By: Ian Santana Moore
Last August, a team of astronomers at the European Southern Observatory announced a discovery that forever changed how we view our place in the Universe. On nearby Proxima Centauri–a red dwarf star found within a ternary star system containing two much larger blue giant stars–scientists discovered an exoplanet in the habitable zone, which is the range of a star system at which an earth-like atmosphere and liquid water are likely to be found. What differentiates this discovery from past ones, however, is the Centauri system’s proximity to our own. These three stars are 4.37 light years from our sun, so by making use of innovations in observation and propulsion techniques, we will likely have information about this exoplanet, Proxima Centauri B, within our lifetimes. However, the viability of red dwarfs’ ability to harbor life-bearing or even liquid water-bearing planets has been called into question, and scientists across many disciplines have chosen sides on the issue. Observations of Proxima B could lead earth-like planets around Red dwarfs to be accepted or rejected as potentially habitable in general, decreasing our probability of finding biotic planets in the universe by up to a factor of one thousand [1]. To understand the significance of the discovery of Centauri B in the context of extrasolar astronomy, it is important to gain an appreciation of how incredibly difficult it is for astronomers to find exoplanets in the first place.
How to Detect an Exoplanet
Man’s search for earth-like characteristics on other celestial bodies is as old as Galileo himself, who named the deep, dark formations that he observed on the moon’s surface “maria” (seas). Since April 2014, when the Kepler team announced the discovery of the first rocky, earth-sized exoplanet in the habitable zone of another star [2], over three thousand exoplanets as well as countless unconfirmed candidates have been observed [3]. Because the light that earthlike exoplanets reflect is so much fainter than their parent stars, astronomers have been forced to come up with various indirect methods for observing them.
The most popular method for detecting exoplanets, and the one that was used to find Proxima B [4], we will call the Doppler method. When an orbiting planet’s gravitational pull acts on its parent star, it causes the star to wobble around an equilibrium position. A Doppler shift occurs when an object emitting waves (eg. light, sound, etc.) moves toward or away from an observer—in this case, the astronomers on earth—causing the wavelength of these waves to shorten or lengthen accordingly. By taking advantage of the miniscule, yet detectable, Doppler shift caused by this sort of wobbling, astronomers can measure the mass and velocity of a potential exoplanet.
Another method, called transit photometry, is the most popular method in general and can be used to detect and measure the radius of an exoplanet. With this method, one measures the degree to which a star’s brightness drops when a planet comes between the star and the earth, or when it transits the star. A large planet close to the star will cause a larger change in brightness than a small planet far away.
A final, much less effective but still used method is gravitational microlensing. Gravitational lensing occurs when the gravity of one massive body that is in front of a light source causes the light source to appear larger to an observer as a result. In the case of exoplanets, both bodies are stars. Gravitational microlensing as an exoplanetary detection method makes use of the tiny amount that a planet orbiting the obstructing star contributes to the lensing effect. These three complicated methods are still more effective than the direct imaging [4].
Once astronomers identify a planet and measure its velocity, they are able to perform a trivial calculation to find the distance with which it orbits its parent star in order to determine whether it falls into that stars habitable zone. The best candidate for an exoplanet that could support life is one of around the earth’s mass in the habitable zone of a star of around the sun’s mass. Sun-like stars, or G type stars, however, are around one-tenth as common as Red dwarf stars like Proxima Centauri [5], which are by far the most common type of star in the universe.
Life around a Red Dwarf Star
Due to their incredible prevalence in the night sky and their size (less than half of that of our sun), which makes their Doppler wobbles much more pronounced than those of larger stars [6], the number of exoplanets found in the habitable zones of Red dwarfs far exceeds those found around any other type of star. Various arguments against Red dwarfs’ ability to maintain the conditions for life on their planets’ surfaces do, however, persist.
