While some planets were discovered by accident, we actually sought out Pluto. In 1906, a wealthy astronomer named Percival Lowell, who founded an observatory in Flagstaff, Arizona, USA, began an extensive search for a planet beyond Neptune.
He tasked the staff at the observatory with looking for what he called ‘Planet X’, a large planet causing perturbations in the outer planets’ orbits. Pluto was photographed many times without astronomers knowing what it was, known as prediscoveries.
Finally a young astronomer named Clyde Tombaugh found the planet in 1930, after a year of examining photographs. Suggestions for a name came in from around the world, and an 11-year-old English girl named Venetia Burney suggested Pluto, receiving GBP 5 (the equivalent of GBP 234 or USD 376 today) as a reward. Although Pluto was too small to be Lowell’s ‘Planet X’ – the so-called perturbations were explained later by more accurate measurements – it was still considered the ninth planet in our Solar System until 2006.
Pluto lies 5.87 billion kilometres (3.67 billion miles) from the Sun on average, but its highly elliptical orbit makes it come as close as 4.44 billion kilometres (2.76 billion miles) and as far away as 7.31 billion kilometres (4.54 billion miles). During its 248-year trip, sometimes it’s closer to the Sun than Neptune. Pluto doesn’t orbit in the ecliptic – the flat, circular plane of reference around the Sun – the way that the planets do. Instead, its orbit is inclined about 17 degrees. The dwarf planet’s orbit is also chaotic and difficult to predict over time. While we can create models of planetary orbits millions of years from now, Pluto’s small size makes it very sensitive to influences from other bodies in the Solar System.
A day on Pluto lasts just over six Earth days and, much like Uranus, Pluto rotates while tipped on its ‘side’, with an axial tilt of 120 degrees. This results in extreme weather during its solstices; one-quarter of the surface is always in daylight and another quarter is in darkness. Pluto’s distance from the Sun really has more to do with its seasons than the tilt, though. Summer lasts about 50 years, and occurs when Pluto is closest to the Sun. The usually frozen gases vaporize during this time. Otherwise, it’s essentially a long, cold winter. Spring and fall are short transitions between the two.
Because Pluto is so tiny and far away we simply don’t know all that much about it – and much of what we do know is speculative. Our best images of Pluto have come from the Hubble Space Telescope, but if you’re used to seeing clear images of planets, you may be taken aback by the mottled photographs of this rocky icy dwarf planet. Pluto is smaller than the Earths Moon with a diameter of around 2,300 kilometres (1,430 miles). Its mass is estimated to be around a sixth of the Moon’s mass. Pluto also has its own satellites – five of them. The largest, Charon, is about half the diameter of Pluto.
So, if Pluto has all of these planetlike characteristics, why did the International Astronomical Union (IAU) change its classification? For one, we’ve learned a lot about the composition of the Solar System since its discovery. Pluto is located within a region called the Kuiper belt, just beyond the orbit of Neptune and extending about 7.5 billion kilometres (4.6 billion miles) from the Sun. Discovered in 1992, this massive area may contain as many as 100,000 objects left over from the formation of the Solar System. Pluto is probably the largest object in the belt, but there are others that rival it in size. It’s also one of a group of Kuiper belt objects (KBOs) known as plutinos, which share similar characteristics that include being in a 2:3 orbital resonance with Neptune.
In 2006, the International Astronomical Union voted to redefine the meaning of the term ‘planet’. A planet has to orbit the Sun, have enough gravity to be spherical and be the dominant object in its orbit. Pluto meets the first two requirements, but not the last because it’s surrounded fairly closely by objects of a similar size. However, the reclassification of Pluto caused a lot of controversy among astronomers, with many disagreeing with the IAU’s decision. And many non-astronomer types have a difficult time considering Pluto anything but a planet, even if purely for sentimental reasons.
Pluto inside and out
Since we’ve never visited Pluto, all of our knowledge about it comes from data recorded by ground-based telescopes or the Hubble Space Telescope (HST). Even the most powerful and largest telescopes on Earth show what looks more like a star than anything else. The HST, however, has given us maps showing changes in the brightness and colour of Pluto’s surface, and we also have infrared data. This information has helped astronomers make rough estimates of the size of Pluto’s polar regions, and to see that it has wide variations in colour and darkness. Pluto’s colours include white, dark reddish orange and grey black. The differentiation and changes in colours over time can probably be attributed to seasonal changes. The pole most exposed to sunlight tends to brighten as the frozen gases on the surface melt. For example, the northern pole became brighter between 1994 and 2002, while Pluto overall became a darker red-orange colour from 2000 to 2002.
