positions. Its 248-year orbital period means that any seasonal change is very slow to take place.
Magnetic Field
Pluto may have a magnetic field, but it would not be strong. The New Horizons probe may be able to take measurements of any field that exists.
Moons of Pluto
In 1978 the American astronomer James Christy noticed that Pluto had an elongated shape in photographs. A search through previous images also showed a similar shape. This observation led to the discovery of a moon orbiting Pluto. The moon was named Charon and it was found to orbit Pluto at a distance of 19,700 km over a period of 6.39 days. It turned out that Pluto and Charon rotate synchronously, with Charon always facing Pluto. Astronomers were able to observe the two bodies rotating around each other during 1985 and 1990 when the two were edge-on to Earth. Such observations enabled astronomers to determine that Pluto’s diameter is 2370 km and Charon’s 1270 km. Because these sizes are close, some astronomers referred to the two bodies as a double-planet.
The average distance between Charon and Pluto is one-twentieth the distance between the Earth and our Moon. The combined mass of Pluto and Charon amount to less than one four-hundredth of Earth’s mass.
The best pictures of Pluto and Charon have come from the New Horizons space probe that passed by these bodies in July 2015. Both objects are thought to consist of rock and ice. Charon’s surface is probably covered with dirty water ice, which is why it doesn’t reflect as much light as Pluto does. Photos from the New Horizon probe showed Charon has a dark north pole and a prominent impact crater near its equator that is ringed by bright rays. Deep canyons surround the crater, one larger than Earth’s Grand Canyon. Charon’s terrain appears to be younger than Pluto’s surface (see Fig. 13.4).
Fig. 13.4Pluto’s largest moon, Charon, as seen by the New Horizon’s probe in July 2015. Charon has a swath of cliffs and troughs which stretch about 1000 km from left to right, which suggests widespread fracturing of its crust (Credit: NASA/JHUAPL/SwRI).
In late 2005, a team of scientists using the Hubble Space telescope discovered two additional moons orbiting Pluto—they are now known as Nix and Hydra. These two tiny moons are roughly 5000 times fainter than Pluto and between 48,000 km and 65,000 km away from Pluto.
A team led by Mark Showwalter from the SETI INSTITUTE in California discovered Pluto’s smallest moons, Kerberos and Styx, in 2011 and 2012 respectively. Both were first seen in lengthy exposures of the Pluto system taken by the Hubble Space Telescope (see Fig. 13.5 and Table 13.2).
Fig. 13.5Pluto and its five moons as seen by the Hubble Space Telescope. The centre of the image was blocked out (then reinserted) to allow for a longer exposure to capture the moons (Credit: NASA).
Table 13.2Details of the moons of Pluto
Name
Distance from Pluto (km)
Period (days)
Diameter (km)
Discovered
Charon
19,571
6.39
1207
1978
Styx
42,393
20
20
2012
Nix
48,708
24.8
26
2005
Kerberos
57,729
32.1
28
2011
Hydra
64,698
38.2
30.5
2005
Other Kuiper Belt Objects
Since the 1980s, hundreds of icy bodies have been detected in the Kuiper belt. Most of these objects are much smaller than Pluto. Once the orbit of a KBO is determined it is given an official number by the Minor Planet Centre of the IAU. Once sufficient details of the body are determined, the object is given a name. Examples of KBOs with names include Asbolus, Bienor, Chaos, Chariklo, Chiron, Cyllarus, Deucalion, Elatus, Huya, Hylonome, Ixion, Nessus, Okyrhoe, Pelion, Pholus, Quaoar, Phadamanthus, Thereus, and Varuna. Full orbital details are known of only a few of the KBOs (see Table 13.3).Table 13.3Main Kuiper Belt objects
Name
Diameter (km)
Orbital radius (AU)
Orbital period (years)
Orbital eccentricity
Number of moons
Pluto
2320
39.5
248
0.249
5
Ixion
650
30–49
250
0.242
0
Varuna
900
43
281
0.056
0
Quaoar
1110
43
286
0.039
1
2002AW197
750–768
41–53
325
0.132
0
Haumea
1240
43
283
0.195
2
Orcus
760–810
30–48
245
0.227
1
Makemake
1430
45.8
308
0.159
0
Ixion (2001 KX76) is a Kuiper Belt Object discovered on 22 May 2001. Its estimated diameter is 650 km and its distance from the Sun varies between 30 AU and 49 AU because of a highly elliptical orbit. It has a reddish colour and spectroscopic data suggests its composition is a mixture of water ice, dark carbon and tholin (a tar-like substance formed by irradiation of water and carbon-based compounds). Ixion and Pluto follow a similar but differently oriented orbit. Ixion’s orbit is below the ecliptic, whereas Pluto’s is above it. Ixion takes about 250 years to orbit the Sun.
