Red Dwarf Planets, No Place to Call Home?

In several blogs, as well as in an upcoming article in Astronomy Magazine, I have been bubblingly optimistic about the potential abundance of habitable planets around red stars that are smaller and cooler than our sun.

For starters they are the most numerous stars in our Milky Way galaxy. For every star like our sun there are as many as ten red dwarfs. Red dwarf stars burn for many billions of years longer than our sun, and so they provide all the time in the universe for life to evolve into complex organisms.

One qualifier is that for a planet to be in a dwarf’s habitable zone, where liquid water could remain stable on the surface, it would likely be so close to the star to be tide-locked. The gravitational pull of the parent star would keep one hemisphere of the planet permanently aimed at the star, just as the moon keeps one hemisphere facing Earth.

In a previous blog I dismissed this situation as being tolerable if a planet had a thick atmosphere capable of moderating temperatures between the permanently day and night sides. But now I’m a bit more chagrin after reading a recently published study by J.M. Griebmeier of the Observatory of Paris, and colleagues, who conclude that life is very tough on earthlike planets orbiting red dwarfs stars. Many may not be habitable at all.

A tide-locked planet would slowly rotate because its spin would be synchronized to its orbital period. The consequence is that it would have a very weak magnetic field because the metallic dynamo core would slowly spin too, at least according to some models. Reduce this magnetic shielding and the planet is irradiated with galactic cosmic rays and charged particles from the star. The right dose of cosmic rays can accelerate the rate of biological mutations and give evolution a kick in the pants. But too high a flux of cosmic rays squelches evolution with a hammer blow.

Whatever weak magnetic field exists on the planet would be pounded and squashed like a windsock in a hurricane by a dense stellar wind from the close by red dwarf. Add to that titanic coronal mass ejections where a star explosively belches out a dense cloud of subatomic particles. These would erode the planet’s atmosphere and perhaps even dry up nascent oceans. On an Earth-mass planet or larger, plate tectonics (considered a driving force for biological evolution) could grind to a halt without the cooling effects and lubrication of an ocean.

However, there may be hope for a massive super-Earth (five to ten times Earth’s mass) that manages to retain a deep global ocean. It would have a very thick and moist cloudy atmosphere that would absorb the shower of secondary particle from cosmic ray strikes in the upper atmosphere.

Several super-Earths have already been found around red dwarfs. Two were discovered only 20 light-years away around the star Gliese 581. They are only five and seven times Earth’s mass. The smaller planet lies in the red dwarf’s habitable zone.

Astronomers are looking forward to using advanced space telescopes, like the upcoming James Webb Space Telescope, to look for biomarkers – the chemical byproducts of life such as oxygen and methane -on a super-Earth near a red dwarf.

But the atmospheric chemistry could be confusing and complicated given the high radiation environment. Cosmic rays would break apart nitrogen molecules and form nitrous oxides (which are also biomarkers under the right circumstances). So life on the planet could die laughing. But this will be no laughing matter for astrobiologists who will try to deduce whether the exotic atmospheres on these tortured worlds are modified by life.