At the moment – September 2022 – Jupiter is the brightest object in the night sky apart from the Moon. It rises in the Eastern sky early in the evening and is visible throughout the night. 

Jupiter has been known among by every group of humans to wander our home planet going back to ancient times. What’s become known more recently however is just how lucky all life on Earth is to have Jupiter is exactly where it is – or are we?

For many years astronomers theorised that Jupiter and the massive gravity of the enormous planet either flung comets or small rocky bodies from outer Solar System away from the inner planets like the Earth or Jupiter directly absorbs the impacts itself.  This theory gained a significant boost when several large bodies were directly observed hitting the Jupiter.

First recorded impact ?

The most famous impact on Jupiter was the direct hit of pieces of comet Shoemaker-Levy 9, but there’s a good chance that the dark spots recorded by Giovanni Cassini  – the same bloke famous for discovering the caps in the rings of Saturn –  were also an impact. He observed a darkened region on the surface of Jupiter in December 1690 that lasted some 18 days. Given what we’ve seen of impacts in recent times this dark spot on the Jupiter that faded over a period of some days could have easily been an impact event.

Luckily a few hundred years later in 1994 telescope technology had improved slightly since 1690. We were in a much better place to observe a major impact on Jupiter. A couple of years before the impact, Comet Shoemaker–Levy 9 broke in several large pieces. This was due to the effects of Jupiter’s gravity as the comet had been orbiting the planet for some time when it was discovered. Comets aren’t solid, rocky bodies like asteroids and are easily ripped apart. However, their total mass can be significant.

Comet Shoemaker–Levy 9

The impact of the 21 pieces of Comet Shoemaker–Levy 9 into Jupiter over a period of several days in July 1994 was widely observed and famously captured by the Hubble Space Telescope (HST). Some debated whether there was going to be anything to see at all given the small size of the cometary fragments compared to vast size of Jupiter, but the dark scars left on Jupiter were an example of the energy contained in comets orbiting at high velocities.

Dark scars left on Jupiter by Comet Shoemaker–Levy 9 in 1994/NASA HST image

Other notable impacts on Jupiter, such as the one in 2010 have even been discovered by amateur astronomers, long time BINTEL customer Anthony Wesley and renowned planetary imager Christopher Go.

Image via HST/NASA

There’s even been video taken in Brazil of an object striking Jupiter and put on YouTube by keen amateur astronomer José Luis Pereira. (Click here for more of his work)

While events like the Comet Shoemaker–Levy 9 impact are observed rarely from on a human timescale, it’s clear Jupiter gets “whacked” by bodies of a wide range of bodies on a regular basis.  Given the number we’ve observed, does this mean that Jupiter has been hit by millions of objects in the last few billion years?

If this the case, does Jupiter act as a giant vacuum cleaner that minimizes the chance of the Earth being hit by comets or asteroids? Does Jupiter being hit on such a regular basis scoop up rouge bodies whizzing around that otherwise would cause life disturbing havoc on Earth?

The answer is yes, but there’s a catch.

In a strange twist, researchers, Professor Jonti Horner and Professor Barrie Jones decided to look in detail at Jupiter’s influence on objects heading our way by simulating via computer a more or less massive Jupiter.

Firstly, the considered objects thrown towards us from the asteroid belt. These are thought to be the source of about 75% of all objects that actually do hit the Earth.  Does the influence of Jupiter protect us from these objects or increase the risk of them hitting Earth by fling them towards us?

Much their surprise, when they “scaled” Jupiter, they found that rather protecting Earth as its mass increased, it actually increased the rate of impact varied. With no Jupiter at all, there’s far less impacts than there is at its actual size.  But if Jupiter’s mass of slightly less than Saturn, the rate of impact would be far higher.








Impact rates on Earth as Jupiter’s mass increases (Credit: Horner et al. (2020).)

They next simulated the effect of impacts in Earth for the two main groups of comets.

Short period comets are those already in the inner Solar System, orbiting on the same plane as the planets and taking usually less than 200 years to complete their orbits. They’ve been nudged into the inner Solar System by gravitational influences of the outer planets from a region past Neptune called the Kuiper Belt. They part of a group of objects called trans-Neptunian objects

Long periods comets often highly inclined to the plane of the planets and orbits taking much long than 200 years. These could be making their first or possibly only trip into the inner Solar System from the Oort cloud, a yet unobserved but theorised region way beyond the orbit of Pluto that could contain untold trillions of cometary objects.  Almost all of the spectacular comets of history including Comet McNaught, which put on a show in early 2007 and Comet Hale-Bopp, are long term comets.

Illustration of the Oort Cloud. Image via NASA

For both groups of comets, Professors Horner and Jones found very much the same sort of results – counterintuitively there’s fewer impacts if there’s no Jupiter at all, and there would be many more impacts by comets if Jupiter were smaller.

What does this all mean?

From the simulations it appears that if there no Jupiter at all, there would have been fewer impacts on Earth than there have been. However, Jupiter does absorb a considerable number of impacts itself and disrupt the path of many objects headed towards us. We are lucky that Jupiter was not smaller as this would have dramatically increased the number of impacts Earth has received.

Almost any significant impact on Earth would have terrible effects on current society which is jittery at the best of times. However, in the past, impacts have brought resources to the Earth, changed climate and more. The series of events some 65 million years ago wasn’t ideal for the dinosaurs but turned out pretty well for mammals.

Either way, the Earth and life on it wouldn’t be like it is today without Jupiter. Get out there and enjoy looking at it tonight!


Earl White






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