In the search for new planets, there has been a lot of focus on finding some living in the so-called habitable zone. This is the region within the planet’s orbit that receives enough starlight to keep liquid water, but not so much light that it all boils off as steam. Planets in the habitable zone are expected to have the best prospects of sustaining life as we know it from Earth.
But it is important to note that residential area does not mean residential. If the atmosphere of the habitable zone is bright enough, it may end up as a frozen snowball. If your planet has enough greenhouse gases, you could end up in a hell like Venus.
Now, a team of European researchers has identified another factor that could have a major impact on habitability: the star’s magnetic field. Under the right conditions, planets close to a star will experience a strong but variable magnetic field, which can cause induction heating. In the case of a system with many habitable planets, the induction heating can be strong enough to turn them into magma oceans.
Induction heating is a technique sometimes used on Earth. It is based on the absorption of magnetic fields across metallic objects. These fields collect moving electrons, creating small eddies of current, called “eddy currents.” Those currents experience some degree of electrical resistance, which turns the entire metal into a weak heater (think of the red, shiny coils in a portable space heater). With a strong magnetic field, heating can be intense.
The same effect can work in non-metals. Obviously, the value of the current will be small, but the resistance of the object will be large.
This is not an issue in our solar system, because planets and moons are too far from other sources of magnetic fields to experience significant heating (with the possible exception of Jupiter’s moon Io, where any induction heating is is swamped by tidal heating) . But many exosolar systems have planets in close proximity to their host stars, which may have strong magnetic fields.
To get induction heating to work, however, you need a rapidly changing magnetic field. From a star, this requires a specific set of circumstances: the star itself has to rotate rapidly, and its magnetic poles must be slightly offset from its rotational position, just as the Earth’s magnetic poles do not have local poles. With this combination, the star’s rotation will transfer its magnetic field across any planets orbiting it.
Real exoworld examples
The European team behind the new news focused on M dwarf stars. Because these are small, relatively cool objects, their habitable zones are close to the star and well within the region where the star’s magnetic field is very strong. They also have strong magnetic fields to begin with, sometimes in the region of thousands of Gauss. The Sun’s magnetic field is typically 10 to 1,000 times weaker.
Not all M dwarfs spin fast enough for this to be important. Proxima Centauri, which hosts the closest exoplanet, takes more than 80 days to complete one revolution. But there is a nearby M dwarf that completes a cycle in only three days: TRAPPIST-1, which hosts at least seven planets, three of them in the habitable zone. Therefore, the team decided to model how induction heating could have an effect on these tissues.
TRAPPIST-1 has measured its magnetic field at 600 Gauss, but the authors used an earlier estimate of its orbital period (1.4 days vs. the current estimate of 3.2 days). That is probably an exaggeration of today’s heating but may reflect earlier conditions, since stars tend to slow their rotation as they age.
The unknown key, however, is the composition of the planet. The possible induction heating rate is very sensitive to hot materials, but at present there is no way to determine what these planets do. The team assumed an Earth-like composition, but the results could change significantly depending on the actual materials that formed the TRAPPIST exoplanets.
In this model, the innermost orbit, which takes 1.5 days, is a rough match for the star’s 1.4-day rotation. As a result, the magnetic fields get past it slowly, so induction heating is not a significant factor. At the current rate of star rotation, there are no planets that will end up seeing this opportunity. The three outer planets are so far away from the star that they don’t see much heating, either. That leaves three spaces between these groups, one of which lives in a residential area, to be fully explored.
Here, induction heating has a counter-intuitive effect. The star’s magnetic fields are at their strongest at the top; as it moves inward, the eddy currents induced in the layers above act as a shield, limiting the strength of the magnetic field. As a result, these planets end up being warmer from the inside than from the core-out like on Earth. Maximum heating actually occurs only 10 percent of the way to the Earth’s core.
And the heating is rather substantial. For TRAPPIST-1c, the third planet from the star, induction heating reaches more than 60 percent of the heat released in the planet by radioactive decay. That’s enough to melt the entire surface, turning it into a magma ocean in almost all different model conditions. Similar conditions are possible on TRAPPIST-1d, one in the habitable zone, where induction heating can exceed half the amount of heat released by radioactive decay.
The process, however, can be self-limiting to a degree. Molten minerals tend to be better conductors, which means stronger eddy currents. Although this means more energy is released at the surface, it limits the depth to which the magnetic field can penetrate. As a result, the total heating ends up lower, and the prospects for a magma ocean go down a bit. Other conditions, such as material cycling through plate tectonics, can moderate the effect of induction heating. An active Earth’s magnetic field would also provide some protection (researchers think none exist).
In any case, the models make a convincing case that induction heating will increase volcanic activity on these worlds. And that will have a complicated relationship to accommodation. Volcanic activity can bring water and carbon dioxide to Earth’s surface, restoring air lost from previous explosions on nearby stars. That atmosphere, however, would create a strong greenhouse effect, making these planets too hot for habitation, because they are near the inner edge of the habitable zone.
The large number of assumptions about composition and internal structure required to make this model work means that it should be viewed as an indication of what can happen on exosolar systems like this. It is also a general precaution; it’s easy to get excited about habitable local planets, but we have a lot of work to do to understand whether they are truly habitable. That work includes fully understanding their stars, planets, and planet composition. And, unfortunately, there’s no technology on the horizon that’s going to get us in the world anytime soon.
The Nature of Astronomy2017. DOI: 10.1038/s41550-017-0284-0 (About DOIs).