The point to which you refer to your question is what we know as the point of Lagrange 3 (L3), which is a space balance point, located on the opposite side of the sun with respect to the earth. It would be the antipodal point in the orbit of our planet. In the spheroid that is the earth, the antipodal points of many Spanish locations are in New Zealand or in the waters of the Pacific Ocean that surround it, although historically there has been talk of Australia as “the antipodes.”
L3 is an unstable equilibrium point, which means that any object located in it would move from its position if a constant force is not applied to keep it there. For that reason, among others, we do not launch satellites to L3 and yes we throw them to other points of Lagrange —L4 or L5 – because once we place them in one of those two places, there they stay, which would not happen in L3. In L3 the gravitational attraction between the sun and the earth is balancing; But, as I told you, it is a very unstable balance because any other next celestial body would unbalance it.
That point is completely invisible from Earth, so direct communications from our planet would be blocked by the Sun. I suppose that for that reason the possibility you ask in your question you place it there, because if there were a light blue object in that place, we could not see it. But that we cannot see it directly does not mean that we are not able to detect whether or not there is any light blue body there.
As we could detect, I can answer you no, there is no planet or any massive massive object at that point. And we know for several reasons.
First, a land or any other planet located there would exert a gravitational force and that force would influence everything that moves, so we would have measured it. For example, all the probes that we have sent to Venus, to Mars or any other site, could not land if there was a land in that place because for the navigation calculations of these probes it would have been necessary to introduce their gravitational mass; That would not be negligible. And since the probes have come without problems, this is the first argument. We can rule out objects of more than one hundred kilometers in diameter. Below that diameter, the force of gravity would be invaluable, but above we should have noticed it.
That is the first reason why we can rule out the presence of what is commonly known as a counter-earth. There is another argument: we have also released space probes to observe the sun, which would have seen something that was there. For example, NASA Stereo probes, launched in 2006, would have detected that hypothetical land because the L3 was visible to them.
And finally, the third argument is that being an unstable equilibrium point, an object at that point would end up moving. For example, Venus approaches that position every few months; And therefore, it would move any massive object from that antipodal point to another orbit in which it would be visible from the earth. So the answer is clear, there is no other land in L3.
In general, we have a fairly complete census of the solar system for bright asteroids, although it is not totally exhaustive, especially since small, weak and distant bodies are difficult to detect. With small bodies I mean objects one hundred meters in diameter or minors. With asteroids and that type of smaller objects, the census is increasingly complete. However, the monitoring and registration of them has been doing relatively little time – a couple of decades – so of the objects of sixty or fifty kilometers in diameter we know less because they are more difficult to detect.
The large bodies, the planets, we have them well measured, although there is the possibility that there is what we call the planet 9 or planet X, it would be in the confines of the solar system. Its existence is inferred from the movement of a certain group of asteroids in the Kuiper belt. To explain the movements of this group of asteroids there is the hypothesis that there could be a planet beyond the orbit of Neptune; and that it would be a relatively small body, but larger than the earth. It is still looking for, but the problem of a planet in an orbit so far from the earth is that it has very long periods; And it is very difficult to detect an object that takes so many years to complete its orbit around the sun.
Eva Villaver Sobrino She is astrophysics, research professor and deputy director of the Institute of Astrophysics of the Canary Islands (IAC).
Coordination and writing: Victoria Toro.
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