Chapter 26. The Kuiper Belt and the Oort Cloud

26.1 Pluto

26.1.1 The hunt for Planet X

Mercury, Venus, Mars, Jupiter, and Saturn are all visible with the naked eye, and can be distinguished from stars because they move around the sky in a different way. The orbits of the planets were determined in the 1600s, and confirmed with telescopes, which had just started to be used in astronomy (discussed in Chapter 3). There was no reason to think that planets stopped existing beyond what we can see with the naked eye, and so the race was on to discover more.

Consequently, Uranus was discovered by British astronomer William Herschel in 1781,[1] and four more planets, Ceres, Pallas, Juno, and Vesta, were discovered between 1801 and 1807.[2] These were found to be orbiting between Mars and Jupiter. In the 1840s and 1850s, astronomers realised that Ceres, Pallas, Juno, and Vesta were not planets, but part of a larger body of objects - the asteroid belt.

German astronomer Johann Galle discovered Neptune in 1846, following calculations made by French mathematician Urbain Le Verrier.[3] Shortly after this, astronomers suggested that there should be a ninth planet beyond Neptune’s orbit. This was because something more massive than Neptune appeared to be affecting Uranus’ orbit.

American astronomer Percival Lowell began searching for this planet, which he called ‘Planet X’, at the Lowell Observatory in 1906. This involved using a telescope to photograph the sky every night and then looking for anything that appeared to be moving relative to the stars. Lowell continued this search until his death in 1916.[4]

Lowell’s death prompted a legal battle over his estate, and so the search for Planet X did not begin again until 1929, when Lowell’s former assistant, American astronomer Vesto Slipher, appointed the task to American astronomer Clyde Tombaugh.[4]

Tombaugh found an image of something that appeared to be moving in February 1930, and he published his discovery the following month.[5] Planet X was then renamed ‘Pluto’ after the god of the underworld. The name was first suggested by British pupil Venetia Burney, and then put to a vote by members of the Lowell Observatory.[6]

Pluto’s mass was soon predicted to be less than 1/10th the mass of the Earth.[7] It was not massive enough to affect Uranus’ orbit in the way that Lowell thought, and so astronomers continued to search for Planet X. This search continued until 1992, when data from Voyager 2 showed that Neptune is less massive than previously thought, and new calculations showed that Planet X was no longer needed.[8] Although the term is still used to refer to any hypothetical new planet found in the Kuiper Belt.[9]

26.1.2 Observations from Earth

Pluto’s orbit was mapped shortly after its discovery, and it was found to be very different to the other planets in the Solar System.[10] Pluto’s orbit is slightly more elliptical than Mercury’s, and far more elliptical than the orbit of the other planets, which are almost circular. While the orbit of Neptune, for example, varies by about 0.5 AU (where 1 AU is the distance between the Earth and the Sun), Pluto can be between about 30 and 50 AU from the Sun, and at its closest, it is closer to the Sun than Neptune.

Pluto’s orbit is also highly inclined. This means that it does not obit in the flat plane occupied by the rest of the planets and the asteroid belt. Relative to this plane - known as the ecliptic - Pluto orbits at an angle of about 17°.

Diagram showing that Pluto’s orbit is much more elliptical than the orbits of Neptune and the other planets.

Figure 26.1
Image credit

The orbits of the planets and Pluto.

Diagram showing that Pluto’s orbit is not in the same plane as the planets.

Figure 26.2
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The orbits of the planets (blue) and Pluto (purple).

Finally, Pluto is tilted on its axis by about 120°, and so, like Venus and Uranus, it could be considered to be upside down. This means that it experiences seasons differently, with some parts in continuous darkness, and some in continuous daylight, which at Pluto’s distance is comparable to twilight on Earth. All of this suggests that Pluto has undergone some sort of collision or close interaction with another object that has affected its orbit.

