The image above shows the relative differences in the diameters of habitable zones around stars of different masses. The solid green line within the broader green area around each star indicates where an Earth-like planet might be best placed for water to exist in a liquid state.
It is known that red dwarf stars account for about 75% of all the stars in the Milky Way, and astronomers generally agree that red dwarfs are likely to occur in similar numbers in most galaxies throughout the Universe. Therefore, it is likely that there are more red dwarf stars in the Universe that support planets than any other type of star, and in fact, many exoplanets have been discovered around red dwarfs, including the seven roughly Earth-sized planets of the TRAPPIST -1 planetary system.
At first glance then, it might seem that since red dwarfs live for several trillion years, as opposed to the several-billion-year lifetimes of more massive stars, the habitable zones around red dwarf stars might offer the perfect environment for life to develop. However, new research that was announced at the beginning of April 2018 at the European Week of Astronomy and Space Science in Liverpool seems to suggest otherwise.
The core of the problem involves the very nature of red dwarf stars. Most, if not all red dwarfs are flare stars, which means that although red dwarfs are relatively cool, these stars often erupt violently, emitting large quantities of charged particles along with large chunks of their coronas, in explosions known as Coronal Mass Ejections. Moreover, since red dwarfs mostly exist in a range of temperatures centered on about the 70,000 F, any planets on which life might develop have to orbit their stars very closely, and herein lays the problem.
To assess the likelihood that coronal mass ejections might be harmful to nascent life on planets around red dwarf stars, astronomer Eike Guenther and a team of associates from the Thüringen Observatory in Germany launched a research program to observe the effects that the violent eruptions which red dwarf stars are known for might have on the planets around them.
Thus, in February 2018, the researchers observed a flare on a red dwarf known as AD Leo, about 16 years away in the constellation Leo. This was particularly interesting, since AD Lee is known to host a large exoplanet at a distance of only 190,000 miles (300,000km) away, which is about 50 times closer than Earth is to the Sun. Moreover, AD Leo is also thought to host a few more Earth-sized planets a little further away, but still within the star’s habitable zone.
Since the observed flare was not accompanied by a coronal mass ejection event, and therefore left the large planet largely undamaged, the concomitant blast of X-ray radiation that followed the eruption was powerful enough to have reached down to the surface(s) of any less massive planets that were unfortunate enough to be in its path.
In practical terms, this means that the entire hemisphere of an Earth-sized planet facing an X-ray bombardment of this magnitude would be sterilized in matter of minutes, even in the absence of a coronal mass ejection event. While the research team is still fine-tuning its results to ensure that it is reliable, Guenther commented:
“With sporadic outbursts of hard X-rays, our work suggests planets around the commonest low-mass stars are not great places for life, at least on dry land”.
Representatives of the Royal Astronomical Society (RAS) reacted to the announcement of the initial results, saying that, “If they [Guenther et al] are right, then talk of ‘Earth 2.0’ [being located around a red dwarf] may be premature”.