A solar flare, imaged by NASA’s Solar Dynamics Observatory.
Among the many scientific fields born, or reborn, by the rise of astrobiology and its search for life beyond Earth is the study of stars, including our own Sun. Now that we know that planets — from the large and gaseous to the small and rocky — are common in our galaxy and number in the many, many billions, there is suddenly vast amount of real estate where life potentially could arise.
We already know that many of those planets large and small are not candidates for habitability for any number of reasons, and that makes the understanding of what general conditions are required for life all the more pressing.
And as the astrobiological effort speeds ahead, it has become clear that the make-up, behavior and location of the stars that host exoplanets is central to that understanding.
Many stellar issues are suddenly important, and perhaps none more so than the nature, frequency and consequences of the constant stellar eruption of flares, superflares and coronal mass ejections.
Created as intense bursts of radiation coming from the release of magnetic energy following reconnections in stars’ coronas, flares and related coronal mass ejections are the largest explosive events in solar systems. The energy released by a major flare from our Sun is about a sixth of the total solar energy released each second and equal to 160,000,000,000 megatons of TNT
The current focus of study is flares coming off red dwarf stars — much smaller and less energetic than our Sun, but the most common stars in the galaxy, by a lot. Many are already known to have multiple rocky planets within a distance from the star termed the “habitable zone,” where in theory water could sometimes be liquid.
But red dwarf stars universally experience intense flaring in their early periods, and the planets orbiting in the those red dwarf habitable zones can be 20 times closer to their stars than we are to the Sun.
The crucial question is whether those flares forever sterilize the planets in their systems, which is certainly a possibility. But a related question is whether the flares might also deliver amounts of ultraviolet radiation that may be essential to the formation of the chemical building blocks of life.
Not surprisingly, this is a subject of not only intense study but of heated debate as well.
Violent stellar flares from young red dwarf stars, as illustrated here, could potentially evaporate the atmosphere of a planet. (NASA, ESA and D. Player (STScI)
The subject of stellar flares has just gotten some significant new data from a team at University of Colorado Boulder. The paper, to be published soon in The Astrophysical Journal, uses data from NASA’s Transiting Exoplanet Survey Satellite (TESS) space observatory to dig deeper into the behavior of flares.
What they found from TESS observations of 440 large flares from 226 stars is that their flares are at times significantly more energetic than earlier understood. The difference in detected radiation at their peak is 2 to 3 times greater than what had earlier been measured.
And since the amount of radiation received by planets orbiting red dwarfs is already very high compared to those coming from our Sun, doubling or tripling that amount of radiation results in quite a lighting up of any planet in the flare’s path.
Ward Howard is a a postdoctoral researcher at CU Boulder
“What we found is that many flares have brief and extremely energetic peaks that were getting lost in previous measurements,” said Ward Howard, lead author of the paper and a post-doctoral researcher at Boulder. “And these superflare events are repeated many times in the lives of these planets.”
“Can an atmosphere of a planet survive that? Can any microbes survive?”
He said that even the extremophile bacterium D. Radiodurans, found in hostile environments here on Earth such as nuclear reactors, would be strongly affected by these flares. “At maximum brightness,’ he said, “more than a third of the flares we observed would kill 90% of a culture of D. Radiodurans.”
While frequent and intense superflares can erode the ozone layer of an Earth-like atmosphere and allow lethal amounts of ultraviolet radiation to reach the surface, Howard said that ultraviolet radiation from flares that reach planets in the habitable zone is also known to be highly significant for cellular biology, as studied in labs.
Indeed, some researchers have theorized that ultraviolet radiation may play an essential role in the formation of compounds that over time can become a planet’s building blocks of life — complicating the superflare and habitability issue further.
These images from NASA’s Solar Dynamics Observatory show the first moments of an X-class flare in different wavelengths of light – seen as the bright spot that appears on the left limb of the sun. Hot solar material can be seen hovering above the active region in the sun’s atmosphere, the corona. (NASA/SDO)
Science works through a process of collecting ever more data that may or may produce results consistent results with earlier findings, and in that process a fuller understanding is gradually achieved.
And in the case of red dwarf superflares and habitability, another paper released last year based based on TESS data found reason to be optimistic about the effects of superflares on red dwarf exoplanets.
Ekaterina Ilin, a Ph.D. student at the Leibniz Institute for Astrophysics Potsdam and lead author of the study and her team studied four huge flares launched near the poles of red dwarf stars. On the Sun, flares generally launch from the equator and Ilin proposed that this difference has implications for the planets that orbit those red dwarf stars.
“Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own solar system, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system,” she said in a statement.
“This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the solar system,” she said.
Howard’s research into flares and habitability led to a somewhat different opinion.
“This is still early science and so we don’t have a clear answer,” he said. “But what we found does turn the knob a little towards those planets not being habitable.”
However, intense flaring doesn’t necessarily spell eternal doom for life on red dwarf exoplanets, Howard said.
“Previous laboratory work indicates at least one in ten thousand bacteria may survive these flares, raising the possibility that flare-resistant bacteria could evolve on planets orbiting flare stars rather than being entirely wiped out. Furthermore, planetary atmospheres, oceans, and other sources of UV protection may mitigate the effects of the flare radiation.”
“Over time, small planets are thought to re-emit atmospheres through volcanism. Even if the star strips the original atmosphere off the planet, a second atmosphere may form once the star is older and less active.”
And with red dwarf stars, there can be lots and lots of time.
Proxima Centauri is the nearest star to us, just over four light-years away. As imaged by the Hubble Space Teleacope, it is a red dwarf star and a planet has been detected orbiting it. The star, however, is known to experience frequent flaring. (NASA/ESA)
Red dwarf stars — also known as M dwarfs — comprise about 70 to 75 percent of all stars in the Milky Way. They’re much cooler, dimmer and smaller than Sun-like stars. But because they burn up their fuel so much more slowly than larger stars, scientists say they can survive up to 100 billion years, compared with 10 billion years for our Sun.
But since many lack the internal layers of Sun-like stars, red dwarf interior churning and fast rotation make them prone to extreme magnetic activity, resulting in flares. They shoot out from the star’s corona when the stellar magnetic fields get twisted up and then snap back into alignment, exerting high-energy radiation of all wavelengths in the process.
Not only are early-stage red dwarf flares extreme in their radiation, but the new CU Boulder study shows the flares to be “super complicated,” said Meredith MacGregor, assistant professor of astrophysical and planetary sciences and a co-author on the paper. “They have all sorts of weird structure in the light curves, which indicates that some of them are bursting multiple times.”
“We have historically had a very simple picture of stellar activity, where one loop breaks and we have one outburst of energy, and then it slowly dies away, and then we think about the frequency of that,” she continued. “That’s the model that’s been fed into everything we think about stars and their impact on planets, and it’s clearly just flat-out wrong.”
The TESS mission looks at the whole sky and was designed to search for nearby bright stars and their exoplanets. But as with all space observatories, scientists find they can be repurposed when in operation. And so TESS has become a great tool for superflare research.
Until recently, the TESS mission collected data on exoplanets in a two minutes cadence, which is enough to determine if a planet is transiting in front of the star. The CU Boulder team asked if data could be collected every 20 seconds instead in an effort to better understand the structure and behavior of the flares, and the TESS mission was able to accommodate the request.
That resulted in our first large-scale analysis of solar-flare data collected at 20-second intervals, and the scientific insights that followed as a result.
The findings are soon to be published in The Astrophysical Journal.
