I’ve always assumed that so-called super-Earths would be the first place to go hunting for extraterrestrial life beyond the solar system.
Statistically, there should be more of them than “normal” Earths. We live on a puny space rock only 8,000 miles across.
Super-Earths could range from two to as much as ten times Earth’s mass. They could hold onto thick atmospheres, have rigorous volcanism, and deep oceans.
But we are able to live on Earth’s surface only because it has a 60,000-mile diameter protective “force field” that is almost straight out of a science fiction movie. Earth’s magnetic field traps energetic particles blazing out from the sun. This million-mile an hour subatomic blast would irradiate life on Earth.
We are unique in that all the other terrestrial planets in the solar system, Mercury, Venus and Mars have weak magnetic fields. The consequences are that Venus’ atmosphere is continuously being stripped off into space by the solar wind. Mars’ atmosphere was largely lost into space as the Red Planet’s magnetic field petered out billions of years ago.
We can’t see what’s going on in the core of the Earth but conventional wisdom suggests that our magnetic field requires a convective flow of a molten nickel-iron outer core that bubbles like oatmeal cooking on the stove.
Guillaume Morard of the Institute of Mineralogy and Physics of Condensed Matter in Paris, France, and his team say that super–Earth interiors may be gummed up. Their simulations show that the core is under so much crushing pressure it is solid, and therefore can’t sustain flowing convection cells.
If this is true, life might have a tough time surviving on a super-Earth. At first glance this would reinforce the Rare Earth hypothesis: that everything about Earth, including its size and unique geologic history, is precisely right for the emergence of complex life.
But we are only talking about the surface of an alien world. Some models of super-Earths predict very deep oceans. Water would shield ocean life from particle radiation on a world lacking a protective magnetosphere.
Therefore, a number of inhabited planets in the galaxy might have creatures living underground or in an ocean. This could make some atmospheric biotracers harder to detect on worlds light-years away.
However, 2 billion years ago, long before multi-celled life appeared on land, Earth’s atmosphere was building up excess of oxygen due to photosynthesis by blue-green algae in the ocean and tidal pools. This would have been measurable by alien astronomers at our level of technological development.
Vlada Stamankovic of the German Aerospace Center in Berlin says that it’s too premature to rule-out out molten iron cores inside monster Earths. Their interiors might get so hot that iron still melts and flows under enormous pressures.
In theory this question could be answered by listening for evidence of auroras on super-Earths. Earth’s magnetosphere generates radio waves. An exoplanet possessing a magnetic field would transmit weak radio emissions during auroral activity. This would require a radio space telescope 100 times more sensitive than what we already have, according Joseph Lazio at the Naval Research Laboratory in Washington D.C. But it would be another way to characterize the habitability of super-Earths.
Artwork credit: David Agular, NASA