And if so, how would this hypothetical life form function?
Life on Earth uses either chemical energy or light energy as a life-sustaining energy source. Both these energy sources provide roughly the same amount of energy for an organism to thrive. Louis Irwin from the University of Texas at El Paso and I addressed this in a recent paper titled Life Unknown: Preliminary Scheme for a Magnetotrophic Organism. We questioned whether other life forms in the Universe could use another form of energy, particularly magnetic fields, and if so, how would this hypothetical life form function?
The most fundamental issue with this question is whether life on Earth uses chemical energy (from either inorganic or organic compounds) and light because it’s plentiful on our planet or because it’s simply a universal restriction. The first approach to get insight into this issue is to look at how much energy a life form could obtain from magnetic fields. The most straightforward way to extract energy from magnetic fields is via charge separation, either by the Lorentz force or induction. In most cases, the Lorentz force will be the stronger force. If you make the calculations using Earth´s magnetic field and a unit charge (such as an electron or ion), then the free energy that can be obtained is roughly 11 orders of magnitude lower than the energy that can be obtained from one chemical reaction or one photon.
At first glance, this is a pretty devastating result for a putative life form extracting energy from magnetic fields. Even if millions of those charge separations are going on simultaneously, we are still many orders of magnitude off from what chemotrophy or phototrophy could provide as energy. However, we have to keep in mind that Earth’s magnetic field is pretty weak compared to some other interstellar bodies, such as neutron stars or magnetars (neutron stars with extremely powerful magnetic fields). For example, the magnetar SGR 1806-20 has been reported to have a magnetic field strength of about 10E+15 Gauss, or 10E+11 Tesla, which is nearly sixteen orders of magnitude larger than Earth´s magnetic field. If there is a planet in orbit around this star, any life form on it could theoretically obtain more energy from the magnetic field than chemical or light energy. In fact, the first exoplanet discovered in 1992 was found in orbit around a neutron star, so planets around those stars with huge magnetic fields are a definite possibility.
But could life function with magnetic fields? Although Earth has a relatively weak magnetic field of less than 1 Gauss, many species on our planet are sensitive to magnetic fields and even use them for orientation and reproduction purposes. A striking example is magnetic anomalies and imprints that homing pigeons use as navigational cues and which allow turtles, salmons, and some other birds to find their natal areas. Even plants are sensitive to magnetic fields and alter their gene expression in response to varying magnetic fields. The most amazing example, however, are magnetotactic bacteria that produce magnetosomes, iron-rich magnetic particles, that enable them to move along magnetic field lines toward preferred oxygen concentrations. Chains of magnetite crystals have even been found in the Martian meteorite ALH84001, which could indicate bacteria on Mars oriented themselves to an early magnetic field on Mars (but this is something for another blog to delve into!). The point here is that life on our planet is sensitive and responsive to magnetic fields even though Earth’s magnetic field is rather weak.
If there is a planet orbiting a neutron star or a magnetar with extremely strong magnetic field, could life use all the excess energy? The expected magnetic forces would affect cellular functions, especially the movement of paramagnetic elements such as oxygen. It would also affect the diffusion of charged ions such as sodium, potassium, and calcium and polarize a broad variety of particles. It is not clear whether life could even exist under these circumstances but given the enormous adaptability of life under challenging environmental conditions, we should not exclude the possibility.
Assuming that these challenges can be resolved, Louis Irwin and I provided three examples of how magnetotrophic organisms could harvest energy from a magnetic field. All three examples of hypothetical organisms use the charge separation from the Lorentz force and rely on the tendency of living cells, whether freely rotatable or growing on a stable substrate, to become oriented to a magnetic field. Aside from obtaining their life-sustaining energy from magnetic fields, these hypothetical organisms might use familiar biochemistry to the one we observe on Earth, with certain adjustments to the high magnetic field strength that these life forms might be exposed to.
In their book Life Beyond Earth, Gerald Feinberg and Robert Shapiro went a step further and suggested a hypothetical replication mechanism for a magnetotrophic life form based on an ordered chain of magnetic particles. The starting point of their scheme is a chain of atomic magnets with their magnetic moments aligned in variable directions. When a randomly directed magnet approaches the chain, it lines up with its direction parallel to that of the nearest magnet. If the process were continued for many magnets, they hypothesized that a new chain would eventually be formed that duplicates the original chain in the directional arrangement of the magnets. If the magnets placed along the chain can retain their alignment and be protected from the re-magnetization by an exterior field, then they suggested that such an informational string could be a possibility for replicating and transmitting biological information.
Personally, I find their suggestion very imaginative and intriguing. It is the only proposed reproduction scheme I know of that is not based on a chemical compound such as DNA or RNA. However, it has several practical problems, most of all how to protect the alignment of the magnets from a re-magnetization by the exterior field. But even if our magnetotrophic life form would only meet its energy demand from a magnetic field and otherwise utilize an Earth-type biochemistry (including reproduction), we would still call this life form a magnetotrophic organism in analogy to phototrophic life that uses light energy but otherwise relies on a similar biochemistry as chemotrophic life on Earth. Also, this life would still fully satisfy the traditional definition of life as it would exhibit growth, reproduction, metabolism, and environmental adaptation.
Again, we need to emphasize that this is only scientific speculation of a life form we don´t know yet. In reality, it may not exist at all. However, given the vast Universe and the available resources that life uses, there is no principal physical or chemical reason that it could not exist. If we want to find alien life in the Universe, we have to become imaginative and think about life as we don´t know it.