Moons of Pluto
It may not be classified as a real planet any more, but Pluto still has a very planet-like feature – five satellites, or moons, of its own. The system may have been formed from an impact between Pluto and another body in the Kuiper belt. Unlike Pluto, each of the dwarf planet’s satellites has a greyish, lunar appearance.
It took almost 50 years after the discovery of Pluto for astronomers to find its largest moon, Charon. Also known as (134340) Pluto I, the moon was discovered by James Christy at the United States Naval Observatory Flagstaff Station (NOFS) in 1978. Christy examined magnified images of Pluto and noticed a bulge in the rotation that indicated it must be accompanied by a satellite. Until the Hubble Space Telescope, we didn’t have any images showing Charon as separate from Pluto. Advances in ground-based telescopes have allowed us to see the moon more clearly.
Charon is almost half the diameter of Pluto at about 1,200 kilometres (750 miles). Charon and Pluto are tidally locked – with the same sides of each body facing the other. The barycentre, or centre of mass, is outside either of them. On average, they are 19,570 kilometres (12,200 miles) apart and revolve around each other every 6.39 days. It has 11.65 per cent the mass of Pluto, and appears to have a surface of water ice. However, scientists are conflicted as to Charon’s internal structure. Some believe that it is uniformly rocky, while others think that it is differentiated, with a rocky core and an icy mantle.
Nix, or (134340) Pluto II, and Hydra, or (134340) Pluto III, were discovered at the same time, in 2005, by the HST Pluto Companion Search Team. Both appear to be almost identical, and both are in near-resonance with Charon. Nix has an orbital period of 24.9 days and has been estimated to have a diameter between 46 and 137 kilometres (29 and 85 miles). Hydra has an orbital period of 38.2 days, with a diameter between 61 and 167 kilometres (42 and 104 miles). If the two moons are similar in brightness to Charon, they’ll be on the smaller size; if they are dark like other bodies in the Kuiper belt, they may be larger.
In 2011, the HST Pluto Companion Search Team discovered Pluto’s fourth moon, designated P4 or S/2011 (134340) 1. The moon has an estimated diameter of 13 to 34 kilometres (8 to 21 miles) and an orbital period of 32.1 days. Its orbit lies between those of Hydra and Nix. In 2012, the team discovered the most recent moon – P5 or S/2012 (134340) 1. P5 is extremely tiny, at between 10 and 25 kilometres (6 and 16 miles) in diameter. It has an orbital period of 20.2 days and orbits 42,000 kilometres (26,100 miles) away. The orbit lies between those of Charon and Nix.
The main obstacle to sending a spacecraft to Pluto has been its distance, hence why much of what we know about it has been gathered from Earth-based telescopes and the Hubble Space Telescope. Voyager 1 could have continued on its trajectory to fly by Pluto, but instead it conducted a flyby of Saturn’s moon, Titan. NASA considered a space mission called the Pluto Kuiper Express to visit Pluto and its moons, but cancelled the mission in 2000 due to costs and delays.
A new NASA mission launched successfully in 2006. The New Horizons spacecraft left Earth when Pluto was still classified as a planet, and many members of the team still consider it to be one. New Horizons launched at the highest-ever speed for a spacecraft at 58,536 kilometres per hour (36,373 miles per hour) and was pointed towards Jupiter to get a gravity assist on its way to Pluto. It reached Jupiter in 2007, and the gravity assist increased the probe’s speed by about 14,000 kilometres per hour (8,950 miles per hour). While near Jupiter, New Horizons took some images of the planet and as of January 2013, the spacecraft had passed Uranus’s orbit.
The probe began sending back images of Pluto in 2006. It should come within 10,000 kilometres (6,200 miles) in February 2015, and by May begin sending back images better than the HST has been able to provide. New Horizons will fly by Pluto and each of its moons, and then hopefully fly by one or more objects within the Kuiper belt. The controllers aren’t sure yet exactly which Kuiper belt objects (KBOs) the probe will visit – they’re still searching for ones with the right size and at the right distance for New Horizons to check out after it flies by the Plutonian moons.