Written by Apostolos Christou

As far as astronomers know, the Earth is alone in its annual trek round the Sun – that is, apart from its natural satellite: the Moon. But our current understanding of how the solar system formed suggests it may not have been always so. For instance, according to the Giant Impact hypothesis the Moon owes its existence to a body, perhaps as big as the planet Mars, smashing into our planet some 4 billion yr ago. This planet-sized object is sometimes referred to as Theia. Also, the largest and oldest craters we see on the Moon and Mars were caused by similar-sized objects hitting those planets. 

Artist’s impression of the titanic collision between Theia and the early Earth. Image Credit: NASA/JPL-Caltech

Nowadays, we have catalogued most large asteroids near the Earth and found them all to be quite small, a few km across at most. So, where did those larger objects go? To answer this question, we must try to imagine how those planet-like objects might have been lost (assuming they were there in the first place!). Essentially, there are two ways: either physical destruction, or migration to some other corner of the solar system far from the Earth. 

Current wisdom tells us that both mechanisms were important in creating the solar system we see today. Indeed, one of the lessons learned from examining the lunar samples and other data from the Apollo programme was that the early solar system was a very chaotic place, with planets smacking on one another and being pulverised out of existence until, in the end, only four rocky objects remained this side of Jupiter: Mercury, Venus, Earth and Mars.

As attractive as it is to invoke collisions however, these do not – by themselves – solve the problem because, if that were the case, we would still expect to see the fragments of those planets sitting along the Earth’s orbit, but we don’t!

What about migration then? If we place material along the Earth’s orbit, we might reasonably expect it to stay there for as long as the Earth does but, actually, that’s not true: as the largest planet inside the orbit of Jupiter, the Earth exerts quite a respectable gravitational pull on any material in its vicinity. Such debris would therefore feel the combined pull of not just the other planets but the Earth as well, the question then becomes how long can it stay there.

In a recent paper we published in Monthly Notices of the Royal Astronomical Society with my colleague Nick Georgakarakos at NY University in Abu Dhabi, United Arab Emirates, we tried to answer this question by running dynamical simulations of the solar system. Simply put, a simulation is where the scientist gets to “play god”, running a model of the solar system millions of years backwards or forwards in time and watches what happens. Here, we found that the Earth’s orbit behaves like a leaky car radiator, losing a little bit of material at a time so, after billions of years, the “radiator” runs dry. Then, by assuming that the last object was lost right before humanity started to observe and record the solar system, we can calculate what scientists call an “upper limit”, that is the minimum number of Earth companion objects the real solar system started with at the beginning.

And the answer Nick and I came up with is .. between 3 and 12 objects! This is a rather wide range but – remember – this information was extracted from the observation that we now see zero objects. By combining computer simulations with statistics, we are literally getting something from nothing! Also, our estimates appear quite reasonable given what other information we have about the formation of the Earth. On one hand, we would like at least one such object to have existed in order to hit the Earth and form the Moon, on the other if there are too many objects available to collide with Earth, our planet might never have formed!

Orbit of “horseshoe” asteroid 2010 SO16 relative to the Earth (“E”) and the Sun (“S”). The diamond indicates the asteroid’s location on the 1st of January 2010. The asteroid is now moving away from the Earth along the red path; it will reach the opposite end of the horseshoe in the year 2185.

Though these planet-sized siblings of the Earth are now long gone, the same cannot be said – at least not yet – for any debris they may have left behind. Finding even one piece of this debris would effectively open a window into our planet’s deep past. Today, we know of 150 or so small asteroids along the Earth’s orbit. Some of these objects follow uniquely complicated paths relative to the Earth and, in this sense, can be regarded as true attendants or even satellites of our planet, albeit temporary. One example is asteroid 2010 SO16; its orbit, shown in the above Figure as the red line, resembles a horseshoe with the opening at the Earth’s location.

