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. 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, and four more planets, Ceres, Pallas, Juno, and Vesta, were discovered between 1801 and 1807. These were found to be orbiting between Mars and Jupiter.
In the 1840s and 1850s, many more objects were discovered between Mars and Jupiter, and astronomers realised that Ceres, Pallas, Juno, and Vesta were part of a larger body of objects - the asteroid belt.
In the 1700s, it had been suggested that the Solar System formed from a large cloud of gas that begun rotating. This caused it to flatten from a sphere into a circle. Material close to the centre fell towards each other due to gravity, creating the Sun, and smaller clumps of material further out formed smaller objects, the planets. It was suggested that the asteroid belt represented material that did not collapse into a planet. Ceres was finally reclassified as a dwarf planet in 2006.
French mathematician Urbain Le Verrier and German astronomer Johann Galle discovered Neptune in 1846. 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[7a].
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[7b].
Tombaugh found an image of something that appeared to be moving in February 1930, and he published his discovery the following month. 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.
Pluto’s mass was soon predicted to be less than 1/10th the mass of the Earth. 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.
2. Observations from Earth ↑
Up until a few weeks ago, almost everything we knew about Pluto came from data gathered by telescopes on, or orbiting, the Earth.
Pluto’s orbit was mapped shortly after its discovery, and it was found to differ greatly from that of the other planets in the Solar System. Firstly, it was found to be much more elliptical than the orbits 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 different from other planets because Pluto does not orbit in the same flat plane as the rest of the planets and the asteroid belt.
Finally, Pluto differs from all of the other planets, except Uranus, because it orbits on its ‘side’. 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.
The orbits of the planets and Pluto. Image credit: NASA/Public domain.
The orbits of the planets and Pluto. Image credit: Lookangmany/Todd K. Timberlake/Francisco Esquembre/CC-SA.
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. 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.
American astronomer James Christy discovered Pluto’s largest moon in photographs of Pluto taken in 1978[15a]. 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.
Pluto and Charon, image from New Horizons. Image credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute/Public domain.
Astronomers could measure the mass of Pluto and Charon by studying Charon’s orbit. Pluto was found to be about 0.2% as massive as the Earth, which makes it more massive than Ceres, but less massive than the Moon. Charon was found to be about 12% as massive as Pluto. This means that both Pluto and Charon have very low gravitational fields[15b].
The size of Pluto could be determined from its apparent size and its distance, which could be determined using parallax. Pluto was found to have a diameter of about 2,370 km[16a]. 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[16b]. 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 gaseous planets, like Jupiter and Saturn, and 'ice giants', like Uranus, and Neptune, but is less dense than the other planets in the Solar System, 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. 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 gas giants like Jupiter and Saturn.
Charon is less dense than Pluto, and so probably has a similar composition, but with more water relative to rock.
The predicted internal structure of Pluto. Image credit: NASA/Pat Rawlings/Public domain.
Pluto and its moons, image from the Hubble Space Telescope. Image credit: NASA/Public domain.
The first high-resolution images of Pluto and Charon, which were taken by the Hubble Space Telescope (HST) in the early 2000s and 2010s, showed that Pluto has four smaller moons: Nix, Hydra, Kerberos, and Styx. Nix and Hydra were discovered in 2005, Kerberos in 2011, and Styx in 2012.
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.
3. Beyond the planets ↑
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 came from.
In 1932, Estonian astronomer Ernst 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. This is because comets can take much longer to orbit the Sun than Pluto. 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, and this sphere became known as the Oort Cloud.
The Oort Cloud is now thought to begin at about 5,000 AU. This is about 100 times the distance between the Sun and Pluto. It then extends to about 100,000 AU. This is about 1.6 light-years, and is over a third of the distance to the closest stars to the Sun, which are just over 4 light-years away. Oort Cloud objects are thought to have formed closer to the Sun, but were ejected by interactions with Jupiter and other massive planets.
In 1980, Uruguayan astronomer Julio Fernández showed that short-period comets cannot originate from the Oort Cloud. Short-period comets are comets that reappear within 200 years, like Halley’s Comet, which has a period of about 76 years. This led further evidence to the idea that Pluto exists within a belt of comets and asteroids within the Oort Cloud, but beyond the orbit of Neptune. This idea had first been suggested shortly after the discovery of Pluto in 1930.
