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.
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!
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.