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| Artist's impression of Pluto showing one of its large moons that never sets |
In our family, this mnemonic was how we remembered the planets and their order in terms of distance from the sun. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. My eldest brother Maurice has a sensitive stomach so this saying was close enough to the truth to make it easier to remember.
Imagine my upset when in 2002 new data processing technologies discovered a series of planet-like objects orbiting the sun close to the orbit of Pluto. This included Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna and Orcus. Indeed, one of them Eris is about the same size as Pluto. However, Pluto is only 1/400th the size of Earth and is even smaller than our moon. This bunch of objects along with Pluto were renamed dwarf planets instead of planets. Planets were redefined as objects that met three critical requirements
(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 cleared the neighbourhood around its orbit.
It is the last of these requirements clearing the space around it that these dwarf planets fail to do. One look at the Kuiper Belt indicates a failure of good housekeeping, it is far too messy. The strong gravitational field around a real planet serves to either capture, attract or perturb smaller objects around it. Those objects that pass the first two criteria but fail the third were renamed dwarf planets.
No one really cared about these newcomers being downgraded to dwarf status but for some reason, a lot of people got really upset about Pluto no longer being a proper planet. Pluto was found to have five moons of its own but did have an unusual orbit. All the other planets have orbits that lie as if on the surface of a plate extending out around the sun but Pluto has this weird tilted orbit going on.
The Kuiper Belt where Pluto resides is one of the largest structures in our solar system. It is like a huge donut, vast, mysterious and cold and dark. Much, much further out there is another structure called the Oort Cloud, a spherical region of icy comet-like bodies and both the Kuiper Belt and the Oort cloud are sources of comets.
Obviously, Pluto this black sheep of the family, was not only devoid of good housekeeping but deliberately ignoring the basic rules of the solar system.
All those years ago I resented that my family mnemonic had been messed about by forces beyond our control. But actually, comets have been having an incredible impact on our solar system over history. Take for example the Chicxulub comet which impacted 66 million years ago in Mexico. It created a crater 93 miles across and 12 miles deep. It caused the Cretaceous-Paleogene mass extinction of three-quarters of the plant and animal species on Earth. A recently published scientific paper has managed to connect 5 out of 6 mass extinctions of life with times of enhanced impact cratering on Earth. The author concluded with the frank but frightening statement, “This cosmic cycle of death and destruction has without doubt affected the history of life on our planet.”
Worryingly Jupiter, which is 300 times the size of the Earth, has been found to act a bit like a huge pinball machine in defecting incoming long-period comets into orbits close to the sun and our vicinity. Halley’s comet, which comes from the Oort Cloud revisits every 75/76 years and was first recorded in 240 BC and has been recorded since on innumerable other occasions like 1066 on the Bayeux tapestry. Mark Twain famously said he came into the world with the arrival of Halley’s Comet and would go out with the next. In his autobiography, published in 1909, he said,
“I came in with Halley's comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don't go out with Halley's comet. The Almighty has said, no doubt: Now here are these two unaccountable freaks; they came in together, they must go out together.”
Twain died on 21 April 1910, the day following the comet's subsequent perihelion.
There are also many other striking features of comets that should be recorded.
The Hale Bopp Comet is large at 35 km in diameter and has a really long orbit. It passed Earth in 2215 BC and didn't return until 1997. The next time it comes our way will be 4385, so a long wait. In the 6th dynasty reign of the Pharaoh Pepi II which coincided with the 2215BC comet appearance, his pyramid has text mentioning a star appearing. This star is particularly noticeable, after all, it is 6 times the size of Halley's Comet and in its 1997 flyby, it became the brightest comet for decades and was visible for twice as long (18.5 months) as the Great Comet of 1811. Sadly such evident celestial signs in the sky are often misinterpreted here on Earth. In the case of the Hale Bopp Comet, 39 members of the Heaven's Gate Movement in the US committed mass suicide in March 1997 with the goal of teleporting to a spaceship they believed to be behind the comet.
In Jan 2004 NASA’s Stardust probe flew within 236km of comet Wild2, which is 5km in diameter, and managed to capture samples from its trail. These were returned to earth and found to contain glycine a fundamental building block of life. Perhaps comets as well as taking life have also contributed life to planets?
On 4th July 2005, NASA’s Deep Impact probe fired a washing machine-sized object into the path of the comet Tempel 1. It created a hole in the comet the size of a football stadium. This comet is 6km in size and orbits every 5/6 years.
The European Space Probe in 2014 managed to put a lander on the comet Churyumov-Gerasimenko and there is actually a video of what it captured on the surface of this comet! It blew my mind to see this clip.
Given that comets and asteroids are prone to come flying Earth’s way with depressing regularity it is heartening to see someone practicing what to do if we manage to spot one on a collision course towards us in time. Just last year in 2022, after 10 months of flying in space, NASA’s Double Asteroid Redirection Test (DART) successfully impacted the asteroid Dimorphos. This asteroid was no danger to us but it proved that it was possible to move an asteroid in space and it succeeded in deflecting its course. You can watch a video of this collision here https://www.nasa.gov/press-release/nasa-s-dart-mission-hits-asteroid-in-first-ever-planetary-defense-test. Perhaps there is a part of you, like me, hoping they don’t nudge one into a collision course with earth while practicing!
Mind you we shouldn’t resent these impacts as there is evidence that a powerful impact on Earth probably created the moon around 4.1-3.8 billion years ago. Without the moon’s tidal impacts on our sea, life as we know it may not have even begun. As a recent Scientific American article put it, “…the lineages that ultimately gave rise to humans were at first intertidal.” Without those specific regions of the seashore, that are covered at high tide and uncovered at low tide, the viable condition for life may not have occurred. Even more compelling is current scientific theories suggesting that all the water on Earth was acquired from water-rich comets or asteroids hurled here by the influence of Jupiter. When we talk of the ‘water of life’ perhaps we need to be grateful for Jupiter’s pinball nature. Jupiter’s huge gravitational field in some ways also protects Earth as it also acts as the vacuum cleaner of the solar system sucking in some asteroids and comets. One confirmation of this is the fate of the comet Shoemaker Levy-9 which hit the giant planet Jupiter in 1994. This struck with the force of 300 million atomic bombs and left traces of water in Jupiter’s atmosphere reinforcing both Jupiter’s protection role but also proving the ability of comets to donate water to planets.
Possibly, amidst all the chaos of flying objects and collisions, both destructive and constructive, life finds a way. Perhaps, like in the rest of the Cosmos, things can arrive in our own lives unexpectedly and devastate what we hold dear. We just have to hope that something in that pain and chaos may contribute to our future growth and development.
“… this endless universe is like the human body, and … all its parts are connected one with another and are linked together in the utmost perfection.”
‘Abdu’l Bahá