These horseshoes turn out to have a special significance in the search for Earth’s ancient companions. Our study also showed that, if any asteroids could have survived over the 4 billion yr lifetime of our solar system, those would be these horseshoes. Unfortunately,  the very orbits of horseshoes also makes them difficult to spot from the Earth. This is because they prefer to linger close to the Sun where telescopes cannot look. To find them, we must either wait until they approach the Earth (which they do, every few hundred yr!), or move the search out to space and physically chase after them. With missions like the European Space Agency’s Gaia surveying the sky and new initiatives such as NASA’s NEO Surveyor, the search is definitely on and we may not have long to wait until a definite answer to this question is at hand, one way or the other.

1 Comment

Freddie · November 10, 2022 at 07:05

With the ‘Giant Impact’ of Moon and Earth, how did the Moon escape the Roche Limit? Why did the resulting debris not simply fall back to Earth or if the debris ended up further out, not form rings like Saturn?
Many Thanks,
Freddie · August 22, 2022 at 16:02

Courtney Allison · August 27, 2022 at 13:42

Dear Freddie,
Many thanks for your excellent question. To try and answer it, I would like to ask you to imagine the neighbourhood of the Earth right after
that colossal impact that created the moon. That impact created a lot of debris, indeed the Earth itself probably came close
to be broken up into a ring of its own debris orbiting the Sun!

Anyway, the key point is that different parts of the debris “cloud”, as it were, were moving with different speeds with respect to the Earth.
In this way, the slower-moving debris would indeed have rained back down on our planet in fairly short order, while faster-moving debris went into orbit about the Earth, just like an artificial satellite. Computer simulations show that the Moon was created from this fast-moving debris over a few thousand years, a very short time astronomically speaking.

However, not all of this debris contributed to the Moon’s formation since, as you correctly pointed out, debris inside the Roche limit (approx some ten thousand miles from the Earth’s surface) could not accumulate to form a larger body because Earth’s gravity would immediately pull such an object apart. Therefore it must have been debris *outside* the Roche limit that came together to form the Moon.

Still, the closer debris would have remained in orbit around the Earth for some time, until either friction with the early Earth’s atmosphere
or the tide raised by the Moon on the Earth pulled it in. Incidentally, it is this latter tide that is responsible for the modern observation that the Moon is receding from the Earth, at the rate of 4 cm every year. If we use a mathematical model to backtrack this recession 4 billion yr to the time the Earth and Moon formed, we find that the Moon was much closer to the Earth than it is today, perhaps only a tenth of its current distance.
That Moon would have spanned a whopping 5 degrees across the Earth’s sky (compared to the paltry 1/2 degree it displays nowadays), sadly though there would have been no intelligent beings to witness that spectacle..as far as we know!

The rings of Saturn lie within that planet’s Roche’s limit, therefore no large satellite can form there. That doesn’t mean the rings are static though,
the recent Cassini mission to Saturn showed that ring particles – essentially snowball-sized blocks of water ice – do frequently come together to form larger “super-particles”, but these are not long-lived and eventually disintegrate back after some months or years into their constituent particles, like snowballs coming apart in mid-flight as you throw them.

Hopefully this will make some sense. Do keep those questions coming!


Good Morning Courtney,
Thank you very much for your response and for the opportunity to keep the ‘questions coming’.
Regarding the ejecta beyond the Roche Limit, would it not be dissipated by the solar wind for example? Does the modelling support moon formation beyond the Roche Limit? Besides, would the ‘dust cloud’ on condensing not automatically increase pressure, which in turn would negate gravity and therefore prevent the moon forming at all?
I think it is vital to source relevant information. I am sure you agree. To that end I have located an article from the magazine, Journal of Creation (creation.com -please see below), which highlights just some of the problems with the theory of the origin of the moon. Interestingly almost 10 years ago now the Royal Society declared the giant impact theory was ‘highly unlikely’.

