Pluto is in a handout image made up of four images from New Horizons’ Long Range Reconnaissance Imager taken in July 2015, combined with colour data from the Ralph instrument. Photo: NASA/JHUAPL/SwRI/Reuters Handout.
Pluto has always been quite the enigma body in the Solar System – not least because of its distance from our home planet. It has only recently begun to divulge its secrets, slowly, as NASA’s New Horizons flew by it in July 2015.
Until the late 20th century, many of us learnt by heart that there are nine planets in the Solar System and that Pluto was the most distant. Humans soon started sending probes to explore the Moon, Venus, Mars and other planets – but Pluto, being the farthest, remained unobserved and shrouded in mystery for a long time. Until the 21st century, that is.
The International Astronomical Union decided to redefine the term ‘planet’ when it found that there were other objects similar to Pluto in the outer Solar System. So in 2006, during its 26th general assembly, the body redefined the term ‘planet’ in such a way that Pluto ceased to be one, and became a dwarf planet instead. But its allure didn’t wear off.
William Herschel discovered Uranus in 1781. By 1845, astronomers had observed Uranus move through its whole orbit – and they realised that its path didn’t follow the one predicted by celestial mechanics. Instead, the orbit showed some perturbations. Scientists are usually not happy with things that disobey nature’s laws, so they naturally imagined the perturbations to be induced by another body, probably a planet, that may be still further away.
This hypothesis led to the discovery of Neptune in 1846. However, astronomers soon realised that even Neptune’s path couldn’t fully explain Uranus’s, so they began looking for yet other objects that might be responsible.
Percival Lowell, a wealthy businessman and amateur astronomer in the US, mounted a sincere effort in this regard, with help of William H. Pickering. In 1909, they suggested several possible coordinates for a body they called ‘Planet X’. Lowell passed away in 1916. Fortuitously, Lowell had set up an observatory in Flagstaff, Arizona, in 1894, and the observatory’s surveys had managed to capture two faint images of Pluto on March 19 and April 7, 1915. However, Lowell and Pickering had missed them. In fact, the Lowell Observatory had spotted Pluto 14 times by then.
By the Solar System’s standards, Pluto is very small. Its diameter – 2,376 km – is the approximate distance from Jammu to Kochi. But because it’s so far from Earth, astronomers have had a hard time studying it. Its orbital plane is also very tilted relative to the other planets, and its orbit sometimes brings it closer to the Sun than Neptune.
To see Pluto, we need a telescope with an objective lens at least 30 cm wide. Even moderately powerful telescopes only see a pinpoint of light – quite like a star.
Pluto has five satellites. The one closest to Pluto is Charon, around 600 km wide – which is about twice the distance between Bengaluru and Chennai. Its discovery in 1978 prompted astronomers to reduce Pluto’s estimated size because the estimate was based on the observed brightness, which now turned out to be due to both objects.
Astronomers spotted two more satellites, named Nix and Hydra, in 2005 and 2006, with help from the Hubble Space Telescope. Two still smaller satellites were spotted when a team from NASA was examining the region to plot New Horizons’ flight path.
NASA had launched New Horizons in 2006. The probe spent eight years in hibernation to conserve power, and ‘revived’ in 2014 to prepare for its July 2015 flyby of Pluto. When it happened, New Horizons became the first human-made probe to explore Pluto at close quarters.
The probe sent us much more than photographs of Pluto from different angles. It also showed us a body that is rather active, with mountains of frozen nitrogen and giant splinters of methane ice that rise up into the sky to remarkable heights. A feature published by Nature likened its “icy nitrogen cliffs” to “the rugged coast of Norway”.
The probe flew by Pluto at 52,000 km per hour and so could photograph only a part of the surface; the other part was in darkness. However, this ‘dark’ portion had been photographed earlier, so the flyby effectively provided us a full picture of Pluto.
Despite these efforts, there is still a lot more we need to study and discover about Pluto. At the same time, what the Hubble Space Telescope, New Horizons and other instruments have found about Pluto have proved quite fascinating.
V. Sasi Kumar is a scientist formerly at the Centre for Earth Science Studies, Thiruvananthapuram.