A detailed look at the possibilities of Life forming around the most common stars in the Universe.
With planets around other stars discovered in numbers of thousands, our search for Exoplanets has exploded within the past decade. Unlike past discoveries of mostly Jupiter-like large planets, there has recently been a foray of newly discovered Earth-like planets.
Earth-like only means that the planet is terrestrial in nature and has enough mass to hold on to its atmosphere. By that definition, Venus is also Earth-like. What we are really more interested in though are the habitable planets, where Life can possibly exist. Much of the limelight in that regard has now been taken by new discoveries around Red Dwarf stars:
- Proxima b: A potentially habitable Earth-like planet found around the nearest star to our Sun, a mere 4.3 light years away.
- TRAPPIST-1: Seven Earth-like planets found around the star TRAPPIST-1, 3 of which are in the Habitable Zone!
Finding habitable planets around Red Dwarf stars is interesting because:
- Red Dwarf stars live for more than a trillion years! (Source)
- Red Dwarf stars are also the most common type stars, accounting for ~75% of in the Galaxy.
- Multiple investigations suggest strongly that there is at least 1 planet for every Red Dwarf star. (Source)
So the question that matters is whether the habitable zone planets around Red Dwarf stars are truly habitable or not? Here is everything we know about the habitability of such planets.
#1: Habitable Zones of planets around Red Dwarf stars are very close
Red Dwarf stars are smaller, cooler and emit much less energy than Sun-like stars. For a planet to be in its Habitable Zone thus requires a much closer distance than our Earth is to the Sun.
While Proxima b and many such planets do fall in the Habitable Zones of their stars, being so close has its implications.
#2: Habitable Zone planets around Red Dwarf stars are tidally locked
Being very close to their parent stars means that such planets are likely tidally locked i.e. one side of the planet always faces the Sun and other side is in constant darkness. The sun-facing side is hot enough for water to boil off, while the far side is cold enough for water to form ice.
If the planet has an atmosphere though, the heat can be distributed more evenly among both the halves. Despite that, relatively large differences in temperature will give rise to strong winds & potentially massive storms blowing from the warmer side of the planet to the colder one (Source). The only hope for Life to live on such planets seems to be the Twilight Zone (the line dividing day and night), where the temperatures would be moderate. Tackling planet-wide strong winds is another story though.
#3: Red Dwarf stars emit energetic flares
NASA’s GALEX mission has shown us that Red Dwarfs constantly produce stellar flares like our Sun, some even stronger ones. These stellar flares emit a lot of high-energy Ultra-Violet (UV) and X-ray radiation. Constant stellar flares can ultimately strip away a planet’s atmosphere, more so easily when a planet is so close as in the case of Red Dwarf stars. UV light penetrating such a planet’s surface can also damage any existing Life forms.
Red Dwarf stars are also known to produce Mega flares, which are thousands of times brighter than what our Sun produces! These mega flares are more common when a Red Dwarf star is quite young. The constant bursts of strong UV radiation from such strong flares can prevent Life from developing on a planet in the first place.
#4: Planets of Red Dwarf stars face oxygen loss in habitable zones
Scientists developed models to understand the rate at which stellar flares from Red Dwarf stars can eliminate Hydrogen and Oxygen from a habitable planet’s atmosphere. And the results are bad. The strong radiation from flares first break molecules into atoms and then eventually ionize the atoms. The separated electrons and ions become light enough to escape the planet’s atmosphere (Source).
Due to frequent and strong flaring (especially in the early days of a Red Dwarf star), the model suggests that a planet in the Habitable Zone would lose its Hydrogen & Oxygen in a matter of ~10–100 million years. This would make the planet devoid of water, making is difficult for Life to arise or sustain.
#5: Earth-like Planets are less likely to be around Red Dwarf stars
Planets with too much water (Ocean planets) lead to an unstable climate, whereas planets with too little water become Venus-like i.e. too arid for Life. Therefore, an Earth-like water content is more suitable to sustain Life as we know it.
Planetary formation simulations for planets around both Red Dwarf and Sun-like stars suggest that Earth-like planets with Earth-like water contentare more likely to be found around Sun-like stars than Red Dwarf stars (Source). This is potentially a very pessimistic news considering that Red Dwarfs make up ~75% of the stars in the Galaxy.
#6: Cloud cover can make a planet more habitable
Not all is bad news though. Tidally locked planets around Red Dwarf stars can have substantial cloud cover on their sun-side. Clouds can reflect sunlight keeping the planet cooler and at the same time absorb the planet’s infrared radiation to keep things warm. Cloud simulations on such plants suggest that the heating and cooling effects due to this can keep the planet warm enough to sustain Life (Source).
The upcoming James Webb Space Telescope (JWST) will be easily able to verify this after it launches in 2018. If cloud cover is present, the temperature measured on the sun-side would be lower instead of the expected higher and vice versa. If true, this has positive implications on possibilities of Life around Red Dwarf stars.
#7: The Search for Intelligent Life on Planets around Red Dwarf stars is live right now
The organization dedicated to searching for Intelligent Life elsewhere in the Universe — Search for Extraterrestrial Intelligence (SETI) — has started one such survey already in 2016. The 2-year search uses an array of 42 Radio Telescopes (Allen Telescope Array) to detect Radio emissions from Intelligent civilizations around the nearby 70,000 Red Dwarf stars.
The specific radio frequencies being looked for are related to mathematical constants that an Intelligent civilization is expected to beam if it wants to grab attention (Source).