This article is from
Journal of Creation 30(1):14–15, April 2016
Confusion over moon origins

Moon too similar to Earth to be caused by a giant impact
Computer models have been invoked to simulate the giant impact, but they have always had difficulty in correctly simulating the impact origin of the moon, although there has been a little success in ‘modelling’ physical parameters that must be explained.2,3 However, the identical isotopes of various elements between the earth and the moon indicate that the giant impact hypothesis has serious problems.4,5

In September of 2013, researchers gathered at the Royal Society to do an in-depth review of the origin of the moon and concluded that the giant impact hypothesis is highly unlikely based on the geochemical and other problems:
“Following almost three decades of some certainty over how the Moon formed, new geochemical measurements have thrown the planetary science community back into doubt. We are either modelling the wrong process, or modelling the process wrong.”6

Astronomers are discovering more and more that the geochemistry of the moon is almost exactly that of the earth:
“A crisis in the field has been created by the growing realization that the Moon and Earth are exceptionally similar in composition—so similar, in fact, that the emerging constraints are difficult for the giant impact hypothesis to meet. … The Earth and Moon seem to share identical isotopic signatures in oxygen, iron, hydrogen, silica, magnesium, titanium, potassium, tungsten and chromium. … That all these isotopic compositions are the same on the Earth and Moon, to high precision, places stringent constraints on physical scenarios for making the satellite.”7
Such exactness defies the giant impact hypothesis because models have concluded that most of the moon should have been created from the debris of the impactor, and therefore the geochemistry would be significantly different.7

Many models … no solution
Many models have attempted to form the moon from a giant impact by varying the parameters, such as size, velocity, and impact angle, of the impactors.8,9,10 After many model runs, an acceptable isotopic similarity between the moon and Earth has been simulated. The models had to rely on a special Earth–moon–sun resonance to decrease substantially the very high angular momentum of the early Earth.
However, these moon origin simulations are simple models, and adding more complexity to the models will be a major challenge.3 For instance, after the collision a homogenous vapour is supposed to have evolved with the same isotopic ratios of many elements. However, some elements, such as titanium, would condense out too quickly to produce the same isotopic ratio between the earth and moon.3,6 The decrease in angular momentum of the Earth–moon system by resonance with the sun depends upon the ‘thermal state’ of the system, which can only be guessed at.3
Moreover, there are other problems with the simple idea of resonance: “The tidal heating and flexing of the hot young moon so near the earth may, however, prevent capture into these orbital resonances”.7 Where does that leave naturalistic theories on the origin of the moon? Apparently, there is no credible alternative at the moment, and extreme, untested physics seems to be required:
“The simulations of a Moon-forming impact have yet to produce a moon that fits all the puzzle pieces, geochemical and otherwise. … We are attempting to model processes of physics that are extreme as compared to current Earth conditions. We have never observed these processes in nature or in the laboratory.”7

Posted on homepage: 19 January 2018
References and notes
2. Elliott, T. and Stewart, S.T , Shadows cast on Moon’s origin: a chip off the old block, Nature 504(7478):90, 2013 | doi:10.1038/504090a. 
3. Ref 2.
4. Oard, M.J., Problems for ‘giant impact’ origin of moon, J. Creation 14(1):6–7, 2000; creation.com/moonimpact.
5. Samec, R.G., Lunar formation—collision theory fails, J. Creation 27(2):11–12, 2013; creation.com/lunar-collision. 
6. Elkins-Tanton, L.T., Occam’s origin of the Moon, Nat. Geosci. 6:996–998, 2013 | doi:10.1038/ngeo2026. 
7. Elkins-Tanton, ref. 6, p. 997. 
8. Cuk, M. and Stewart, S.T., Making the Moon from a fast-spinning earth: a giant impact followed by resonant despinning, Science 338(6110):1047–1052, 2012 | doi: 10.1126/science.1225542. 
9. Canup, R.M., Forming a Moon with an Earth-like composition via a giant impact, Science 338(6110):1052–1055, 2012 | doi: 10.1126/science.1226073. 
10. Halliday, A.N., The origin of the Moon, Science 338(6110):1040–1041, 2012 | doi: 10.1126/science.1229954. 

I have further questions and comments which I will relay to you later.
In anticipation of your reply, many thanks.

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