Scientists have recently been able to identify many exoplanets that reside within a ‘Goldilocks Zone’, which is the region around a host star where it is thought possible a planet with sufficient atmospheric pressure could sustain liquid water on its surface. Though these planets are within the right orbit of the star, their ability to sustain life is not guaranteed; these exoplanets may be missing their magnetic shielding, which would make them susceptible to damaging radiation.
The magnetic field that protects Earth extends from the inner core to where it meets charged particles coming from the Sun, also known as solar wind. The magnetic field deflects most of these particles, which otherwise would strip off our ozone layer, the layer of our atmosphere responsible for protecting Earth from ultraviolet radiation. This field is generated and maintained through a rotating, convecting and electrically conducting fluid at its core called a geodynamo.
Mars and Venus do not have magnetic fields. Stellar winds are thought to have stripped away most of Mars’ atmosphere after the magnetic field dissipated. A comparison of the effect of the solar wind on Mars and Earth was possible when a planetary alignment occurred on 6 January 2008. Scientists compared the loss of oxygen from the two planets’ atmospheres when the same stream of solar wind hit them. Though the solar wind’s pressure increased at each planet by similar amounts, the increase in the rate of loss of Martian oxygen was ten times that of Earth’s increase. This difference in rates, over billions of years, helps explain the tenuous state of the Martian atmosphere today.
Venus may have had oceans deep in its past that were vaporised once the runaway greenhouse effect took hold. Any water would have been broken down by photons; with no magnetic field the free hydrogen was expelled into space by the solar wind. If an exoplanet had surface water, this water could get blasted away by stellar winds if the planet does not have a strong magnetic field.
A team of scientists, led by Jorge Zuluaga at the University of Antioquia in Colombia, used up-to-date dynamo scaling laws to calculate how long it would take for a rocky planet to cool so much that the geodynamo would stop working. They then compared these results against three well-known exoplanets believed to be potentially habitable: Gliese 581d, HD 40307g and GJ 667Cc . The team assumed an Earth-like composition for the three planets and found that the geodynamos of all of them would be extinct or close to being shut down. Gliese 581d was the best protected against the solar wind. GJ 667Cc has no protection from solar wind and has most likely suffered massive atmosphere loss. Any potential life on the planet is doomed.