A-1 - Encouraged by What You Read?

It seems that all too frequently we read another exciting announcement that an extrasolar planet has been detected, one that is in the “habitable zone” and would likely be able to host life. The announcement is usually accompanied by an artist’s conception of the planet with oceans, continents, and an atmosphere with clouds that appear decidedly Earth-like.

Figure 1: Estimated habitable zones in the solar system compared to Kepler-452 and Kepler-186. Image credit: NASA

Defining “Habitable”

The circumstellar habitable zone (HZ) was given a rigorous definition in a 1993 paper by geoscientist James Kasting (Pennsylvania State University) and his team. This definition was then updated in 2013.1 As defined, an HZ has an inner edge where atmospheric water breaks down and hydrogen escapes. This results in a runaway greenhouse effect. At the outer edge, CO2 condenses into clouds and accelerates cooling, resulting in all surface water freezing. For the solar system, the HZ model has an inner edge at 0.99 astronomical units (AU) and an outer edge at 1.70 AU.2 Earth is just barely inside the inner edge, and Mars is just inside the outer edge at 1.52 AU.

Although often overlooked, the definition of an HZ is extremely dependent on the composition of an exoplanet’s atmosphere. Kasting’s model assumed not only an Earth-like atmosphere, but also a carbon-silicate feedback cycle that requires a fine-tuned mix of oceans, continents, plate tectonics, and steady volcanic outgassing of CO2. One exoplanet researcher notes, “Without knowledge of the major molecules of an exoplanet’s atmosphere, we can only speculate whether it resides in the habitable zone for liquid water.… Declaring a freshly detected exoplanet to be in the ‘habitable zone’ amounts to little more than media spin if its atmospheric composition is unknown.”3

How the Definition Is Changing

Recent studies demonstrate how fragile a planet’s atmosphere can be when subjected to steady stellar radiation and occasional coronal mass ejections. Several Earth-size exoplanets around M dwarf stars have been found in the classically defined HZ. However, the close orbits of these exoplanets lead to tidal locking, which results in the atmospheres being stripped over time by a constantly blowing stellar wind.4 NASA’s MAVEN spacecraft orbiting Mars indicates that the lack of a protective magnetic field may have resulted in a similar stripping of the planet’s atmosphere. Even planets like Venus that retain a dense atmosphere lose their water unless they have a strong magnetic field.

But how has Earth maintained its magnetic shield for billions of years? Recent work done by a French team informs us that complex gravitational interactions between the earth and the moon are responsible for the earth’s long-lasting geodynamo and protective magnetic shield. The team writes (emphasis added):

Finally, because the Moon appears to be a necessary ingredient to sustain the magnetic field, and because a magnetic field is needed to shield the Earth’s atmosphere from erosion by solar wind, the habitability of an Earth­like planet may be subordinated to the existence of a large satellite. While more than 1,000 exoplanets have already been observed, the detection of an accompanying exo­moon is rare. Hence, our model could have major implications in future planetary missions as exoplanets with orbiting moons would more likely host extraterrestrial life.5

This research puts another severe constraint on planet habitability.

So What Does Habitable Really Mean?

The carbon-silicate cycle is a key mechanism in keeping Earth habitable. Now, strong planet-moon gravitational interactions join the list of necessary properties for habitable planets, and also the list of things scientists cannot yet measure. The Earth-Moon system is really a double planet and therefore a relatively rare planetary configuration.6 This new result serves as a strong reminder that “habitable,” as currently defined, really has no connection with Earth’s abundant capacity to support a diverse, thriving array of life. It may also mean that Earth is unique in its ability to do so.

Check out more from Reasons to Believe @ Reasons.org

About The Author

Jeff Zweerink

Since my earliest memories, science and the Christian faith have featured prominently in my life - but I struggled when my scientific studies seemed to collide with my early biblical training. My first contact with RTB came when I heard Hugh Ross speak at Iowa State University. It was the first time I realized it was possible to do professional work incorporating both my love of science and my desire to serve God. I knew RTB's ministry was something I was called to be a part of. While many Christians and non-Christians see the two as in perpetual conflict, I find they integrate well. They operate by the same principles and are committed to discovering foundational truths. My passion at RTB is helping Christians see how powerful a tool science is to declare God's glory and helping scientists understand how the established scientific discoveries demonstrate the legitimacy and rationality of the Christian faith. While many Christians and non-Christians see the two as in perpetual conflict, I find they integrate well. • Biography • Resources • Upcoming Events • Promotional Items Jeff Zweerink thought he would follow in his father's footsteps as a chemistry professor until a high school teacher piqued his interest in physics. Jeff pursued a BS in physics and a PhD in astrophysics at Iowa State University (ISU), where he focused his study on gamma rays - messengers from distant black holes and neutron stars. Upon completing his education, Jeff taught at Loras College in Dubuque, Iowa. Postdoctoral research took him to the West Coast, to the University of California, Riverside, and eventually to a research faculty position at UCLA. He has conducted research using STACEE and VERITAS gamma-ray telescopes, and currently works on GAPS, a balloon experiment seeking to detect dark matter. A Christian from childhood, Jeff desired to understand how the worlds of science and Scripture integrate. He struggled when his scientific studies seemed to collide with his early biblical training. While an undergrad at ISU, Jeff heard Hugh Ross speak and learned of Reasons to Believe (RTB) and its ministry of reconciliation - tearing down the presumed barriers between science and faith and introducing people to their personal Creator. Jeff knew this was something he was called to be a part of. Today, as a research scholar at RTB, Jeff speaks at churches, youth groups, universities, and professional groups around the country, encouraging people to consider the truth of Scripture and how it connects with the evidence of science. His involvement with RTB grows from an enthusiasm for helping others bridge the perceived science-faith gap. He seeks to assist others in avoiding the difficulties he experienced. Jeff is author of Who's Afraid of the Multiverse? and coauthor of more than 30 journal articles, as well as numerous conference proceedings. He still serves part-time on the physics and astronomy research faculty at UCLA. He directs RTB's online learning programs, Reasons Institute and Reasons Academy, and also contributes to the ministry's podcasts and daily blog, Today's New Reason to Believe. When he isn’t participating in science-faith apologetics Jeff enjoys fishing, camping, and working on home improvement projects. An enthusiastic sports fan, he coaches his children's teams and challenges his RTB colleagues in fantasy football. He roots for the Kansas City Chiefs and for NASCAR's Ryan Newman and Jeff Gordon. Jeff and his wife, Lisa, live in Southern California with their five children.

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