The paradox of the red sky will make you question our very place in the universe


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On the grand cosmic scale, our little corner of the Universe is not that special – this idea is at the heart of the Copernican principle. However, there is one major aspect of our planet that is really special: our Sun is a yellow dwarf.

Because our original star is what we know most intimately, it would be tempting to assume that yellow and white dwarf stars (FGK dwarfs) are common elsewhere in the cosmos. However, they are far from being the most numerous stars in the galaxy; this particular feather belongs to the cap of another type of star – the red dwarf (M dwarfs).

Not only do red dwarfs make up up to 75% of all stars in the Milky Way, they are much cooler and outlast stars like the Sun. Much, much longer lived.

We expect our Sun to live about 10 billion years; the red dwarfs are expected to live on trillions. So long, in fact, that none have yet reached the end of their main sequence life in the 13.4 billion years since the Big Bang.

Since red dwarfs are so abundant and so stable, and since we shouldn’t automatically think of ourselves as cosmically special, then the fact that we aren’t revolving around a red dwarf should come as somewhat of a surprise. And yet, here we are orbiting a not so common yellow dwarf.

This, according to an article by Columbia University astronomer David Kipping, is the Red Sky Paradox – a corollary of the Fermi Paradox, which wonders why we haven’t yet found other intelligent life forms. , over there in the grand universe. .

“Solving this paradox,” he writes, “would reveal directions for targeting future remote sensing experiments of life and the limits of life in the cosmos.”

Artist’s impression of the planetary system orbiting the red dwarf TRAPPIST-1. (Mark Garlick / Science Photo Library / Getty Images)

Red dwarf stars are an attractive prospect for the search for alien life. They don’t burn as hot as the Sun-like stars, which means all orbiting exoplanets have to be closer to reach habitable temperatures. In turn, this could make these exoplanets easier to find and study, as they orbit their stars more frequently than the Earth does the Sun.

Indeed, astronomers have found quite a few rocky exoplanets – like Earth, Venus, and Mars – orbiting red dwarf stars in this habitable zone. And some of them are even relatively close. This is tantalizing stuff, and it certainly looks like red dwarf stars should harbor life at least somewhere, which is why astrobiologists are researching.

In his article, Kipping exposes four resolutions to the Red Sky Paradox.

Resolution I: an unusual result

The first is, well, we’re just a creepy eccentric. If the rates of emergence of life around the two types of stars are similar, then Earth is an outlier, and our emergence into orbit around the Sun was a fluke, a one in 100 chance.

This would create tension with the Copernican principle, which states that there are no privileged observers in the Universe, and that our place there is fairly normal. For us, being outliers would suggest that our place is not so normal.

This answer is not impossible, but neither is it particularly satisfactory. The other three resolutions provide answers that are not only more satisfying, they might in fact be testable.

Resolution II: Life inhibited under a red sky

As part of this resolution, Kipping argues that yellow dwarfs are more habitable than red dwarfs, and as a result, life appears much less often around red dwarfs – around 100 times less. There is a lot of theoretical evidence to support this idea. Red dwarfs, for example, tend to be rowdy, with a lot of flare activity, and don’t tend to have Jupiter-like planets.

“Much theoretical work has questioned the plausibility of complex life on M dwarfs, with concerns about tidal blocking and atmospheric collapse, increased exposure to the effects of stellar activity, prolonged phases of Jupiter’s pre-main sequence and the scarcity of potentially beneficial size. companions, ”Kipping wrote.

“Based on this, there is good theoretical reasoning to support Resolution II, although we stress that it remains observably unverified.”

rocket red dwarfArtist’s impression of a red dwarf triggering a megaflare. (NASA / S. Wiessinger Goddard Space Flight Center)

Resolution III: A Truncated Window for Complex Life

The argument here is that life simply hasn’t had time to emerge around the red dwarf stars.

It may sound counterintuitive, but it has to do with the main pre-sequence phase of the star’s life, before it begins to fuse with hydrogen. In this state, the star burns hotter and brighter; for red dwarfs, it lasts about a billion years. Meanwhile, a permanent and uncontrollable greenhouse effect could be unleashed on all potentially habitable worlds.

This could mean that the window of emergence of complex biology on rocky planets on white and yellow dwarfs is much longer than on red dwarfs.

Resolution IV: A Shortage of Pale Red Dots

Finally, while around 16% of red dwarfs with exoplanets are listed as harboring rock exoplanets in the habitable zone, these worlds may not be as common as we thought. Our surveys sample the most massive red dwarfs because they are the brightest and the easiest to study; but what if the titchy, about which relatively little is known, do not have rocky exoplanets in habitable areas?

Since low-mass red dwarfs are actually the most numerous, this could mean that rocky exoplanets in the habitable zone are 100 times less common around red dwarfs than they are around yellow dwarfs.

“In this case, intelligent life is rare in the cosmos and reproduces universally between M and FGK dwarves, but habitable worlds are at least two orders of magnitude less common around M dwarves than FGKs,” Kipping wrote.

“Two orders of magnitude is a huge difference, which makes this an especially interesting explanation. It would require that the vast majority of the many Earth-sized temperate planets known around the M dwarfs to be somehow inhospitable to life, or than the late type M (low mass late) dwarves rarely harbor habitable worlds. “

world of proximityArtist’s impression of a habitable world orbiting the red dwarf Proxima Centauri. (Mark Garlick / Science Photo Library / Getty Images)

It is even possible that the answer can be found in several of these resolutions, which would allow the effect in any area to be less pronounced. And maybe we can get confirmation soon. As our technology improves, for example, we will be able to see lower mass red dwarf stars better and search for planets orbiting them.

That done, if we find any rocky exoplanets, we can take a closer look at their potential habitability, determine if they are orbiting the habitable zone, and if life may have been hampered by stellar processes.

“Ultimately,” Kipping wrote, “resolving the red sky paradox is of central interest to astrobiology and SETI, with implications for which stars to devote our resources to, as well as asking a question. fundamental on the nature and limits of life in the cosmos. “

The research was published in PNAS.

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