In 1976, astronomers at the University of Hawaii calculated Pluto’s albedo – this is a measure of how reflective it is, which is related to its brightness.[11] Pluto was found be much more reflective than Earth, which means that it’s probably covered in ice. The composition of the surface and atmosphere of a planet can be determined using spectroscopy, and Pluto’s surface was found to be composed of frozen methane. In the 1990s, it was also shown to contain frozen nitrogen and carbon monoxide, with a thin atmosphere composed of all these gases.[12]

American astronomer James Christy discovered Pluto’s largest moon in photographs of Pluto taken in 1978.[13] The moon was named Charon, both after Christy’s wife Charlene, and after the mythical figure who ferries the dead to the underworld. Charon and Pluto were found to be in a very close orbit, with an average distance of about 20,000 km between them, this is about twenty times closer than the distance between the Earth and Moon.

This has led to Pluto and Charon becoming tidally locked, so that one side of Charon always faces Pluto – just like one side of the Moon always faces the Earth. In the Pluto-Charon system, however, this goes both ways, so that one side of Pluto always faces Charon too.

Astronomers could measure the masses of Pluto and Charon by studying Charon’s orbit (discussed in Chapter 5). Pluto was found to be about 0.25% as massive as the Earth. This makes it more massive than Ceres but less massive than the Moon. Charon was found to be about 12% as massive as Pluto.[13]

The size of Pluto could be determined from its apparent size and its distance, which could be determined using parallax (discussed in Chapter 3). Pluto was found to have a diameter of about 2,370 km.[14] This is similar to the distance between New York and Las Vegas, and gives Pluto a surface area of about 17 million km2, which is roughly the surface area of Russia. Charon has a diameter about half the size of Pluto’s.[14] This makes it the largest moon in the Solar System relative to the size of its planet, and so Pluto and Charon are sometimes referred to as a binary system.

The density of a planet can be found by comparing its mass to its size, and once the density of Pluto was known, astronomers could predict what it was made of. Pluto is denser than the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune, but is less dense than the terrestrial planets, which are mostly made of rock.

It was suggested that Pluto is made of rock and water, which have separated so that the water forms a thick layer above the rock and below the frozen surface.[15] The layer of water may be frozen, or it may have been heated by the decay of radioactive elements to form an ocean, which could be about 10 times as deep as the ocean on Earth. Similar oceans are expected to exist on the moons of Jupiter and Saturn. Charon is less dense than Pluto, and so probably has a similar composition, but with more water relative to rock.

Diagram showing that Pluto is thought to be composed of rock, covered with a layer of water ice, and then frozen nitrogen.

Figure 26.3
Image credit

The predicted internal structure of Pluto.

The first high-resolution images of Pluto and Charon were taken by the Hubble Space Telescope (HST) in the early 2000s and 2010s. These showed that Pluto has four smaller moons: Nix, Hydra, Kerberos, and Styx. Nix and Hydra were discovered in 2005,[16] Kerberos in 2011,[17] and Styx in 2012.[18]

By this time, other objects had been discovered orbiting the Sun from a similar distance to Pluto, and it had become increasingly obvious that Pluto exists within a larger system, just as Ceres, Pallas, Juno, and Vesta were discovered to exist within the asteroid belt.

Pluto and its moons.

Figure 26.4
Image credit

Pluto and its moons, image from the Hubble Space Telescope.

Pluto Fact Sheet[19]

Designation = Dwarf planet
Mass = 1.5×1022 kg (0.25% mass of Earth)
Radius = 1185 km (18.6% radius of Earth)
Density = 2095 kg/m3 (38.0% density of Earth)
Length of Day = 153.3 hours
Length of year = 90,560 Earth-days (248 Earth-years)
Days per year = 14,178 days on Pluto per year on Pluto
Distance from the Sun = 5.9×109 km (39.5 AU)
Orbital Velocity = 4.7 km/s
Orbital Eccentricity = 0.244
Obliquity (tilt) = 122.5°
Mean Temperature = -225 °C
Moons = 5 (Charon, Nix, Hydra, Kerberos, and Styx)
Ring System = Yes
Photograph of Pluto.

Figure 26.5
Image credit

Pluto, image from New Horizons.

Photograph of Charon.

Figure 26.6
Image credit

Charon, image from New Horizons.

The Norgay Montes (Norgay Mountains) on Pluto, image taken by New Horizons from about 18,000 km.

