In their search for extraterrestrial life, scientists are beginning to look for worlds that revolve around stars, like our Earth - around the Sun. After all, the constant warmth that the glowing yellow ball in the sky gives us, and made life on Earth possible.
But as astronomers continue to discover thousands of planets, they realize that if (or when) we find signs of extraterrestrial life, it is highly likely that these aliens will orbit a star vastly different from our Sun. It will be redder, colder, smaller and lighter than our star. Thus, in the search for extraterrestrial life, many astronomers turn their gaze to such small stars known as red dwarfs or M-dwarfs.
For starters, it's worth noting that astronomers never cared much about M-dwarfs. After the discovery of the first planet outside the solar system in 1995, scientists began the hunt for Earth's true counterparts: rocky planets similar to ours that revolve around stars similar to our Sun. The search for just such systems has guided astronomers for much of the 2000s, says astronomer Phil Muarhead of Boston University.
Then astronomers realized that it might be technically easier to search for planets around M-dwarfs. Finding another planet is difficult, and astronomers rely on two main methods. First, they look for the reduction in star brightness that occurs when a planet passes in front of it. Second, astronomers measure a small wobble in a star, caused by the slight gravitational pull of another planet. Both of these methods work great with a planet orbiting an M-dwarf. In addition, it rotates more often, which increases the chances of its detection.
M-dwarfs have received a major boost thanks to the Kepler space telescope, launched in 2008. Peering into a small patch of sky, the telescope looks for sudden blackouts in stars that occur when planets pass in front of them. Thus, the telescope discovered more than a thousand planets. “Kepler changed everything,” says Muarhed.
Because M-dwarf systems are easier to detect, many planets in their orbits have been discovered through selection effects. But, as Muarhed rightly points out, Kepler is also engineered to search for Earth-sized planets orbiting sun-like stars. Only now the numbers still hint that we need to look for life on planets near M-dwarfs.
"You'd rather find a potentially habitable planet near an M-dwarf than near a sun-like star," says astronomer Courtney Dressing of Harvard.
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She analyzed how many Earth-sized planets - that is, with a radius of one to half Earth's - revolve around M-dwarfs in a potentially habitable zone (the Goldilocks zone, the region around a star in which liquid water can exist on the planet's surface). According to her latest calculations, one in four M stars has such a planet.
This is higher than the estimated estimate for earth-like planets orbiting analogs of the sun, the scientist says. An analysis by astronomer Eric Pettiguere of the University of California at Berkeley showed that less than 10% of sun-like stars have planets with a radius of one to two Earths.
M-dwarfs have another important property. They are the most abundant stars in the galaxy, accounting for about 75% of the hundreds of billions of stars in the Milky Way. If Dressing's estimate is correct, our galaxy could have about 100 billion Earth-like planets orbiting in the potentially habitable zone of M-type stars.
Please note that these estimates have many limitations. They depend on what is meant by a potentially habitable zone, and this is not yet very well defined. As a rule, the habitable zone is where it is not too hot, not too cold and there may be liquid water. But there are also many reservations about how well the planet's atmosphere can hold water (Venus, if anything, is also in a potentially habitable zone).
With more general estimates extending the potentially habitable zone, the Pettiguere numbers for Earth-like planets in sun-like stars could be as high as 22% or more. But Dressing's numbers could go up too.
Initially, astronomers were skeptical about M-dwarfs, because they did not think that such a star could have any habitable planet. On the one hand, M-dwarfs are more active, especially during the first billion years of their life. They can bombard the planet with deadly ultraviolet radiation. They can emit powerful stellar flares that rob the planet of its atmosphere.
And since the planet's orbit will lie close to the M-dwarf, the star's gravity can change the planet's rotation around its axis. If such a planet were tidally blocked, there would be an eternal day on one side of the planet and an eternal night on the other. The light part of the planet will be roasted, while the dark part will freeze completely - not the most hospitable environment for life.
Extraterrestrial life
None of these questions are fully resolved, and some astronomers do not consider them serious problems at all. For example, Eomawa Shields from the University of California, Los Angeles. For example, habitability may depend on specific types and frequency of outbreaks, which are still not well understood. Computer models have also shown that the atmosphere can redistribute heat, preventing the dark side of the planet from freezing.
In some respects, the planet around the M-dwarf may be even more hospitable than it seemed. An inhabited planet must contain a lot of water and ice, and Shields analyzed how the starlight of an M-dwarf interacts with the atmosphere and icy surface of such a planet. The M dwarf produces more infrared radiation than a sun-like star, and since the atmosphere and ice of a planet in orbit absorb infrared light well, the planet will not freeze as quickly as a sun-like star. And even if it freezes, it will quickly thaw.
This kind of stable climate can give a flourishing life more time to develop without disturbances like rapid cooling or heating. Nonetheless, Shields adds that the frozen planet doesn't have to be inconvenient for life. The Earth, after all, could have gone through the Snowball Earth period over 600 million years ago.
While some astronomers continue to observe M dwarfs, others still want to study sun-like stars. Currently, researchers are increasingly turning to the study of M-type systems. Kepler's mission is drawing to a close, but astronomers await the Transitin Exoplanet Survey Satellite, which begins in 2017. TESS will search for planets around bright stars, including many M dwarfs.
The James Webb Space Telescope, the successor to the Hubble cause, to be launched in 2018, could even analyze the atmospheres of such planets. However, according to Muarhed, this telescope will only be able to target M-dwarfs; new missions will be needed to target sun-like stars.
Ultimately, as resources become increasingly scarce, astronomers will have to choose between focusing on M-dwarfs and sun-like stars. The solution will depend on what they find in the next few years. Astronomers are confident they will find a potentially habitable planet no matter what.
As for life, this question is more complicated. “I don’t know when this will happen, but I wish it would happen sooner rather than later - and I’m sure it will,” Shields says. "The only question is whether and when there will be funding."