Red dwarf exoplanets not as habitable as once believed
Freshresearch is pouring cold water on the hopes of discovering life ondistant exoplanets orbiting red dwarf stars. It had previously beenthought that Earth-sized planets orbiting in the habitable zones(HZs) of the small stars could provide a possible haven for life, butsophisticated computer models suggest that these planets are likelyrendered uninhabitable by their super-dense atmospheres.
Reddwarfs, otherwise referred to as M dwarfs, are relatively cool main sequence stellarbodies that are thought to make up around two thirds of the starscurrently populating the Milky Way. Smaller and less powerful thanour Sun, astronomers estimate that there are billions ofexoplanets orbiting red dwarf stars in our galaxy alone.
Studies carried out byobservatories such as Kepler and the TRAPPISTtelescope have revealed a number of Earth-sized planets orbiting inthe HZ of red dwarf stars. Astronomers naturally prize Earth-likeexoplanets as a potential breeding ground for life, due to the factthat they are a relatively close analog of our own planet.
Furthermore,owing to the low luminosity of red dwarf stars, their HZs arerelatively close to their cores compared to the HZ of our Sun, which makes it much easier to detect orbiting terrestrial-size planets viaeither the transit or Doppler methods. And because of the abundanceof targets, red dwarfs are becoming ever more attractive in the huntfor exoplanets capable of hosting life.
However,it is established that many of the exoplanets orbiting red dwarfs arecreated with dense atmospheres composed of hydrogen and helium thatcould make up as much as 1 percent of a planet's total mass. Whilstthis may sound relatively insignificant, it is worth noting thatEarth's atmosphere accounts for around a millionth of its mass.
Anatmosphere of 1 percent of a planet's mass would create a runawaygreenhouse effect,making the planet far too hot to sustain liquid water, which is one ofthe key building blocks of life. Planetary scientists had harboreda belief that over a prolonged period of time, these ultra-denseatmospheres would be thinned out by a bombardment of ultraviolet andX-ray radiation being thrown out by the nearby red dwarf, allowingthe planets to cool to a more hospitable temperature.
However, accordingto computer simulations carried out by researchers from ImperialCollege London and the Institute for Advanced Studies, Princeton,Earth-sized planets are massive enough to resist shedding theiratmosphere to space, and would remain uninhabitable.
Theteam used hydrodynamic models that took into account the stellarevolution of a red dwarf, as well as the thermal evolution of theplanet, alongside a myriad of other factors, coming to the conclusionthat previous studies had dramatically overestimated atmospheric massloss rates.
Theresearchers are keen to point out that just because Earth-likeplanets orbiting M dwarfs are incapable of supporting life, it doesnot mean that it is impossible for life to exist on lower massplanets existing in the HZ. It is possible that the gravity of aplanet with a mass approximating that of Venus or Mars would be tooweak to hold on to the majority of its atmosphere.
Intime, the intense greenhouse effect heating the low mass exoplanetsmay subside, resulting in surface conditions amenable to the presenceof liquid water. Ongoing and future studies making use of both ground-based andorbital observatories are expected to provide definitive evidenceregarding the habitability of exoplanets orbiting red dwarf stars.
The team has published their findings in the Monthly Notices of the Royal Astronomical Society.
Source:Royal Astronomical Society