More and more planets are being discovered orbiting around stars far from Earth. When we think about the possibility of their supporting life there is one requirement we seldom consider. Those planets will probably need to have a magnetosphere somewhat like that of the Earth’s.
Earth’s magnetosphere is that region of space around the planet that is influenced by its magnetic field. First I’ll explain why Earth has a magnetic field, how the resulting magnetosphere behaves and finally why the magnetosphere is vital to life.
Earth is a solid planet, as are Mercury, Venus and Mars. But beneath Earth’s surface, at depths ranging from about 3 to 30 miles, the solid rock gives way to a transition zone leading to a very dense, molasses-like fluid region called the mantel.
The crust amounts to little more than 1 percent of the Earth’s volume, while the mantle is 1,800 miles thick and makes up 84 percent. It is made up of very hot silicate rock rich in iron and magnesium.
The mantle envelopes a solid core composed primarily of iron and nickel. Extreme pressure at these depths makes the core a solid. The core accounts for the remaining 15 percent of the Earth’s volume.
The mantle moves very slowly in regular patterns of flow. In part, those patterns are circular convections currents similar to those generated in a large pot of water being heated on the stove. Hot regions of material rise toward the surface while cooler regions fall toward the core.
Because the Earth is spinning on its axis, these swirling currents are acted on by a phenomenon called the Coriolis force. This force imposes further regularity to the currents of the mantle. These patterns of flow generate distinctive magnetic fields that extend beyond the atmosphere far into space. Consequently, the Earth acts like a large dipole magnet. It has both north and south poles that are only approximately aligned with the rotational axis of the planet.
These fields are a protective shield warding off harmful radiations largely generated by the sun. Without this shield, or if it had been too weak, it’s unlikely life would have arisen or persisted on Earth.
The solar radiation imposing this threat is called the solar wind. It arises from the approximately 1 million tons of material blown off from the Sun every second. These emissions are a consequence of the nuclear reactions driving the sun.
It is composed of charged particles, primarily protons and electrons. This material constitutes what is called a plasma. Since plasmas are made up of charged particles, they have their own magnetic fields, which add to the magnetic fields of the sun itself.
The region of space influenced by the sun’s solar wind and accompanying magnetic fields is called the heliosphere. The Earth’s magnetosphere interacts with the heliosphere as it hurtles along in its orbit about the sun.
At the distance the Earth lies from the sun the solar wind is not particularly dense. However, interactions with it distort the magnetosphere and cause it to take a tear drop shape with a long tail. The leading edge where the solar wind is slowed down and deflected from its path toward the Earth is called the bow shock.
Without this redirection of the solar wind the Earth’s surface and organisms living there would be pummeled by an intensity of radiation that would be life threatening.
In fact, there are a number of identifiable regions where the solar wind interfaces with the magnetosphere. The boundaries between these regions are in a constant state of flux. Those changes depend on variations in the solar wind arising from solar flares and routine variations in solar activity.
The Earth’s ionosphere makes up the boundary between the atmosphere and magnetosphere. Its distance above ground ranges from about 53 to 370 miles. Atmospheric gases in the ionosphere are ionized, striped of atomic electrons, by energetic electromagnetic radiations from the sun.
There are a number of layers making up the ionosphere. Some layers act as reflectors of radio waves, thus enabling long distance communication. Since charged particles in the ionosphere are greatly influenced by what happens in the magnetosphere, solar wind variations can cause havoc with communications.
The other solid planets of the Sun have very weak or nonexistent magnetic fields. The gas giants, Jupiter, Saturn, Neptune and Uranus have very strong magnetic fields, but are not conducive to life for other reasons.
It is doubtful the basic make up of life on planets orbiting other stars would be radically different from that on Earth. Hence, we can anticipate the need for shielding from intense solar wind-like radiation from their stars. This puts a significant constraint on the type of planets that will support life.
That protection might be provided by other means, but magnetic shielding is the most likely. For fields to arise having sufficient strength to provide protection, a dense solid planet with a molten interior of iron is most plausible.
This, along with a whole host of other conditions, narrows the search for habitable planets. Planets close to their stars are generally solids that have had their atmospheres blown away by intense radiation from their star. More distant planets are typically gaseous planets.
What we know for sure is the Earth’s conditions have been appropriate for life. As astronomers have found more stars with planets, they’ve also developed methods to determine the size of those planets. Given this information, along with the size and type of star and the distance between it and its planets, we can infer a great deal about whether life may have emerged elsewhere.
Steve Luckstead is a medical physicist living in Walla Walla. He can be reached at firstname.lastname@example.org.