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 differently. The orbits of the planets were determined in the 17th century 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 William Herschel in 1781,^[1] and four more planet , Ceres , Pallas , Juno , and Vesta , were discover 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.

In 1846, Johann Galle discovered Neptune using calculations made by 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.

The american astronomer is began Percival Lowell begin search for this planet , which he call ‘ planet x ’ , at the Lowell Observatory in 1906 . This is involved involve using a telescope to photograph the sky every night and then look for anything that appear to be move relative to the star . Lowell is continued continue this search until his death in 1916 .^{[ 4 ]}

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

Tombaugh is found find an image of something that appear to be move in February 1930 , and he publish his discovery the follow month .^{[ 5 ]} Planet x was then rename ‘ Pluto ’ after the god of the underworld . The name was first suggest by the british schoolchild Venetia Burney and then put to a vote by member 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 observation 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 orbit 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°.

finally , Pluto is tilt on its axis by about 120 ° , and so like Venus and uranus , it could be consider to be upside down . This is means mean that it experience season differently , with some part in continuous darkness and some in continuous daylight , which at Pluto ’s distance is comparable to twilight on Earth . All is suggests of this suggest that Pluto has undergo some sort of collision or close interaction with another object that has affect its orbit .

In 1976 , astronomer at the University of Hawaii calculate Pluto ’s albedo – this is is is a measure of how reflective it is , which is relate to its brightness .^{[ 11 ]} Pluto was found to 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]

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

This is led has lead Pluto and Charon to become tidally lock so that one side of Charon always face Pluto – just like one side of the Moon always face the Earth . In the Pluto – Charon system , however , this is goes go both way so that one side of Pluto always face 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 is is similar to the distance between New York and Las Vegas and give Pluto a surface area of about 17 million km², which is roughly the surface area of Russia. Charon has a diameter that is 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 it is less dense than the terrestrial planets, which are mostly made of rock.

It was suggest that Pluto is made of water and rock , and that these have separate so the water form a thick layer above the rock and below the frozen surface .^{[ 15 ]} The layer of water may be freeze , or it may have been heat by the decay of radioactive element to form an ocean , which could be about 10 time as deep as the ocean on Earth . similar ocean are expect to exist on the moon of Jupiter and Saturn . Charon is is is less dense than Pluto , and so probably has a similar composition but with more water relative to rock .

The first high-resolution images of Pluto and Charon were taken by the Hubble Space Telescope (HST) in the 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 object had been discover orbit the Sun from a similar distance to Pluto , and it had become increasingly obvious that Pluto exist within a large system , just as Ceres , Pallas , Juno , and Vesta were discover to exist within the asteroid belt .

Pluto Fact Sheet^[19]

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, 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, the 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 is mean would mean that there is another belt of comet and asteroid within the orbit of the Oort Cloud but beyond the orbit of Neptune . In 1951 , Gerard Kuiper is made made a similar observation but suggest that the rest of the belt would be sweep away by the orbit of Pluto and so would no long 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, the 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 ]}

The british astronomer is began David Jewitt and the vietnamese – american astronomer Jane Luu begin search for Kuiper Belt object 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 think 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 think to contain hundred of thousand of object 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 ]}

The closest of these is Pluto, which was discovered by Clyde Tombaugh and his colleagues in 1930^{[ 5 ]} and was referred to as a planet until 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 ]}

Eris Fact Sheet^{[ 30 ]}

Once it was accepted that the Kuiper Belt exists and that Pluto is a Kuiper Belt object, then it was suggested 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 definition

“A planet is a celestial body that,

• is in orbit around the Sun ,
• has sufficient mass for its self – gravity to overcome rigid body force so that it assume 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 force so that it assume 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 is was was just a few month before Voyager 1 fly past Neptune , the furth planet to be explore . Engineers is began at NASA begin to look into the idea in 1990 , and several plan were then devise and subsequently cancel or reject , includePluto 350.

After the discovery of Kuiper Belt object in 1992 , plan were made for a mission to flyby other Kuiper Belt object after Pluto , and this mission became know as thePluto Kuiper Express. The Pluto Kuiper Express was cancelled in 2000. Shortly after this, the Greek-American space scientist Stamatios “Tom” Krimigis and the 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.

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 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 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 mountain , as well as a disproportional amount of carbon monoxide . It has been name the Tombaugh Regio ( Tombaugh Region ) .^{[ 44 ]}

Pluto ’s icy plain have been name the Sputnik Planum ( Sputnik Plain ) . They is contain contain segment that are about 20 km wide and resemble mud – crack on Earth .^[45] These are surround by shallow trough and hill and are border by area contain flow nitrogen ice , which are similar to glacier on Earth .^{[ 46 ]}

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

Pluto ’s moon is has Charon is has also has mountain^[48] as well as chasms^[49] and canyons.^{[ 44 ]} Charon ’s great canyon is is is about 8 km deep . This is is is over four time 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 expect to remain work for at least another 10 year but may exceed expectation and work for decade like the Voyager probe have . If it ’s still work in 2038 , then New Horizons is travelled will have travel about 100 AU from the Sun , and it will leave the Kuiper Belt and travel towards the Oort Cloud .

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 close 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 theorise by Ernst Öpik in 1932 , within two year of the discovery of Pluto . Öpik is showed show that comet must originate from a hollow sphere of icy material that orbit the Sun from well beyond the orbit of Pluto .^[20] This is is is because comet can take much long to orbit the Sun than Pluto does . Comet Hale is reappears – Bopp , for example , reappear every 2500 year or so , and Pluto ’s orbit was calculate to take just 248 year . öpik ’s idea was refine by Jan Oort in 1950^[53] and this sphere became know as the Oort Cloud . The Oort Cloud is is is so far away that the Pioneer , Voyager , and New Horizons probe will not pass into it for hundred of year .

relative distance in the Solar System

If it takes you 1 hour to travel from the Sun tothe Earth, then assume you travel at the same speed , it is take would take about9.2 second to travel to the Moon, and ,

23 minutes to travel from the Sun toMercury,

43 minutes to travel from the Sun toVenus,

1.5 hours to travel from the Sun toMars,

2.8 hours to travel from the Sun toCeres,

5.2 hours to travel from the Sun toJupiter,

9.6 hours to travel from the Sun toSaturn,

19.2 hours to travel from the Sun touranus,

1.3 day to travel from the Sun toNeptune,

1.6 days to travel from the Sun toPluto,

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

2.7 days to travel from the Sun toEris,

4.2 days to travel from the Sun tothe end of the scatter disc,

208 day to travel from the Sun tothe beginning of the Oort Cloud,

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

30 year to travel from the Sun tothe close extrasolar star.