Some general criteria for a star’s ability to maintain an earth-like environment on a star in its habitable zone are: (1) The speed with which a star goes through its lifetime; (2) the size of the star’s habitable zone; (3); the frequency and strength of electromagnetic emissions caused by changes in the star’s magnetic field; (4) the frequency and strength of solar flares; and (5) the possibility for Oxygenic Photosynthesis [7].
Under the first criterion, red dwarfs are a great environment for habitable planets to exist. To our current knowledge, they burn consistently and indefinitely, not exhibiting the same changes we see in larger stars. For example, sun-like G type stars eventually run out of hydrogen fuel and begin to burn helium instead, turning red and becoming much larger in the process, moving its habitable zone outward in the process. For blue giant stars, this process occurs much more quickly, and, even around planets in the habitable zone, life might not have enough time to appear before the planet becomes uninhabitable.
Due to the latter four criteria, however, some scientists believe that life around a red dwarf is relatively unlikely. Their habitable zones are significantly smaller and closer to the star itself than those of larger stars.[8] Planets in the habitable zone have such a high proximity to their parent star that they could become tidally locked (always facing the star with their same side, like the earth and the moon), which would produce an atmosphere incapable of keeping surface water in the liquid state. The fact that most of the electromagnetic radiation produced by red dwarfs is in the red and infrared is enough for many to rule out the possibility for photosynthesis [1]. These stars are also known to have high levels of magnetic activity and to frequently produce solar flares. When it was around 100 million years old, our own Sun exhibited similar characteristics, which resulted in Venus’ losing all of its water and Mars’ developing freezing surface temperatures. The fact that the earth survived this period intact pro vides support for proponents of biotic Red dwarf exoplanets.
Many astronomers are not convinced that these latter four criteria are enough to rule out the possibility of life. Some have used mathematical atmospheric models to prove that under certain conditions, a tidally locked planet could harbor liquid water on its surface. Others have pointed out that proximity to the parent star does not always result in tidal locking, as in the case of Mercury, which rotates three times for every two orbits around the Sun [8]. Furthermore, on a tidally locked planet, the effect of solar flares and electromagnetic radiation would be minimal at the terminator (the border between the day side and the night side of the planet), which would allow life to thrive there if not elsewhere on the surface. The issue of photosynthesis could be solved either if plants developed to utilize infrared radiation rather than visible light or if we allow that the amount of visible light produced by the red dwarf is enough for photosynthesis [1]. For these reasons, red dwarfs may be viable hosts for life after all.
The scientific community really cannot make up its mind on red dwarfs, which is all the more reason to be excited about the discovery of Proxima B. What better way to solve this debate than by directly observing an earthsize planet in the habitable zone of a red dwarf? The exoplanet is as close to us as it can possibly be, and astronomers’ current telescopic arsenal is not sufficient to image the planet’s surface, so the question becomes, not if, but when we will visit it. Recent initiatives in interstellar travel, such as the StarShot project, which would accelerate a small probe to relativistic speeds using lasers beams, mean that we could see images of the planet’s surface within our lifetimes. The significance of this discovery will not be completely understood until we have more data, but it has already begun to change how and where we hunt for life outside of our solar system.
Ian Santana Moore ‘19 is a sophomore in Eliot House.
WORKS CITED
[1] Gale, J.; Wandel, A. Astrobio. 2016, 1-9
[2] Quintana, E. V. et al. Sci. 2016, 6181, 277-280
[3] NASA Exoplanet Archive. http://exoplanetarchive.ipac.caltech.edu/docs/counts_detail. html (accessed Sep. 26, 2016).
[4] Hatzes, A. Nature 2016, 536, 408-409.
[5] Glenn, L. J. of Roy. Astro. Soc. of Can. 2001, 95, 32.
[6] Baraffe, I. et al. Astron. and Astroph. 2003, 402, 701-712.
[7] Cuntz, M.; Guinan, E. F. Astroph. J. 2016, 827, 1-26.
[8] Tarter, J. C. et al. Astrobio. 2007, 7, 30-65.
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