105,283
0.936
18
2013
Proteus
118,000
1.12
420
1989
Triton
354,800
5.88
2700
1846
Nereid
5,510,000
360
340
1949
Halimede
15,728,000
1880
48
2002
Soa
22,422,000
2914
48
2002
Laomedeia
23,571,000
3167
48
2002
Psamathe
46,695,000
9115
28
2003
Neso
48,387,000
9373
60
2002
The inner four moons, Naiad, Thalassa, Despina and Galatea orbit within the ring system (see Fig. 12.7). Larissa, S/2004N1, Proteus, Triton, Nereid and the five small, outer moons orbit beyond the ring system. Four of Neptune’s moons have retrograde motions—Triton, Halimede, Psamathe, and Neso. Many of the names of Neptune’s moons come from the Nereids or water spirits of Greek mythology (Fig. 12.8).
Fig. 12.7The south pole region of Neptune's largest moon Triton. Credit: NASA.
Fig. 12.8Proteus is a dark, cratered moon of Neptune (Credit: NASA).
Nereid’s orbit is the most eccentric in the solar system. Its distance to Neptune ranges from about 1,353,600–9,623,700 km. The unusual and inclined orbit of Nereid suggests that it may be either a captured asteroid or Kuiper belt object, or that it was an inner moon in the past and was perturbed during the capture of Neptune’s largest moon Triton. Prior to the visit of Voyager, Nereid was thought to be the second largest moon of Neptune. However, when Voyager discovered Proteus, Proteus was found to be larger than Nereid. Apart from Triton, all the moons of Neptune are irregular in shape, all being too small for them to form uniform spheres (they are more asteroid-like).
In July 2013 NASA’s Mark Showalter found a new moon orbiting Neptune by analysing archived photographs the Hubble Space Telescope captured between 2004 and 2009. The moon, designated S/2004N1, is estimated to be no more than 18 km across, making it the smallest known moon in the Neptunian system. It is so small and dim that it is roughly 100 million times fainter than the faintest star that can be seen with the naked eye. It even escaped detection by NASA’s Voyager 2 spacecraft, which flew past Neptune in 1989. The designation ‘S/2004N1’ is provisional; ‘2004’ refers to the year the data was first acquired, not the year of discovery.
Neptune’s Status in the Solar System
From its discovery in 1846 until the subsequent discovery of Pluto in 1930, Neptune was the farthest known planet from the Sun. Upon Pluto’s discovery Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto’s elliptical orbit brought it closer to the Sun than Neptune. The discovery of the Kuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet in its own right or part of the belt’s larger structure. When the International Astronomical Union defined the term ‘planet’ for the first time in 2006, Pluto was reclassified as a ‘dwarf planet’ and Neptune once again became the outermost planet in the solar system.
Further Information
http://nssdc.gsfc.nasa.gov/planetary/planetfact.html
www.space.com/neptune/
https://solarsystem.nasa.gov/planets/profile.cfm (check out Neptune)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_13
13. Beyond Neptune: TNO’s and Comets
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
Pluto is classified as a dwarf planet with at least five moons.
The New Horizons probe flew within 10,000 km of Pluto in July 2015 and sent back some amazing up close images of the body.
Kuiper belt objects that are also dwarf planets include: Pluto, Haumea and Makemake.
Eris is a dwarf planet with one moon located in an outer part of the solar system known as the Scattered disc.
Sedna has the longest orbital period of any known large object in the solar system, calculated at around 11,400 years. It exists in a region known as the Oort cloud.
In 2014, the Rosetta spacecraft became the first spacecraft to orbit a comet and place a lander on its surface—spectacular pictures were returned.
Our understanding of the solar system has changed in the last decade mainly due to a new definition of what constitutes a planet by the International Astronomical Union, and a host of newly discovered objects that exist beyond the planet Neptune. Any object in the solar system that orbits the Sun at a greater distance on average than the planet Neptune has been termed a Trans-Neptunian Object (TNO). By 1 January 2008, astronomers had catalogued over a thousand Trans-Neptunian Objects and more have been detected since this date.
The first astronomer to suggest the existence of a Trans-Neptunian population was Frederick C. Leonard in 1930. In 1943, Kenneth Edgeworth postulated that in a region beyond Neptune, the material from the primordinal solar nebula would have been too widely spaced to condense into planets. From this he concluded that the outer region of the solar system should contain a very large number of smaller bodies that could from time to time, venture into the inner solar system. In the last few decades, astronomers have identified three regions that exist beyond Neptune: the Kuiper belt, the Scattered disc, and Oort cloud.
Trans-Neptunian Objects display a wide range of colours from blue-grey to very red. It is difficult to determine the size of TNOs because they are so far away. For large objects, diameters can be precisely measured during an occulation of a star. For smaller objects, diameters need to be estimated by thermal and relative brightness measurements. Orbital characteristics are also difficult to determine because these objects travel so slow.