Figure 26.7
Image credit

The Norgay Montes (Norgay Mountains) on Pluto, image taken by New Horizons from about 18,000 km.

26.2 The Kuiper Belt

Shortly after the discovery of Pluto, astronomers realised that the Solar System must contain more than just nine planets and an asteroid belt. This is because there was still no explanation for where comets come from. In 1932, Estonian astronomer Ernst Öpik showed that some comets must originate from a sphere of icy material that orbits the Sun from well beyond Pluto’s orbit.[20] This sphere became known as the Oort Cloud.

In 1943, Irish astronomer Kenneth Edgeworth suggested that Pluto may be part of a mass of material that was spaced too far apart to form a single planet, and so formed many smaller bodies, like those that make up the asteroid belt.[21] This would mean that there is another belt of comets and asteroids within the orbit of the Oort Cloud, but beyond the orbit of Neptune. In 1951, Dutch–American astronomer Gerard Kuiper made a similar observation, but suggested that the rest of the belt would be swept away by the orbit of Pluto, and so would no longer exist.[22] Despite his negative prediction, this belt became known as the Kuiper Belt, and people began referring to Pluto as a Kuiper Belt object.

In 1980, Uruguayan astronomer Julio Fernández showed that short-period comets cannot originate from the Oort Cloud, and so probably originate from the Kuiper Belt.[23] Short-period comets are comets that reappear within 200 years, like Halley’s Comet, which has a period of about 76 years.[24,25]

British astronomer David Jewitt and Vietnamese-American astronomer Jane Luu began searching for Kuiper Belt objects in 1987.[26] They discovered what were considered the first two Kuiper Belt objects, other than Pluto and Charon, in 1992.[27]

The Kuiper Belt is now thought to mostly extend from just past the orbit of Neptune, at 30 AU to about 55 AU.[26] This makes the Kuiper Belt about 25 times as wide as the asteroid belt between Mars and Jupiter. Short-period comets, however, can travel up to 100 AU from the Sun, into a region known as the scattered disc.[24,25]

The Kuiper Belt is thought to contain hundreds of thousands of objects over 100 km in diameter,[25] and over 1000 of these have been catalogued.[28] These include at least 375 possible dwarf planets,[29] and at least four confirmed dwarf planets: Pluto, Eris, Haumea, and Makemake.[25]

Eris Fact Sheet[30]

Designation = Dwarf planet
Mass = 1.7×1022 kg (0.28% mass of Earth)[31]
Radius = 2400 km (37.6% radius of Earth)[32]
Density = 287 kg/m3 (5.2% density of Earth)
Length of Day = 25.9 hours
Length of year = 203,400 Earth-days (557 Earth-years)[33]
Days per year = 267,070 days on Eris per year on Eris
Distance from the Sun = 1.0×1010 km (68 AU)
Orbital Velocity = 3.4 km/s
Orbital Eccentricity = 0.43
Mean Temperature = -230 °C[32]
Moons = At least 1 (Dysnomia)[33]
Diagram of objects beyond the orbit of Neptune, where sizes are to scale.

Figure 26.8
Image credit

The Earth, Moon, and objects beyond the orbit of Neptune, sizes are to scale.

Relative distances in the Solar System:

If it takes you 1 hour to travel from the Sun to the Earth, then assuming you travel at the same speed, it would take about 9.2 seconds to travel to the Moon, and:

23 minutes to travel from the Sun to Mercury,

43 minutes to travel from the Sun to Venus,

1.5 hours to travel from the Sun to Mars,

2.8 hours to travel from the Sun to Ceres,

5.2 hours to travel from the Sun to Jupiter,

9.6 hours to travel from the Sun to Saturn,

19.2 hours to travel from the Sun to Uranus,

1.3 days to travel from the Sun to Neptune,

1.6 days to travel from the Sun to Pluto,

2.3 days to travel from the Sun to the end of the Kuiper Belt,

2.7 days to travel from the Sun to Eris,

4.2 days to travel from the Sun to the end of the scattered disc,

208 days to travel from the Sun to the beginning of the Oort Cloud,

11 years to travel from the Sun to the end of the Oort Cloud, and

30 years to travel from the Sun to the closest extrasolar star.

