When searching for something, it often seems the best course of action is to start by looking in the most likely location. If that initial search doesn’t yield results, the next step would be to continue exploring probable sites while trying to uncover clues to help refine the investigation.
The hunt for life-friendly exoplanets has followed a similar logic and, thus, has expanded our understanding of what kinds of stars might host these planets. When scientists began the search, the ideal candidate star was one like the Sun with an Earth-sized planet in an orbit similar to ours.
However, this type of star-planet arrangement is difficult to find for two reasons. First, Earth orbits relatively far away from the Sun, but the techniques astronomers currently use to find exoplanets are far more capable of detecting planets located closer to their host stars. Second, while the Sun is a bachelor star, most stars similar in size to the Sun have a companion. Planets can form around binary systems, but the gravitational interactions of the binary system places considerable constraints on the ability of those planets to host life. These two observations led scientists to realize that a more reasonable place to search for potentially habitable planets would be near M dwarf stars.
Any star with a mass less than 50 percent of the Sun’s mass qualifies as an M dwarf. In the search for planets, M dwarfs provide many advantages. They are the most abundant type of star in our galaxy, composing around 75 percent of all stars. M dwarfs rarely form in binary systems, so any surrounding planets have a much better chance of hosting life. The habitable zone resides closer to M dwarf stars than to Sun-like stars, giving our planet-hunting techniques a greater chance of yielding results. Another benefit that will help in the future is that M dwarf stars emit significantly less light than Sun-like stars making direct detection efforts more likely to succeed. Additionally, M dwarf stars live far longer than Sun-like stars, which leads many secular scientists to believe that life is given more time to originate and evolve in such systems.
However, one key question remains: Do the characteristics of planets in the habitable zone around an M dwarf provide a suitable environment for life? The tidal influences of the M dwarf on its “habitable” planet play an important role in answering this question.
A star exerts a tidal influence on any orbiting planet, just as the Sun (and Moon) do to Earth. The tidal influence grows rapidly as the star-planet distance decreases, affecting a planet’s habitability by changing its thermal, magnetic, and orbital properties. The tides deform a rocky planet, which generates heat. Too much heat leads to excessive volcanic activity that makes the planet uninhabitable. For example, Jupiter exerts powerful tides on its moon Io that make it the most volcanically active body in the solar system. Similar forces acting on Earth would likely cause erupting magma to resurface the planet frequently, sterilizing it in the process.
Tidal influences also affect the magnetic field of the planet by slowing the rotation rate. Once the rotation rate drops below a certain level, the planet no longer generates a strong enough magnetic field to protect the atmosphere from the stellar radiation and wind. In fact, most planets in the habitable zone around M dwarfs would quickly become tidally locked (i.e., the same side of the planet faces the star all the time). The small (or nonexistent) magnetic field resulting from this, coupled with strong flaring activity,1 likely eliminates any atmosphere required for life.
One positive benefit is that tides tend to circularize the orbits of planets. Thus, any eccentricity that a planet starts with eventually disappears resulting in a more uniform illumination from the star. In a small percentage of circumstances (specifically, planets orbiting within 0.07 AU of a 0.1 solar-mass star), this circularization of the orbit can also provide just the right amount of heat so that a planet maintains a molten core that results in a magnetic field. Yet according to recent research, any planets with eccentric orbits will experience “weak magnetic fields, massive eruption rates and prolonged magma oceans” that will render these habitable-zone planets “inhospitable to life.”2
Scientists continue to find new locations and methods of discovering where life might exist, and they relentlessly investigate the viability of those options. Though there is much more searching to do, the results add to the considerable body of evidence revealing how inhospitable the rest of the universe looks compared to the abundant array of life hosted on our fine-tuned Earth.
- Edward F. Guinan and S. G. Engle, “Future Interstellar Travel Destinations: Assessing the Suitability of Nearby Red Dwarf Stars as Hosts to Habitable Life-bearing Planets,” American Astronomical Society 221 (January 2013): http://adsabs.harvard.edu/abs/2013AAS…22133302G.
- P. E. Driscoll and R. Barnes, “Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions,” Astrobiology 15 (September 2015): 739–60, doi:10.1089/ast.2015.1325.