A team of researchers led by Vladimir Airapetian, a solar scientist at NASA’s Goddard Space Flight Center, has developed a model that estimates the oxygen ion escape rate on exoplanets around red dwarf stars, which plays an important role in determining an exoplanet’s habitability. The research is published in the Astrophysical Journal Letters.
This artist’s impression shows Proxima b orbiting Proxima Centauri, which at only 4.23 light-years is the closest star to our Solar System. The double star Alpha Centauri AB also appears in the image between the exoplanet and Proxima itself. Image credit: M. Kornmesser / ESO.
“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,” Dr. Airapetian said.
To determine a star’s habitable zone, astronomers have traditionally considered how much heat and light the star emits.
Stars more massive than the Sun produce more heat and light, so the habitable zone must be farther out. Smaller, cooler stars yield close-in habitable zones.
But along with heat and visible light, stars emit X-ray and UV radiation, and produce stellar eruptions such as flares and coronal mass ejections — collectively called space weather.
One possible effect of this radiation is atmospheric erosion, in which high-energy particles drag atmospheric molecules – such as hydrogen and oxygen, the two ingredients for water – out into space.
The team’s new model for habitable zones now takes this effect into account.
The search for habitable planets often hones in on red dwarfs, as these are the coolest, smallest and most numerous stars in the Universe — and therefore relatively amenable to small planet detection.
“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,” Dr. Airapetian said.
“Now we know these red dwarf stars generate a lot of X-ray and extreme UV emissions at the habitable zones of exoplanets through frequent flares and stellar storms.”
Every day, young stars produce superflares, powerful flares and eruptions at least 10 times more powerful than those observed on the Sun.
Superflares cause atmospheric erosion when high-energy X-ray and extreme UV emissions first break molecules into atoms and then ionize atmospheric gases.
During ionization, radiation strikes the atoms and knocks off electrons.
Electrons are much lighter than the newly formed ions, so they escape gravity’s pull far more readily and race out into space.
Opposites attract, so as more and more negatively charged electrons are generated, they create a powerful charge separation that lures positively charged ions out of the atmosphere in a process called 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 co-author Dr. Alex Glocer, of NASA’s Goddard Space Flight Center.
The model estimates the oxygen escape on planets around red dwarfs, assuming they don’t compensate with volcanic activity or comet bombardment.
Various earlier atmospheric erosion models indicated hydrogen is most vulnerable to ion escape.
As the lightest element, hydrogen easily escapes into space, presumably leaving behind an atmosphere rich with heavier elements such as oxygen and nitrogen.
But when the astronomers accounted for superflares, their new model indicates the violent storms of young red dwarfs generate enough high-energy radiation to enable the escape of even oxygen and nitrogen — building blocks for life’s essential molecules.
“The more X-ray and extreme UV energy there is, the more electrons are generated and the stronger the ion escape effect becomes,” Dr. Glocer said.
“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.”
Considering oxygen escape alone, the model estimates a young red dwarf could render a close-in exoplanet uninhabitable within a few tens to a hundred million years.
The loss of both atmospheric hydrogen and oxygen would reduce and eliminate the planet’s water supply before life would have a chance to develop.
The team’s model has implications for Proxima b, the recently-discovered Earth-mass exoplanet orbiting Proxima Centauri.
Dr. Airapetian and co-authors applied the model to this exoplanet, which orbits its host star roughly 20 times closer than Earth is to the Sun.
Considering the star’s age and the planet’s proximity to Proxima Centauri, the team expects that the exoplanet is subjected to torrents of X-ray and extreme UV radiation from superflares occurring roughly every two hours.
They estimate oxygen would escape Proxima b’s atmosphere in 10 million years.
Additionally, intense magnetic activity and stellar wind exacerbate already harsh space weather conditions.
The authors concluded that it’s quite unlikely Proxima b is habitable.
“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,” Dr. Airapetian said.
“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.”
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Vladimir S. Airapetian et al. 2017. How Hospitable Are Space Weather Affected Habitable Zones? The Role of Ion Escape. ApJL 836, L3; doi: 10.3847/2041-8213/836/1/L3
This article is based on a press-release from NASA.