The closest of these is Pluto, which was discovered by American astronomer Clyde Tombaugh and his colleagues in 1930,[5] and demoted from planetary status in 2006.[34] The most massive is the scattered disc object Eris. Eris was discovered in 2005,[35] and like Pluto, Eris also has a moon, which was named Dysnomia.[36]

Once it was accepted that the Kuiper Belt exists, and that Pluto is a Kuiper Belt object, then it was argued that it should no longer be classified as a planet. This is because if Pluto was considered a planet, then the Solar System might contain hundreds or even thousands of planets, and scientists would need a way to distinguish between the two planetary types.

The International Astronomical Union decided that an object that has not “cleared the neighbourhood around its orbit”[37] would no longer be called a planet in 2006, and Pluto was reclassified as a dwarf planet, along with Ceres and Eris.[34]

International Astronomical Union definitions:

“A planet is a celestial body that

  • is in orbit around the Sun,
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • has cleared the neighbourhood around its orbit.

A ‘dwarf planet’ is a celestial body that

  • is in orbit around the Sun,
  • has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  • has not cleared the neighbourhood around its orbit, and
  • is not a satellite.

All other objects, except satellites, orbiting the Sun shall be referred to collectively as ‘Small Solar System Bodies’.”[37]

Pluto’s strange orbital parameters were explained around the same time. Pluto is now thought to have moved to its current position when Neptune moved further away from the Sun early in the Solar System’s formation. This also led to Neptune capturing at least one Kuiper Belt object, which became its largest moon, Triton.[38] Neptune’s moon Nereid is also suspected to have originated in the Kuiper Belt, as is Saturn’s moon Phoebe.[39]

26.2.1 New Horizons

Scientists and engineers began campaigning for a mission to Pluto in the late 1980s, forming the Pluto Underground in May 1989. This was just a few months before Voyager 1 flew past Neptune, the furthest planet to be explored. Engineers at NASA began to look into the idea in 1990, and a number of plans were then devised and subsequently cancelled or rejected, including Pluto 350.

After the discovery of Kuiper Belt objects in 1992, plans were made for a mission to flyby other Kuiper Belt objects after Pluto, and this mission became known as the Pluto Kuiper Express. The Pluto Kuiper Express was cancelled in 2000. Shortly after this, Greek-American space scientist Stamatios “Tom” Krimigis and American engineer Alan Stern, who had previously been involved with the Pluto Underground, Pluto 350, and the Pluto Kuiper Express missions, suggested a new, lower budget mission, which they called New Horizons.[40]

New Horizons was granted funding in 2001.[41] Krimigis and Stern were joined by most of the Pluto Kuiper Express team, and the New Horizons space probe was launched in 2006, setting the record for the highest launch speed of a human-made object from Earth.[42] New Horizons began to approach Pluto in January 2015, and flew within 12,500 km of Pluto on 14th July 2015.[43] Since then, New Horizons has mapped the surface of Pluto and Charon and measured how they vary in temperature. It has also made observations that can be used to determine the composition of the surface and atmosphere of Pluto and Charon, and observations that can be used to search for any undiscovered moons.

The New Horizons space probe.

Figure 26.9
Image credit

The New Horizons space probe.

Surprisingly, data from New Horizons showed that Pluto and Charon are geologically active.[44] The best evidence for this comes from the fact that both have areas that are not covered in craters. This is particularly evident within the ‘heart’ region of Pluto. There’s no reason why craters would not fall on these regions, and so something must be filling them in.

Craters on other objects in the Solar System can be filled in by the formation of mountains and volcanoes. Movement of the surface of a massive object like the Earth is caused by the Earth’s internal heat, which is mainly caused by radioactivity. Less massive objects, like the moons of Jupiter, can be geologically active because they are heated by gravitational interactions with more massive objects, like Jupiter.

