Are Red dwarfs habitable?
Red dwarfs are the most common type of star in the universe. |
| Red Dwarfs Source: ESA/Hubble / CC |
With their ubiquity, low temperatures, lifetimes spanning trillions of years, and a bounty of rocky planets in their habitable zones, red dwarfs may seem like the most likely place in the universe to find life. But red dwarfs are known for unleashing ultrapowerful flares that could potentially destroy a close planet's atmosphere. However, the two closest red dwarfs to our solar system - Proxima Centauri and Barnard's star - each host potentially habitable planets.
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| Proxima Centauri Source: ESA/Hubble / CC BY |
As our technology improves, these systems will become the first natural laboratories to see if living with a red dwarf star is possible.
Welcome back to my blog "Mystery of Galaxy". In a previous article, we talked about the lifespans of red dwarf stars. The link is given at the end of this article.
They're certainly the most common type of star in the universe, and they are home to planets - many of which exist in their so-called habitable zones. As red dwarfs age, they become more luminous and warmer, reaching sun-like luminosities for up to several billion years.
But that's trillions of years down the road, what about today? Can red dwarfs support life now?
If they can that would make life a relatively common occurrence in the universe. It's estimated that anywhere from 10 to 50percent of all red dwarf stars have at least one planet in their habitable zone. This is the range of distances where liquid water could be stable on the planet's surface. In fact the closest star to our Sun, Proxima Centauri, is only 4.2 light-years away and is host to a planet in its habitable zone. As the planet revolves around the star, its spectrum shifts from red to blue wavelengths and then back again. This repeated oscillation revealed the presence of Proxima's planet, and also showed that the planet has an overall period of about 11.2 days. From Kepler's third law, we know that the planet must be at an orbital distance of about 0.05 astronomical units. In other words, about 5% the distance between the Earth and the Sun. However, Proxima b's distance and orbital period are really the only two things that we know for sure about this planet. Analysis of its radial velocity curve suggests that the planet is at least 1.3 times the mass of the Earth, but it could be larger. And that's because we don't know the system's inclination relative to our line of sight. If we're viewing the system directly edge-on, then the planet is exactly 1.3 times the mass of Earth. But if we're viewing the system at some intermediate angle, then the planet would have to be much more massive to produce the radial velocity measurements that we see. If its mass is closer to the minimum, then it is likely a rocky super Earth. If it's much greater then it could be a mini Neptune.
But a day on Proxima b would be unlike anything we experience on Earth.
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| Proxima b Source: ESO/M. Kornmesser / CC BY |
Tidal forces from the star on the planet would be extremely strong and would slow the planet's rotation down. If Proxima b's is perfectly circular, chances are good that it would be tidally locked to the star, with one side of the planet always facing the star and the other side in perpetual night. Halfway between the day and night sides of the planet, Proxima Centauri would loom large and low on the horizon in a perpetual Twilight Zone. Now keep in mind we don't know anything about Proxima b's composition. We don't know if it has an atmosphere or oceans or anything, but different climate models have been run involving different scenarios to assess Proxima b's potential habitability. In one very simple model, Proxima b is covered in a global ocean with temperatures at the sub stellar point reaching 19 degrees centigrade or 66 degrees Fahrenheit. However the rest of the planet is pretty much a frozen ice ball. In other words the planet would look kind of like a frozen eyeball with a liquid ocean at the substellar point and ice everywhere else. In other scenarios, ocean currents would carry the substellar heat around the planet, but that would leave the sub stellar region that much colder, in most cases just a few degrees Celsius above freezing. However Proxima b's orbit may have a moderate eccentricity. In that case, the planet wouldn't be tidally locked but instead completes three rotations on its axis for every two revolutions around its star. This is called a 3:2 spin resonance and it's exactly the kind of orbit that Mercury has.
