How Red Dwarf Stars Deprive Their Baby Planets of Oxygen
By Judith E Braffman-Miller
Are we alone? The quest to find the Holy Grail of life beyond Earth is certainly one of humanity’s greatest endeavors–and the answer to this profound question could change forever how we view ourselves, and how we perceive our own true place in the cosmic scheme of things. The search for life on other worlds begins in habitable zones–the “Goldilocks” region surrounding stars where the conditions are not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase–because life as we know it can only exist in the presence of liquid water. In February 2017, an interdisciplinary team of NASA scientists announced that they want to expand just how habitable zones are defined by taking into consideration the impact of stellar activity, which can pose a great danger to an alien world’s atmosphere, resulting in oxygen loss. NASA research indicates that habitable zones surrounding small, relatively cool red dwarf stars–the most common type of star in our Milky Way Galaxy–might not be able to support life because of frequent eruptions that hurl enormous storms of stellar material out into space from active, young red dwarf parent-stars.
“If we want to find an exoplanet that can develop and sustain life, we must figure out which stars make the best parents. We’re coming closer to understanding what kind of parent stars we need,” commented Dr. Vladimir Airapetian in a February 8, 2017 NASA Press Release. Dr. Airapetian is lead author of the paper describing the research, and a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
In order to determine a given star’s habitable zone, astronomers have traditionally considered the amount of light and heat the parent-star emits. Stars that are more massive than our own manufacture more light and heat than our Sun. Therefore, the habitable zone belonging to more massive stars must be farther out from the star. Stars that are comparatively small and cool sport habitable zones that are relatively close-in.
The bad news is that, along with heat and visible light, stars hurl out X-ray and ultraviolet radiation. The situation gets even worse because stars also produce eruptions in the form of flares and coronal mass ejections–collectively termed space weather.
One potential effect of this radiation is atmospheric erosion on a vulnerable exoplanet, in orbit around its parent-star. This happens because high-energy particles drag atmospheric molecules–such as hydrogen and oxygen–out into space. Hydrogen and oxygen are the two components that create water. Dr. Airapetian and his team’s new model for habitable zones takes this highly destructive effect into consideration.
Stars do not come in only one size. There are big stars, little stars, and stars of mid-size. The hunt for habitable planets frequently targets small, cool red dwarfs–the most numerous true stars in the Cosmos. These little stars, that shine with a light that is red, are relatively amenable to the detection of orbiting small planets that are about the same size as our Earth.
Unfortunately, for astronomers on the hunt for habitable worlds, “Red dwarfs are also prone to more frequent and powerful stellar eruptions than the Sun. To assess the habitability of planets around these stars, we need to understand how these various effects balance out,” explained Dr. William Danchi in the February 8, 2017 NASA Press Release. Dr. Danchi is a Goddard astronomer and co-author of the research paper.
Therefore, the exoplanet-offspring of a red dwarf star is unfortunate because it has to survive an extreme space environment–in addition to other stresses like tidal locking. Our much more fortunate Earth, in orbit around a Star that is small–but nonetheless more massive than a red dwarf– is well-protected from violent solar eruptions and bad space weather by its magnetic field, which essentially behaves much like the shields of the Starship Enterprise of Star Trek. Our own planet’s magnetic field serves the important function of deflecting approaching, potentially destructive, violent storms of energy. Earth is also protected by its distance from the fiery Sun because it orbits it at a comfortable 93,000,000 miles!
The habitable zone of a red dwarf is much closer to its parent-star than Earth’s more comfortably distant orbit around our Sun, The unfortunate exoplanet offspring of a red dwarf is doomed to endure much more powerful–and therefore destructive–space weather storming out from its merciless, red-hued stellar parent.
There is yet another important habitability factor–the star’s age. The team of NASA scientists determine a star’s age based on observations they have collected from NASA’s planet-hunting Kepler Space Telescope. Every day active young stars emit superflares, powerful flares, and eruptions that are at least 10 times stronger than those emitted by our Sun. This is in dramatic contrast to the red dwarfs’ more mature counterparts that resemble our middle-aged Sun today. For stars like our Sun, similar superflares only occur about once every century. Our Star is about 4.56 billion years old, and it has another 5 billion years to go before it must bid its final farewell to the Universe. Stars of our Sun’s mass “live” for about 10 billion years–which is why our Sun is considered to be in stellar midlife.