Varuna (2000 WR106) is a small KBO named after the Hindu god of the sky, rain, oceans and rivers, law and the underworld. Varuna was discovered in November 2000 by R. McMillan and has a size of about 900 km. It orbits the Sun at an average distance of 43 AU in a near circular orbit. Unlike Pluto, which is in 2:3 orbital resonance with Neptune, Varuna is free from any significant perturbation from Neptune.
Varuna has a fast rotational period of about 6 h. The surface of this body is red but dark compared with other KBOs, suggesting the surface is largely devoid of ice.
One of the largest KBOs is Quaoar (2002 LM60) discovered in 2002 by astronomers Chad Trujillo and Mike Brown in California, USA, using large ground-based telescopes. The name Quaoar is derived from a group of Native American Tongva people, native to the area around Los Angeles, where the discovery was made. Quaoar is reddish in colour and orbits the Sun once every 286 years in a near circular orbit of radius about 43 AU. It has an estimated diameter of about 1110 km, roughly the size of Pluto’s moon Charon and about one-tenth the size of Earth. Although smaller than Pluto, Quaoar is 100 million times greater in volume than all the asteroids combined. Quaoar is spherical and is a possible candidate for classification as a dwarf planet. . Like other KBOs, Quaoar’s composition is thought to be mainly ice mixed with rock. The surface temperature of is estimated at −230 °C making it one of the coldest bodies in the Solar System. From the surface of this body, the distant Sun would appear as bright as Venus does from Earth. A satellite named Weywot, of about 100 km diameter was discovered orbiting Quaoar in February 2007, but little is known about it.
Another KBO is 2002 AW197, which was discovered in January 2002 by a group of scientists led by Mike Brown. This object is about 750 km in diameter and orbits on an elliptical path between 41 AU and 53 AU from the Sun. This body takes about 325 years to orbit the Sun. Spectroscopic analysis shows a strong red colour but no water ice. It is a possible dwarf planet.
One of the strangest objects in the Kuiper belt is Haumea (previously 2003 EL61). This object is only half as large as Pluto but it is oval-shaped like an Australian or American football. It spins end-over-end every 4 h like a football that has been kicked. Haumea appears to be made almost entirely of rock, but with a glaze of ice over its surface. Astronomers have detected two tiny moons (Namaka and Hi’iaka) orbiting Haumea. By following the orbits of the moons astronomers have been able to determine that the mass of Haumea is about 32 % that of Pluto.
The odd shape of Haumea is thought to have been caused by a collision with another object early in its history. This collision knocked some of the original ice away from the surface of the body, leaving behind mostly rock. The impact also caused the body to spin rapidly and take the shape we see today. The small moons orbiting Haumea may have come from debris blown away during the collision.
In March 2007, astronomers announced they had have also found five other icy bodies in orbits similar to Haumea. Such families of objects are common in the asteroid belt, but this is the first group found in the Kuiper belt. All the fragments have a colour and proportion of water to ice similar to Haumea, and each fragment also has a surface that looks like it was once an internal region of the original object (Fig. 13.6).