In this scenario, the planet rotates on its axis once every seven and a half days. That would spread the star's heat more evenly around the planet, leaving the planet with a cold tropical ocean with ice caps reaching all the way down to mid latitudes. Adding land to these models interrupts the flow of heat around the planet. If there's land on the daytime side of Proxima b, that may increase the planet's overall habitability. But the models also predict that the planet would be cloudy most of the time. That's not necessarily bad for life in general, but it would have an effect on the efficiency of photosynthesis. In a recent paper, researchers demonstrated that red dwarfs with masses less than 20% the Sun just don't produce enough of the kinds of light that are necessary for photosynthesis here on Earth. Perhaps a kind of plant life could evolve along the shores of Proxima b's oceans, with large black leaves to absorb as many of Proxima's low-energy photons as possible. However, it's unlikely that such plants could ever produce enough oxygen to sustain complex life. And then there's that little matter of the frequent, powerful, and ultra-deadly flares from red dwarfs. Their convective interiors produce strong magnetic fields and their short rotation periods twist those magnetic fields up until they snap, unleashing massive flares of plasma, X-ray, and UV radiation. Proxima Centauri flares about once every two to three days; if Proxima B is going to maintain its atmosphere, it's going to need a really strong magnetic field to shield itself. But like all red dwarfs, Proxima occasionally unleashes super flares that are up to 10,000 times more powerful than anything the Sun produces. Such an event was witnessed in 2016 when Proxima suddenly brightened by 68 times. It's estimated the Proxima unleashes about five to six of these super flares every year. If Proxima be ever had an ozone layer, it would have been pretty much destroyed after just a few hundred thousand years of these flares. Any life on the surface is going to have to adapt to that as well. Assuming of course its atmosphere wasn't completely destroyed by Proxima Centauri when it was just a few million years old. AU Microscopii and AD Leonis are examples of red dwarfs that are about a hundred million years old and they flare three to four times a day. Not only does a habitable zone planet require an ultra powerful magnetic field, it also requires huge water inventories to resist the relentless onslaught from their host stars. But Proxima b may no longer have any water. Red dwarf stars take very long to form with gestation periods lasting up to a hundred million years. During this time they are actually more luminous than they are when they have reached the main sequence. That means that during this time the planet could be subjected to so much energy, that its atmosphere would have undergone a runaway greenhouse effect. The result may be a planet that isn't cold at all, but rather an ultra hot desert wasteland like Venus. However, Proxima b may have avoided this early fate if it had formed farther away from the star and then migrated it in after the star had settled down a little bit. Then the plant would only have to contend with the ultra-deadly flares from Proxima Centauri. Now none of this is to say that life is impossible on such a planet, but any life that could evolve on Proxima b would have to be adapted to resist extreme cold and extreme radiation. But perhaps there's another scenario that's more favorable to life around a red dwarf.
Barnard red dwarfs:-
Barnard's star is the second closest star system to our Sun. At 8.6 billion years it is nearly twice as old as Proxima Centauri and therefore a lot less active. In 2018, astronomers announced the discovery of a planet orbiting at 0.4 astronomical units from Barnard's star. That's the same distance from Barnard as Mercury is from the Sun. However Barnard's star is only 0.33% the luminosity of the Sun. That means this planet is only getting about2% the amount of radiation Earth gets from the Sun. That puts Barnard b at a frigid -168 °C. However Barnard b is at least 3.3 times more massive than Earth. That would put it firmly in the super Earth category. And it may also mean that it has a very large liquid iron core. If so, that might provide enough geothermal energy to sustain life in subsurface oceans. It's tempting to think that maybe some of that heat could burst through cracks in the surface and dot the surface of the planet with warm springs where more complex life could survive. But we don't really know enough about super-earths to begin with; they may just not have any plate tectonics allowing such cracks and fissures to form. Still, a large liquid convicting core would arm Barnard b with a very strong magnetic field. Combine that with it's already larger distance from the star and a thick enough ice sheet, subsurface life could be very well protected from Barnard's flares. However, after 8.6 billion years, Barnard b may no longer have enough radiogenic heat to prevent the subsurface ocean from freezing. It might seem that the chances for life around a red dwarf star are really not that good, but it may be possible that life still finds a way to eke out an existence in the same way it does in the most extreme environments here on Earth. But such environments don't bode well for complex life; the best we may be talking about are just small microorganisms. However, Barnard and Proxima b both share a very compelling property: they are both the closest known exoplanets to Earth. That makes them ripe for future study with our upcoming telescopes. Barnard b is far enough away from its host star that the James Webb Space Telescope might be able to directly image it in the near infrared. |
| James Webb Space Telescope Source: NASA / Public domain |
And the upcoming Giant Magellan, Thirty Meter and Extremely Large telescopes may also have a shot at imaging it using adaptive optics. Now mind you, these telescopes won't be able to show us pictures of Barnard b's oceans and coastlines. But they should be able to at least get the color information of the planet and possibly obtain the planet spectra. If they can do that then we have a shot at characterizing Barnard b's atmosphere. Assuming of course it has an atmosphere. Unfortunately, Proxima B is too close to its host star to be directly imaged by any of these telescopes. But maybe over time the spectra of the plane twill be able to be teased out from the spectra of the star. Fingers crossed. It seems that the likelihood for life around these red dwarf stars is...pretty unlikely. Then again, life would have plenty of time to form, evolve, get destroyed by a flare, and repeat over and over again for trillions of years. With each iteration, the star becomes less active - literally making life easier on those planets. And then, in the far future, the red dwarf evolves and in many cases becomes very hospitable for life. To see how a red dwarf evolves, make sure you check out my article on red dwarf stars.
here the link Red Dwarfs star
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