Closest rocky exoplanets could support life
Some 4,000 exoplanets – orbiting distant stars – have been discovered so far. They range from hot gas giants larger than Jupiter to smaller rocky worlds like Earth. That’s exciting enough in itself, but one of the big questions is how many of those planets might be habitable. This is of particular significance in regard to rocky exoplanets orbiting in the habitable zones of nearby stars, since they are some of the easiest to study. One problem, however, has been that most of those known planets orbit red dwarf (M dwarf) stars – the most common type of star in the galaxy – which emit very high levels of UV radiation. This high-energy radiation suggests it might be difficult for life to evolve on such worlds.
But now a new study from Cornell University by Lisa Kaltenegger and Jack O’Malley-James makes the case that life could indeed have already survived in such extreme environments. The paper focuses on four of the closest potentially habitable rocky exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b and LHS-1140b, all of which orbit volatile red dwarf stars. The new peer-reviewed paper was published April 9, 2019, in the Monthly Notices of the Royal Astronomical Society.
The intense radiation from red dwarf stars has been considered an obstacle for life to develop on any otherwise habitable planets that orbit them, since that radiation can destroy biological molecules. But that might not always be the case. In the new paper, Kaltenegger and O’Malley-James make the argument that life on planets orbiting red dwarfs should still be possible since life on Earth evolved from organisms that endured an even greater amount of radiation from the early sun than some nearby red dwarf planets experience:
Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars. Our closest neighboring worlds remain intriguing targets for the search for life beyond our solar system.
To help determine the likelihood of early life on M stars, Kaltenegger and O’Malley-James used computer models to simulate the surface radiation environments of all four of those closest exoplanets. The radiation impact can be estimated, even though other specific conditions on these planets are still unknown, as related to temperature, atmospheric composition, etc.
They created models showing a range of possible scenarios for the four planets, from those with atmospheres similar to present-day Earth’s to “eroded” and anoxic (lacking oxygen) atmospheres. Exoplanets with very thin atmospheres would not block UV radiation well. Likewise, those without protective ozone wouldn’t shield their surfaces from UV. Kaltenegger and O’Malley-James then compared their exoplanet models to models of Earth, from early in its history to today.
What they found was interesting. Even though those planets – four of the closest potentially habitable rocky exoplanets to Earth – receive more radiation than Earth does now, the amount of radiation is actually significantly less than Earth received 3.9 billion years ago, when life was still first forming. Clearly, if the early Earth experienced even harsher radiation, and life still managed to evolve and flourish here, the chances for life on some of these other worlds might be greater than had been supposed.
Another aspect of all this is that some studies have shown that UV radiation was actually necessary for life to get kick-started on Earth. Perhaps the same could be true for other radiation-bathed worlds, if the amount of radiation isn’t exceedingly high, since the radiation output varies from star to star.
The researchers tried to assess the habitability of planets with varying rates of radiation influx. To do this, they studied the mortality rates at different UV wavelengths of the extremophile organism Deinococcus radiodurans, one of the most radiation-resistant microscopic organisms known to exist on Earth. As might be expected, different radiation amounts produced different results, as Kaltenegger and O’Malley-James stated:
A dosage of UV radiation at 360 [nanometers] would need to be three orders of magnitude higher than a dosage of radiation at 260 [nanometers] to produce similar mortality rates in a population of this organism.
With this new study, the researchers make a good case as to why potentially habitable planets orbiting red dwarf stars should not necessarily be ruled out for hosting life based on the UV radiation they receive – and Earth’s own history would seem to support that. As O’Malley-James said:
The history of life on Earth provides us with a wealth of information about how biology can overcome the challenges of environments we would think of as hostile.
As Kaltenegger also noted, these worlds, and other similar nearby planets, remain of great interest to scientists searching for evidence of life elsewhere:
Our research demonstrates that in the quest for life on other worlds, our closest worlds are fascinating targets to explore.
Bottom line: It may seem logical that planets orbiting red dwarf stars would be less likely to have life due to the enormous amounts of UV radiation they can receive, but this new study provides some good reasons why we can still be optimistic about finding some inhabited worlds around them – even if that life is just something like Deinococcus radiodurans.