Pluto is thought to be far from any objects more massive than itself, and so it must have an internal heat source, like the Earth does. No one knows how this is possible since Pluto is far less massive than the Earth. It has been suggested that Pluto may be more radioactive than expected, or it may be better than expected at storing heat.

The youngest region on Pluto may be one of the youngest surfaces in the Solar System, and contains ice plains,[45] flowing ice,[46] and mountains, as well as a disproportional amount of carbon monoxide. It has been named the Tombaugh Regio (Tombaugh Region).[44]

Pluto’s icy plains have been named the Sputnik Planum (Sputnik Plain). They contain segments that are about 20 km wide, and resemble mud-cracks on Earth.[45] These are surrounded by shallow troughs and hills, and are bordered by areas containing flowing nitrogen ice, which are similar to glaciers on Earth.[46]

Pluto’s terrain, showing mountains, plains, and glaciers.

Figure 26.10
Image credit

Map showing Pluto’s mountains, plains, and glaciers.

Close-up of Pluto’s icy plains.

Figure 26.11
Image credit

Map showing Pluto’s icy plains.

The Tombaugh Regio contains at least two mountain ranges. These have been named the Hillary Montes (Hillary Mountains), and the Norgay Montes (Norgay Mountains). The Hillary Montes are about 1.6 km tall,[46] which is similar to the height of Ben Nevis in Scotland, or the Appalachian Mountains in North America. The Norgay Montes are over twice as tall as this,[47] and are similar in height to the Rocky Mountains in North America.

Pluto’s mountains are most likely made of frozen water.[44] This is because methane and nitrogen ice would not be strong enough to form mountains this high. Frozen water, on the other hand, should behave like rock at Pluto’s temperatures, which are below -200 °C.

Pluto’s moon Charon also has mountains,[48] as well as chasms,[49] and canyons.[44] Charon’s greatest canyon is about 8 km deep. This is over four times as deep as the Grand Canyon in North America.

Future data will include higher resolution images of Pluto and all of its moons, more information about the composition and temperature of Pluto and Charon, and information that could lead to the discovery of more moons. Meanwhile, New Horizons is still collecting data, on and off, as it continues to move away from the Sun, and it is hoped that it will go on to make similar observations of other Kuiper Belt objects.[50]

New Horizons is expected to remain working for at least another ten years, but may exceed expectations and work for decades, like the Voyager probes have. If it’s still working in 2038, then New Horizons will have travelled about 100 AU from the Sun, and we’ll be able to observe space beyond the Kuiper Belt, as it travels towards the Oort Cloud.

Diagram showing the Oort cloud, the Kuiper belt is deep inside, and the planets orbit the Sun from within the Kuiper belt.

Figure 26.12
Image credit

Diagram showing the Kuiper Belt, which orbits between about 30 AU and 55 AU, and the Oort Cloud, which orbits between about 5000 AU and 100,000 AU.

26.3 The Oort Cloud

The Oort Cloud is a spherical cloud of comets that orbit between about 5000 AU - about 100 times the distance between the Sun and Pluto - and 100,000 AU. This is over 1.5 light-years, 2000 times the distance from the Sun to the edge of the Kuiper Belt, and about a third of the distance to the closest extrasolar star, Proxima Centauri.[51] Comets in the Oort Cloud are thought to have formed closer to the Sun, but were ejected by interactions with Jupiter and other massive planets. They can also be affected by the gravitational force of nearby stars, and this sometimes sends them towards the centre of the Solar System.[52]

The Oort cloud was first theorised by Estonian astronomer Ernst Öpik in 1932, within two years of the discovery of Pluto. Öpik showed that comets must originate from a hollow sphere, or cloud, of icy material that orbits the Sun from well beyond the orbit of Pluto.[20] This is because comets can take much longer to orbit the Sun than Pluto does. Comet Hale–Bopp, for example, reappears every 2500 years or so, and Pluto’s orbit was calculated to take just 248 years. Öpik’s idea was refined by Dutch astronomer Jan Oort in 1950[53], and this sphere became known as the Oort Cloud. The Oort Cloud is so far away that the Pioneer, Voyager, and New Horizons probes will not pass into it for hundreds of years.

26.4 References

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