The Kuiper Belt
The Kuiper belt is a vast region of the solar system beyond Neptune’s orbit (see Fig. 13.1). It is best described as a flat doughnut shaped disc that extends from 30 AU to 50 AU around the Sun. It is similar to the Asteroid belt, although it is much larger and more massive. Like the Asteroid belt, it contains mainly small bodies. But while the asteroids are composed mainly of rock and metal, the objects in the Kuiper belt are made mostly of rock and ices. The temperature of objects within the Kuiper belt is around −230 °C, so they are very cold.
Fig. 13.1The Kuiper belt is a region beyond the orbit of Neptune. The diagram shows the belt as seen from above the solar system and from side on (Credit: NASA).
The Kuiper belt is named after Dutch-born American astronomer Gerard Kuiper, who first predicted its existence in 1951. The objects in this region are called Kuiper belt objects or KBOs. The orbits of many of these objects are highly elliptical and destabilised by Neptune’s gravity.
The classical Kuiper belt appears to be made up of two separate populations. The first, known as the “dynamically cold” population, has nearly circular orbits, with relatively low inclinations to the ecliptic (up to about 10°). The second, the “dynamically hot” population, has orbits much more inclined to the ecliptic (by up to 30°). The two populations not only possess different orbits, but different colours; the cold population is markedly redder than the hot. If this is a reflection of different compositions, it suggests they formed in different regions. The hot population is believed to have formed near Jupiter, and to have been ejected out by movements among the gas giants. The cold population, on the other hand, has been proposed to have formed more or less in its current position, although it might also have been later swept outwards by Neptune. It
has also been suggested the colour difference may reflect differences in surface evolution.
The most well known KBO is Pluto. Pluto was once regarded as a planet, but was reclassified in 2006 as a dwarf planet. Apart from Pluto, the first KBO was discovered by David Jewitt and Jane Luu in Hawaii in August 1992 and is called 1992 QB1. This object was found 42 AU from the Sun. Six months later, these two astronomers discovered a second object, 1993 FW, in the same region.
It is suspected that there may be as many as 35,000 objects in the Kuiper belt with diameters of 100 km or greater and many more smaller objects. Most objects in this belt probably formed at the same distance from the Sun as we find them today.
The Kuiper Belt is also thought to be the home of short period comets (those with periods less than 200 years). Comets are small bodies of ice and rock in orbit around the Sun. Many comets have highly elliptical orbits that occasionally bring them close to the Sun. When this happens the Sun’s radiation vaporises some of comet’s icy material, and a long tail is seen extending from the comet’s head and pointing away from the Sun.
The former planet Pluto and its companion Charon are two of the larger KBOs. Several other large KBOs have been discovered, including Quaoar, Makemake and Orcus.
Pluto
Pluto was once classified as the ninth major planet of the solar system, but was reclassified in 2006 by the IAU as a dwarf planet. The main reason why Pluto was demoted as a major planet was that it has not cleared the neighbourhood around its orbit. Pluto orbits the Sun in the inner Kuiper belt where many other objects also orbit.
Pluto is much smaller than any of the official planets and is even smaller than seven of the moons in the solar system. It is so small and distant that we cannot see any surface detail on the planet through Earth based telescopes. It has a diameter of 2370 km (as measured by the New Horizons probe) and takes 248 years to travel once around the Sun. The other strange thing about this body is that its orbit is on a different plane to those of the major planets.
Fig. 13.2Image of Pluto taken by the New Horizons probe as it flew by Pluto in July 2015. Left of the bright heart shaped region are some impact craters (Credit: NASA/APL/SwRI).
Pluto orbits the Sun at an average distance of about 5913 million km. It is so distant that Pluto’s brightest daylight is less than moonlight on Earth. Pluto is always further from the Sun than Uranus, but every 248 years it moves inside Neptune’s orbit for about a 20 year period, during which time it is closer to the Sun than Neptune. Pluto crossed Neptune’s orbit on 23 January 1979, and remained within it until 11 February 1999.
At its closest approach to the Sun, Pluto is 30 times more distant from the Sun than Earth. At its farthest distance from the Sun, Pluto is 50 times more distant from the Sun than Earth. Pluto will next be at its maximum distance from the Sun during the year 2113. During the coldest 124 years of its orbit, all of Pluto’s atmosphere condenses and falls to the surface as frost. Images taken of Pluto by the Hubble Space Telescope have shown the reflectivity of its surface varies. Lighter areas are probably patches of nitrogen and methane frost as well as exposed regions of water ice.
Early Views About Pluto
In Roman mythology Pluto (Greek: Hades) was the god of the underworld. It also received this name because it was so far from the Sun and was in perpetual darkness.
Early astronomers did not know about Pluto because it could not be seen from Earth by the unaided eye. It is even difficult to locate using Earth-based telescopes.
The discovery of Pluto is an interesting story. Irregularities in the orbits of Uranus and Neptune led to the suggestion by US astronomers Percival Lowell and William Pickering that there might be another body (planet X) orbiting beyond Neptune. Lowell died in 1916, but he initiated the construction of a special wide field camera to search for planet X. In 1930, Clyde W. Tombaugh at Lowell Observatory in Arizona found planet X, which was later named Pluto. As it turned out, Pluto was too small and too distant to influence the orbits of Uranus and Neptune, and the search for another planet continued. The name ‘Pluto’ also honours Percival Lowell, whose initials PL are the first two letters of the name.