Fig. 13.6The object Haumea (2003 EL61) is shaped like a football (Credit: NASA).
Orcus (2004 DW) is a KBO discovered by Mike Brown and David Rabinowitz (USA) in February 2004. This body has an elliptical orbit around the Sun, similar to that of Pluto, and it takes about 245 years to orbit the Sun. At its closest approach it is 30 AU from the Sun, while its greatest distance is 48 AU. With a diameter of between 760 km and 810 km Orcus is smaller than Quaoar. Its surface temperature is around −230 °C. Observations in infrared by the European Southern Observatory give results consistent with mixtures of water ice and carbon-based compounds. Orcus appears to have a neutral colour in comparison with the reds of other KBOs. In February 2007, a satellite, called Vanth, of size between 270 km and 380 km was discovered orbiting Orcus. Under the guidelines of the IAU naming conventions, objects with a similar size and orbit to that of Pluto are named after underworld deities; Orcus is a god of the dead in Roman mythology.
The KBO Makemake (previously 2005 FY9) is a large spherical object with a diameter of about 1430 km. Discovered in 2005 by the team led by Mike Brown, this object orbits the Sun once every 308 years in an eccentric and inclined orbit like Pluto’s. Spectral analysis showed the surface to resemble that of Pluto but is redder. The infrared spectrum indicates the presence of methane, as observed on both Pluto and Eris. The body lacks a substantial atmosphere because of weak gravity. No satellites have yet to be detected around Makemake. In July 2008 Makemake was classified as a dwarf planet.
The Scattered Disc
The scattered disc is a sparsely populated region beyond the Kuiper belt, extending from 50 AU to as far as 100 AU and further. Objects in this region have highly eccentric orbits and are often wildly inclined to the orbital plane of the major planets. Two of the first scattered disc objects (SDO) to be recognised are 1995 TL8 (at 53 AU from the Sun) and 1996 TL66 (at 83 AU). Other objects detected include: 1999 TD10, 2002 XU93 and 2004 XR190 (at 58 AU). Some astronomers prefer to use the term ‘scattered Kuiper belt objects’ for objects in this region.
Many of the SDOs are doomed in the long term because, sooner or later, their highly eccentric orbits will carry them close to the giant planets to undergo more scattering. They may last a few million years or even 100 million years in their current orbits, but eventually Neptune will flip them nearer Uranus, Saturn or Jupiter. These planets will, in turn, fling them outward, far beyond the Kuiper belt and into the Oort cloud, or out of the solar system entirely or closer to the Sun (where they will become comets).
One of the major scattered disc objects is Eris (2003 UB313 and previously known as Xena). The elliptical orbit of this body takes it to within 38 AU from the Sun and as far as 97 AU. The discovery of this object in 2003 prompted astronomers to decide on a definition of a planet. If Eris had been classed as a planet, there may have been as many as 15 planets in the solar system. In the end, the IAU decided on a definition that excluded Eris and also Pluto as major planets, instead classifying them as dwarf planets (Figs. 13.7 and 13.8).
Fig. 13.7Eris, the largest known scattered disc object, and its moon Dysnomia (Credit: NASA).
Fig. 13.8Orbital path of Eris (2003UB313).
Eris has a diameter of 2326 km, which makes it as large as Pluto. It is the largest object found in orbit around the Sun since the discovery of Neptune and its moon Triton in 1846. At times Eris is even more distant than Sedna (see below) and it takes more than twice as long to orbit the Sun as Pluto (560 years). In 2005, a near infrared spectrograph on the Gemini Telescope in Hawaii, showed the surface of Eris to be mainly methane ice. Methane ice suggests a primitive surface unheated by the Sun since the solar system formed. If Eris ever had been close to the Sun, the methane ice would have been boiled off. Unlike the somewhat reddish Pluto and Triton, however, Eris appears almost grey. The interior of the dwarf planet is probably a mix of rock and ice, like Pluto. However, Eris is denser than Pluto.