“When we look at young red dwarfs in our Galaxy we see they’re much less luminous than our Sun today. By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the Sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms,” Dr. Airapetian noted in the February 8, 2017 NASA Press Release.
Superflares result in atmospheric erosion when high-energy X-ray and extreme ultraviolet emissions rip molecules apart into their constituent atoms–and then ionize an unfortunate planet’s atmospheric gases. During ionization, radiation blasts against the atoms, and knocks off their clouds of electrons. Because electrons are considerably lighter than the freshly formed ions, they are able to escape to freedom from gravity’s merciless pull much more easily–and then go screaming out into interstellar space.
As is the case in some human relationships, opposites attract. Therefore as more and more negatively charged electrons are produced, they form a very powerful charge separation that attracts positively charged ions out of the atmosphere in a process termed ion escape.
“We know oxygen ion escape happens on Earth at a smaller scale since the Sun exhibits only a fraction of the activity of younger stars. To see how this effect scales when you get more high-energy input like you’d see from young stars, we developed a model,” explained Dr. Alex Glocer in the February 8, 2017 NASA Press Release. Dr. Glocer is a Goddard astrophysicist and co-author of the paper.
The model calculates the oxygen escape on planets circling red dwarfs, assuming they do not compensate with volcanic activity or the bombardment of rampaging, migrating comets. A number of previous atmospheric erosion models suggested that hydrogen is the most vulnerable to ion escape because it is the lightest atomic element. Because hydrogen is so light, it readily escapes into the space between stars–leaving behind an exoplanet atmosphere richly endowed with heavier atomic elements such as oxygen and nitrogen.
Small, Cool, Red, And Very Plentiful
The Universe is literally filled with red dwarf stars. Astronomers categorize a red dwarf as any true star that is less than 50% the mass of our Sun–down to about 7.5% solar-mass. Red dwarfs cannot be less massive than 0.075 times solar-mass. This is because at that low mass they would be too small to sustain nuclear fusion reactions in their cores–and they would become sad stellar failures. Failed stars, that are termed brown dwarfs, never managed to attain the mass necessary for igniting their nuclear-fusing stellar furnaces.
Everything that a red dwarf star does, it does slowly. Because they are a mere fraction of the mass of our Sun, red dwarfs churn out as little as 1/10,000th the energy of our Star. Basically, this means that they burn their supply of nuclear-fusing hydrogen fuel at a much slower rate than that of a larger star similar to our Sun. The largest known red dwarf shines with only 10% of the luminosity of our Sun.
Our large spiral Milky Way Galaxy sparkles with the stellar fires of at least 100 billion stars–and most of these stars are red dwarfs. There are about 100 red dwarf systems situated within 25 light-years of our planet. These very cool stars are extremely faint, and because they send forth such a relatively small amount of radiation, they can dance around in the space between stars quite secretively–well-hidden within our Milky Way, successfully eluding the peering, prying eyes of curious astronomers.
Red dwarfs are extremely common. Estimates of their abundance range from 70% of all the stars contained by a spiral galaxy to more than 90% of all the stars dancing around within an elliptical–football-shaped–galaxy. Because these very small reddish stars emit only a very weak energy output, they are never visible to the unaided eyes of Earthly observers. The closest red dwarf to our Sun is Proxima Centauri, and it is the sparkling member of a triple system of companion stars. Proxima Centauri–which is also our Star’s nearest stellar neighbor–is much too faint to be viewed from our planet without the aid of a telescope. The closest solitary red dwarf to our Sun is Barnard’s star.
Recently, red dwarf stars have become the target of astrobiologists and astronomers on the hunt of possible life dwelling on the exoplanets belonging to these little stars. A red dwarf possesses the relatively puny mass of only one-tenth to one-half that of our own Star, and determining precisely their characteristics may help scientists calculate the frequency of extraterrestrial life and intelligence.
The planets belonging to the family of a red dwarf star hug their stellar parent very closely. Because of this, these unfortunate planets suffer from powerful tidal heating. Of course, this tidal heating serves as a major impediment to the evolution of fragile living tidbits within these systems. Other tidal effects also render the formation and evolution of life in such planetary systems extremely difficult. This is because there are extreme temperature variations that occur because one side of the habitable zone red dwarf exoplanet is permanently locked facing the star, while the other side is permanently locked away from the star. In addition, there are non-tidal impediments to the formation and evolution of delicate living tidbits on red dwarf worlds, such as small circumstellar habitable zones that are caused from puny light output. Other non-tidal impediments include extreme stellar variation, as well as spectral energy distributions that are shifted to the infrared part of the electromagnetic spectrum relative to our own Star.