At one time it was thought that Pluto may have once been a moon of Neptune, but this now seems unlikely (Table 13.1).Table 13.1Details of Pluto
Distance from Sun
5,745,000,000 km (39.6 AU)
Diameter
2370 km
Mass
1.3 × 1022 kg (0.002 Earth’s mass)
Density
1.8 g/cm3 or 1800 kg/m3
Orbital eccentricity
0.248
Period of revolution
90,740 Earth days or 249 Earth years
Rotation period
6.38 Earth days
Orbital velocity
17,064 km/h
Tilt of axis
122.5°
Average temperature
−220 °C
Number of Moons
5
Atmosphere
Nitrogen, methane
Strength of gravity
0.67 N/kg at surface
Probing Pluto
A spacecraft called New Horizons was launched on the 19 January 2006 on a mission to Pluto and the Kuiper belt. Using a combination of monopropellant and gravity assist, it flew by the orbit of Mars on 7 April 2006, Jupiter on 28 February 2007, the orbit of Saturn on 8 June 2008, and the orbit of Uranus on 18 March 2011. New Horizons flew within 10,000 km of Pluto in mid July 2015. The probe had a relative velocity of nearly 50,000 km/h at closest approach, and came as close as 27,000 km to Charon. After passing by Pluto, New Horizons continued farther into the Kuiper belt. Mission planners are hoping to flyby one or more additional Kuiper belt objects.
Position and Orbit
Pluto orbits the Sun in an elliptical path at an average distance from the Sun of about 5913 million km. Because it is so far from the Sun, Pluto takes a very long 248 years to go around the Sun once. The strange thing about Pluto is that its orbit is inclined at an angle of 17.2° to the orbital plane of the other planets. This means that its orbit rises and drops below the ecliptic plane. Pluto’s plane is so elliptical that for 20 years of its orbital period it is closer to the Sun than Neptune (which follows a near circular orbit).
Like Uranus, Pluto is also tipped over on its side. Its rotational axis is inclined at an angle of 122.5° to the plane of its orbit. This means that Pluto’s equator is almost at right angles to the plane of its orbit. Pluto also rotates in the opposite direction from most of the other planets, with one rotation taking 6 days 9 h and 18 min (Fig. 13.3).
Fig. 13.3Orbital path of Pluto.
Density and Composition
The average density of Pluto (just below 1.8 g/cm3) indicates it’s composition is a mixture of about 70 % rock and 30 % water ice, much like Triton. One theory is that Pluto and Triton formed at the same time in the same part of the solar nebula.
Pluto probably has a large rocky core of silicate materials, surrounded by a mantle rich in ices and frozen water. The extent of Pluto’s crust is unknown, but it is thought to be covered with patches of frozen nitrogen, water, methane and ethane.
The strength of gravity on Pluto is much less than Earth’s gravity (0.67 compared to 9.8 N/kg). This means that a 75 kg person weighing 735 N on Earth, would weigh only 50 N on Pluto.
The Surface
Astronomers know little about the surface of Pluto because the body is so far from Earth. Pluto’s diameter is less than one fifth that of Earth’s so it is difficult to observe anything on a surface that is so far away. The best views of Pluto show a brownish disc with bright and dark areas (see Fig. 13.2). The bright areas are probably covered with frozen nitrogen with smaller amounts of methane, ethane and carbon monoxide. The composition of the darker areas on the surface is unknown but they may be caused by decaying methane or carbon-rich material. Pluto’s interior is probably rich in ices with some frozen water. The central core probably contains solid i
ron and nickel and rocky silicate materials. The New Horizons probe on approaching Pluto in 2015 noticed wide variations in the bright and dark areas on Pluto as well as a large, bright heart shaped feature. The probe found Pluto has a polar ice cap and ice mountains towering to 3500 m above plains of frozen methane and nitrogen. Infrared spectral images taken by New Horizons show dark patches representing concentrations of methane ice with striking differences in texture across different regions.
The Atmosphere
Little is known about Pluto’s atmosphere but it thought to contain about 98 % nitrogen with about 2 % methane and carbon monoxide. The composition was determined from observations made when the planet passed in front of a bright star (an occulation).
The atmosphere is tenuous and the pressure at the surface is only a few millionths of that of Earth. It is thought to extend above the surface by about 600 km. Because of Pluto’s elliptical orbit, the atmosphere may be gaseous when Pluto is near the Sun and frozen on the surface when furthest from the Sun. NASA wanted its New Horizon spacecraft to arrive at Pluto when the atmosphere was unfrozen. The probe found nitrogen escaping from Pluto’s atmosphere.
Pluto’s weak gravity means its atmosphere extends to a greater altitude than does Earth’s atmosphere.
The Solar System in Close-Up Page 22