The elliptical orbit of Eris is tilted at an angle of 44° to the orbital plane of the major planets. Eris has also been found to have a moon, named Dysnomia.
Eris is currently about three times Pluto’s distance from the Sun, following an orbit that is about twice as eccentric and twice as steeply inclined to the plane of the solar system.
The Oort Cloud
The Oort cloud is an immense spherical cloud surrounding the solar system between 1000 AU and 100,000 AU (30 trillion km) from the Sun. This region contains billions of small icy objects probably left over from the formation of the solar system. Sometimes the orbit of one of these objects gets disturbed by other bodies, causing it to come streaking into the inner solar system as a long period comet (one with a period of around 2000 years). In contrast, short period comets take less than 200 years to orbit the Sun and they come from the Kuiper belt. The total mass of comets in the Oort cloud is estimated to be 40 times that of Earth.
One of the major Oort Cloud objects is Sedna (2003 VB12), which was discovered in November 2003 by a team led by Mike Brown at Palomar Observatory near San Diego, California, USA. The object was named after Sedna, the Inuit goddess of the sea, who was believed to live in the cold depths of the Arctic Ocean. Sedna has a highly elliptical orbit that is inclined at about 12° to the ecliptic. Its distance from the Sun varies between 76 AU and 937 AU, so it is best described as an inner Oort cloud object. Sedna will make its closest approach to the Sun (perihelion) about the year 2076 and will be furthest from the Sun (aphelion) in 8207. The shape of its orbit suggests it may have been captured by the Sun from another star passing by our solar system, or its orbit could be affected by another larger object further away in the Oort cloud.
Sedna is an odd body because no astronomers thought they would find an object like it in the empty space between the Kuiper belt and the Oort cloud. Spectroscopy has revealed that Sedna’s surface composition is similar to that of some other Trans-Neptunian objects, being largely a mixture of water, methane and nitrogen ices with tholins. Sedna is the second reddish coloured object in the solar system after Mars. Its size is estimated to be about 995 km. Sedna has the longest orbital period of any known large object in the solar system, calculated at around 11,400 years. Recent estimates put its rotational period at about 10.3 h and its surface temperature at a very cold −250 °C. Sedna appears to have methane ice and water ice on its surface. The object’s deep red spectral slope is indicative o
f high concentrations of organic material on its surface, and its weak methane absorption bands indicate that methane on Sedna’s surface is ancient, rather than freshly deposited. Models of internal heating via radioactive decay suggest that Sedna might be capable of supporting a subsurface ocean of liquid water. A search by the Hubble Space telescope has found no moons. Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe.
Sedna is classified as a Scattered disc object, it currently is not a dwarf planet. Sedna has a Stern–Levison parameter estimated to be much less than 1, and therefore cannot be considered to have cleared the neighbourhood, even though no other objects have yet been discovered in its vicinity. To qualify as a dwarf planet, Sedna also must be shown to be in hydrostatic equilibrium (Fig. 13.9).
Fig. 13.9Orbital path of Sedna.
Another object, 2000 CR105, of diameter 328 km, has a similar but less extreme orbit than Sedna: it has a perihelion of 44.3 AU, an aphelion of 416 AU, and an orbital period of 3491 years. It is considered a detached object.
In November 2014, astronomers announced the discovery of 2012 VP113, an object half the size of Sedna in a 4268-year orbit similar to Sedna’s and a perihelion within Sedna’s range of roughly 80 AU. Its greatest distance from the Sun is around 450 AU. The surface of 2012 VP113 is believed to have a pink tinge, resulting from chemical changes produced by the effect of radiation on frozen water, methane, and carbon dioxide.
The similarity in the orbits found for Sedna, 2012 VP113, and a few other objects near the edge of the Kuiper belt suggests that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations.