Magnetic shielding of exomoons

A new study on magnetic fields around extrasolar giant planets sheds first light on the magnetic environment of extrasolar moons. 

The work, authored by René Heller of the Department of Physics and Astronomy at McMaster University (Canada) and Jorge I. Zuluaga of the FACom group in the Institute of Physics of the University of Antioquia (Colombia), is the first to explore the complex magnetic environment of exomoons and its impact on the habitability of these peculiar bodies.

Regrettably the results are not completely encouraging. Even the most massive moons that can be expected from a formation point of view will be small compared to Earth.

Thus, the only possibility these moons can be magnetically protected from the stellar and cosmic high-energy radiation is that they are encased by their giant planet's magnetosphere.

Yet, in orbits close to the planet, these moons can be subject to enormous tidal heating, potentially making them uninhabitable.

These results represent just the beginning of an interesting research branch, which introduces a new key factor for the habitability of those "Pandora"-like environments.

Probably the first image that comes to our minds when thinking of an inhabited extrasolar moon shows the beautiful landscapes of Pandora, the hypothetical moon of James Cameron's movie Avatar.

But the environments of extrasolar moons seem to be less favored than the idealized version shown on the big screen.

Even if located around planets in the stellar "habitable zone," where the amount of incoming light allows for the existence liquid water and hence life, exomoons are subject to a number of other perturbing effects making things harsher for life than previously thought.

René Heller

In a paper accepted for publication in the Astrophysical Journal Letters, Prof. Jorge I. Zuluaga and Dr. René Heller have studied the relationship among the magnetic field of extrasolar giant planets in the stellar habitable zone and the so-called "habitable edge" around those planets.

This edge defines the minimum distance to the planet at which a moon would just avoid to undergo a runaway greenhouse effect—an uninhabitable state similar to that on Venus.

Heller and Zuluaga studied moons having a mass and size similar to that of Mars orbiting planets with masses and compositions ranging from that of Neptune to Jupiter.

Larger moons, with masses similar to that of Earth, are unlikely to have formed even around the largest planets, while moons lighter than Mars will not be easily detected in the near future.

Structure of the magnetosphere. Dashed lines represent the orbits of moons which are completely shielded (CS), partially shielded (PS) and unshielded (US)

The core of this new work is in the calculation of the size of the magnetospheres of giant planets that are located in the habitable zones of their host stars.

Planetary magnetospheres are "bubbles" made of fields and plasma created by the shock between the stellar wind and the intrinsic magnetic field of the planet.

These bubbles separate the immediate magnetic environment of the planet from the very different environment of the interplanetary space. Magnetospheres could be really huge.

In the solar system, for example, Jupiter is surrounded by a magnetosphere that ends at distances up to 50 times the size of the planet (3.5 million of kilometers or almost 10 times the distance from the Earth to the Moon) on the dayside and stretches out almost as far as the orbit of Saturn in the night or trailing side, creating an "invisible" plasma tail.

Although the magnetosphere and its elongated tail could cover billions of kilometers, the most important part in terms of permanent moon coverage is restricted to a region the size of the dayside radius, or "standoff radius."

Read the full story here