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.
Image credit: NASA/Public domain.
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. Despite his negative prediction, this belt became known as the Kuiper Belt, and people began referring to Pluto as a Kuiper Belt object.
British astronomer David Jewitt and Vietnamese-American astronomer Jane Luu began searching for Kuiper Belt objects in 1987[27a]. They did this by looking at photos of the night sky, in a similar way to how people had searched for Pluto. They discovered what were considered the first two Kuiper Belt objects, other than Pluto and Charon, in 1992.
The Kuiper Belt is now thought to mostly extend from just past the orbit of Neptune, at 30 AU to about 55 AU[27b]. This makes the Kuiper Belt about 25 times as wide as the asteroid belt between Mars and Jupiter.
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:
1.6 hours to travel from the Sun to Mars,
2.9 hours to travel from the Sun to Ceres,
5.4 hours to travel from the Sun to Jupiter,
10 hours to travel from the Sun to Saturn,
20 hours to travel from the Sun to Uranus,
30 hours to travel from the Sun to Neptune,
2 days to travel from the Sun to Pluto,
2.3 days to travel from the Sun to the end of the Kuiper Belt,
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 edge of the Oort Cloud, and
30 years to travel from the Sun to closest extrasolar star.
Thousands of Kuiper Belt objects, hundreds of scattered disc objects, and a hand-full of Oort Cloud objects have since been discovered. These include 375 possible dwarf planets. The largest of these is the scattered disc object Eris. Eris was discovered in 2005, and is even more massive than Pluto. Like Pluto, Eris also has a moon, which was named Dysnomia.
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 types.
Artist's impression of the Earth and Moon, and Eris, Ceres, Pluto, and Charon to scale. Image credit: modified by Helen Klus, original image by NASA/Public domain.
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. Neptune’s moon Nereid is also suspected to have originated in the Kuiper Belt, as is Saturn's moon Phoebe.
A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
A 'dwarf planet' is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
All other objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies'.
4. 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.
New Horizons was granted funding in 2001. 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.
The New Horizons space probe. Image credit: modified by Helen Klus, original image by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Public domain.
New Horizons contains seven scientific instruments, as well as a number of cultural artefacts, including a CD containing over 400,000 names, a Flag of the United States, and about 30 g of Clyde Tombaugh's ashes.
New Horizons began to approach Pluto in January 2015, and flew within 12,500 km of Pluto on 14th July 2015. 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.
Pluto and Charon in natural colour. Image credit: NASA/JHUAPL/SWRI/Public domain.
Pluto, image from New Horizons. Image credit: NASA/JHUAPL/SWRI/Public domain.
Charon, image from New Horizons. Image credit: NASA/JHUAPL/SWRI/Public domain.
While it will probably be over a year before all the data can reach Earth and be analysed, data from New Horizons is already showing that Pluto and Charon are much more interesting than previously thought. This is because they appear to be geologically active, and no one knows why[42a].
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[43a], flowing ice[44a], and mountains, as well as a disproportional amount of carbon monoxide. It has been named the Tombaugh Regio (Tombaugh Region)[42b].
Pluto’s icy plains have been named the Sputnik Planum (Sputnik Plain). They contain segments that are about 20 km wide, and are described as resembling mud-cracks on Earth[43b]. These are surrounded by shallow troughs and hills, and bordered by areas containing flowing nitrogen ice, which are similar to glaciers on Earth[44b].
Pluto’s icy plains. Image credit: NASA/JHUAPL/SWRI/Public domain.
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[44c], 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, and are similar in height to the Rocky Mountains in North America.
Pluto’s mountains are most likely made of frozen water[42c]. 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.
Simulated flight over Pluto’s icy plains. Image credit: NASA/JHUAPL/SWRI/Public domain.
Simulated flight over the Hillary Montes. Image credit: NASA/JHUAPL/SWRI/Public domain.
New Horizons also imaged Pluto’s other four moons, Nix, Hydra, Styx, and Kerbeos. Although very little data has been analysed yet, we already know that Hydra is probably covered in frozen water.
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.
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.