However, “good things come in small packages”. Red Dwarf stars can “live” for trillions of years because of their extremely slow rate of nuclear fusion. In addition, larger stars, like our Sun, contain a core that is encircled by a radiative zone, that is in turn surrounded by a convective zone. Energy can only pass from the core through the radiative zone as a result of emission and absorption by particles within the zone. One lone photon (particle of light) can take over 100,000 years to make this incredibly long journey. Outside of the radiative zone is the star’s convective zone. In this stellar convective zone, pillars of searing-hot plasma carry the intense heat from the radiative zone up to the seething surface of the star.
But little red dwarfs do not have a radiative zone. This basically means that, for the red dwarf, the convective zone descends right down to the star’s core and carries away the heat. This also mixes up the hydrogen fuel and carries away the helium that has been fused as a by-product of the nuclear fusion of hydrogen atoms. Most stars perish at the tragic point when they use up their necessary supply of hydrogen fuel in their searing-hot cores. In contrast, little–and comparatively cool–red dwarfs keep their supply of hydrogen fuel mixed up, and they will only meet their inevitable demise when–at long last–they have managed to use it all up to the very last dribble.
Because of this extremely efficient use of hydrogen fuel, red dwarfs, containing a mere 10% solar-mass, can “live” for 10 trillion years. Because our Universe is “only” about 13.8 billion years of age, it is generally thought that no red dwarf has had time enough to die since the Big Bang. Stars like our Sun can only survive for 10 to 12 billion years, by comparison.
How Red Dwarf Stars Deprive Their Baby Planets Of Oxygen
When the team of NASA scientists took superflares into account, in their new model, they found that the violent storms that characterize young, active red dwarf stars can produce sufficient high-energy radiation to enable the escape of even oxygen and nitrogen–which are building blocks for the essential molecules that make life possible.
“The more X-ray and extreme ultraviolet energy there is, the more electrons are generated and the stronger the ion escape becomes. This effect is very sensitive to the amount of energy the star emits, which means it must play a strong role in determining what is and is not a habitable planet,” Dr. Glocer continued to explain in the February 8, 2017 NASA Press Release.
The new model, when considering oxygen escape alone, estimates that a young red dwarf could cause an unfortunate close-in exoplanet to become uninhabitable within approximately a few tens to a hundred million years. The loss of both atmospheric hydrogen and oxygen would greatly reduce–and actually eliminate–the tragic planet’s supply of life-sustaining water long before tender living tidbits had a chance to emerge and evolve.
“The results of this work could have profound implications for the atmospheric chemistry of these worlds. The team’s conclusions will impact our ongoing studies of missions and would search for signs of life in the chemical composition of those atmospheres,” explained Dr. Shawn Domagal-Goldman in the February 8, 2017 NASA Press Release. Dr. Domagal-Goldman is a Goddard space scientist not involved with the new research.
Modeling the rate of oxygen loss is the first step in the NASA team’s endeavors to expand the classical definition of habitability into what they term space-weather-affected habitable zones. When exoplanets are in orbit around a parent-star that luckily has a mild space weather environment, the classical definition works very well. However, when the parent-star shows violent X-ray and extreme ultraviolet levels that exceed seven to 10 times the average emissions from our own Star, then the new definition applies–because the classical definition cannot work for those more turbulent space weather environments. In the future, the team plans that their work will include modeling nitrogen escape. Nitrogen escape may be comparable to oxygen escape because nitrogen is only just a bit lighter than oxygen.
The new habitability zone model has important implications for the recently discovered exoplanet in orbit around Proxima Centauri. Dr. Airapetian and his colleagues applied their new model to the Earth-sized exoplanet that orbits it, named Proxima Centauri b. Proxima Centauri b is 20 times closer to its parent-star than Earth is to our Sun.
Considering the age of the parent-star, as well as the exoplanet’s close-in orbit to it, the NASA scientists think that Proxima b is badly battered with terrible storms of X-ray and extreme ultraviolet radiation pouring out from superflares occurring approximately every 10 million years. Furthermore, intense magnetic activity and stellar wind–the perpetual outpouring of charged particles from a star–serve to worsen the already deadly space weather conditions.
Dr. Airapetian continued to explain that “We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability. As we learn more about what we need from a host star, it seems more and more that our Sun is just one of those perfect parent-stars, to have supported life on Earth.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.