The hunt for exoplanets circling distant stars beyond our Sun proved to be a difficult endeavor, and their ultimate discovery, beginning in the mid 1990s, arguably represents one of humanity’s finest moments. Today, the discovery of such alien planets has become almost routine–just „business as usual“ for those planetary scientists on the hunt for brave new worlds. Still, the greatest prize sought by planet-hunting scientists is the discovery of habitable worlds, and in March 2014 scientists at the University of Texas at Arlington, hunting for such life-friendly exoplanets, announced that F-type stars should not be ruled out as potential stellar parents in favor of their considerably more abundant, cooler, and more petite kindred stars.
All stars are gigantic balls of roiling, seething-hot, mostly hydrogen gas. Solar system’s, such as our own, form when a relatively dense, small glob, embedded within a cold, dark, and enormous molecular cloud made up of gas and dust, collapses under the weight of its own gravity. Most of the collapsing glob of material collects at the center, and ultimately catches fire as a result of the process of nuclear fusion–giving birth to a fiery new baby star. The remaining material flattens out and becomes what is termed a protoplanetary accretion disk, from which planets, their attendant moons, and other small objects eventually emerge.
Stars on what is termed the main-sequence are in the prime of their stellar lives– maintaining a very delicate, necessary, and precious balance between two battling forces, radiation pressure and gravity. The radiation pressure of a star on the main-sequence pushes everything out and away from the star, and this keeps it bouncy against the crush of its own gravity that tries to pull everything in. A star’s radiation pressure results from nuclear fusion, which progressively fuses lighter atomic elements into heavier ones. The process begins with the burning of hydrogen–the lightest and most abundant atomic element in the Universe–into helium, which is the second-lightest atomic element. This process of fusing heavier atomic elements out of lighter ones is termed stellar nucleosynthesis. All of the atomic elements heavier than helium are called metals in the terminology used by astronomers, and all such metals are continually manufactured in the nuclear-fusing hearts, or cores, of the multitude of fiery stars dwelling in our Universe–or, in the case of the heaviest atomic elements, in the explosive supernova deaths of massive stars.
An F-type main-sequence star is still actively burning its hydrogen fuel into heavier things. It is of spectral type F and luminosity class V. F-type main-sequence stars typically possess from 1.0 to 1.4 times the mass of our Star the Sun, and surface temperatures between 6,000 and 7,600 Kelvin. This particular temperature range causes the F-type stars to be yellow-white in color. Alternatively, F-type stars are sometimes called yellow-white dwarfs. All stars that are still on the main-sequence are called dwarfs.
In general, stars fall into seven lettered categories that are based on their surface temperature. However, stars in the same category can also vary in other characteristics, such as their luminosity, mass, and the number of their particular kind that are dancing around in the Cosmos.
In astronomy, the classification of stars is based on their spectral characteristics. That is, the light emanating from the star is studied by splitting it with a diffraction grating or prism into a spectrum showing the lovely rainbow of colors interspersed with absorption lines. Each line represents an ion of a particular chemical element, with the line strength also displaying the abundance of that ion. The relative abundance of the various ions changes with the temperature of the stellar photosphere. The spectral class of a star is a brief code that summarizes its ionization state, thus providing an objective measure of the photosphere’s density and temperature.
Most stars are classified under the Morgan-Keenan (MKK) system that uses the letters O, B, A, F, G, K, M, L,T and Y. This sequence ranges from the hottest (O type) to the coolest (Y) type. The types R and N are carbon-based stars, the type S is zirconium-monoxide-based stars. Each letter in this sequence is further subdivided using a numeric digit with 0 being the hotterst and 9 being the coolest–forming a sequence from hotter to cooler.
Planetary scientists who are hunting for habitable planets usually focus on the less massive end of the spectrum. This is where our own G-type Sun is located on the main-sequence–as well as the even less massive K and M-type stars.
F-type stars are right at the center of the scale–they are more massive and hotter than our Sun. Their increased ultraviolet radiation emissions are thought to be a limiting factor for sustaining life. Furthermore, F-type stars are comparatively few in number.
The habitable zone surrounding a star is that Goldilocks region where temperatures are such that life-sustaining water can exist in its liquid state. Where liquid water exists, life also has the potential to evolve. This Goldilocks zone of a star is where the temperatures are not too hot, not too cold, but just right for water to exist in its life-loving liquid form.
However, even if a planet is dwelling within its parent star’s habitable zone, this does not mean it is necessarily a habitable world. For example, in our own Solar System, Venus is situated within the habitable zone of our Sun. However, Venus is the victim of a „runaway greenhouse effect“–and is a searing-hot, Earth-sized ball of hell, with surface temperatures hot enough to melt lead. Indeed, the rocks on Venus emit an eerie hot red glow. Earth is the only planet that we know of that hosts life. However, there is certainly a great deal of potential for life to exist on other worlds dancing around elsewhere in our vast Cosmos.
F-Type Stars Are Not Hopeless!
University of Texas at Arlington Physics Professor Dr. Manfred Cuntz commented in a March 2, 2014 University of Texas, Arlington Press Release that „F-type stars are not hopeless.“
„There is a gap in attention from the scientific community when it comes to knowledge about F-type stars, and that is what our research is working to fill. It appears they may indeed be a good place to look for habitable planets,“ he added.
Dr. Cuntz and his graduate student Satoko Sato joined with other astronomers at the University of Guanajuato in Mexico. Their research was published in the International Journal of Astrobiology in March 2014.
The team of astronomers suggest that since F-type stars sport wider habitability zones, they should be getting special attention as potential abodes for life. The scientists also investigated the potential limations caused by ultraviolet radiation by calculating its possible damage to carbon-based macromolecules in the habitable zones surrounding F-type stars. In order to accomplish this, they used DNA as an example, and then compared their calculations of DNA damage on F-type star exoplanets to the damage that would be done to life on Earth by our Sun.
The study included calculations for several varying types of F-type stars, at different stages of their evolution. The researchers met with some encouraging results. In a few instances, the damage estimates were similar to the damage that would occur on our own planet–that is, if Earth did not have an atmosphere. The damage estimate was even less if the planet of an F-type star possessed an atmosphere.
„Our study is a further contribution toward the exploration of the exobiological suitability of stars hotter and, by implication, more massive than the Sun… at least in the outer portions of the F-star habitable zones. UV radiation should not be viewed as an insurmountable hindrance to the existence and evolution of life,“ the study contends.
The research is titled the Habitability around F-type Stars. Study co-authors from the University of Guanajuato were Dr. Cecilia Maria Guerra Olvera, Dr. Dennis Jack, and Dr. Klaus-Peter Schroder.
Dr. Pamela Jansma, dean of the University of Texas Arlington College of Science said in the March 25, 2014 Press Release that „Astrophysics as it relates to habitable planets is an increasingly popular topic, and Dr. Cuntz and his student have enriched that conversation by weaving elements of theoretical biology and planetary science into their outstanding work.“
by Judith E Braffman-Miller