Planets Orbiting Red Dwarfs May harbour Plenty of water to sustain Life

Small, cold stars well-known as red dwarfs are the most common kind of star in the Universe, and the absolute number of planets that may be present around them possibly make them valued places to search for signs of extraterrestrial life.

Though, previous study into planets around red dwarfs proposed that while they may be warm enough to sustain life, they might also totally dry out, with any water they have locked away forever as ice. New study published on the topic discovers that these planets may stay wet enough for life after all. The researchers detailed their discoveries online on November 12 in The Astrophysical Journal Letters.

Red dwarfs, also known as M stars, are approximately one-fifth as massive as the Sun and up to 50 times dimmer. These stars consist of up to 70 percent of the stars in the cosmos, and NASA's Kepler space observatory has found that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.

Red dwarf planets are possibly main places to hunt for life as we know it, not just because there are so many of them, but also because of their unbelievable longevity. Unlike our Sun, which will die in a few billion years, red dwarfs will take trillions of years to burn through their fuel, considerably longer than the age of the Cosmos, which is less than 14 billion years old. This longevity possibly gives red dwarfs a great deal more time for life to grow around them.

Study into whether a distant world might sustain life as we know it typically focuses on whether or not it has liquid water, since there is life almost everywhere there is liquid water on Earth, even miles underground. Researchers normally concentrate on habitable zones, the area around a star where it is neither too hot for all its surface water to boil away, nor too cold enough for all its surface water to freeze.

The habitable zones of red dwarfs are near to these stars since how dim they are, often nearer than the distance Mercury orbits the Sun. This nearness makes them attractive to astrobiologists, as planets near their stars cross in front of them more often, making them easier to notice than planets that orbit farther away.

Though, when a planet circles very near a star, the star's gravitational pull can compel the world to become "tidally locked" to it. When a planet is tidally locked to its star, it will continuously show the similar side to its star, just as the Moon always shows the same side to Earth. This origins the planet to have one perpetual day side and one everlasting night side.

The excesses of heat and cold that tidally locked planets experience could make them extremely different from Earth. For example, prior study speculated the dark sides of tidally locked planets would turn out to be so cold that any water there would freeze. Sunlight would make water on the sunlit side vaporize, and this water vapour could get carried by air currents to the night sides, ultimately leading to sheets of ice miles thick on the night sides and eliminating all water from the sunlit sides. Life as we know it perhaps could not develop on the day sides of such planets. Though they would have sunlight for photosynthesis, they would have no water to serve as the primeval soup for life to swim in.

To see how habitable tidally-locked planets actually are, researchers devised a 3D global climate model of planets that simulated interactions among the atmosphere, ocean, sea ice, and land, as well as a 3-D model of ice sheets big enough to cover entire continents. They also simulated a red dwarf with a temperature of about 5,660 degrees Fahrenheit (3,125 degrees Celsius), and examined whether all the water on these planets would certainly get stuck on their night sides.

Recent discoveries propose that planets in the habitable zones of red dwarf stars could harbour substantial amounts of water. In fact, each planet could own about 25 times more water than Earth.

"I've been interested in trying to make calculations related for M-star planet habitability since being persuaded by astrophysicists that these types of planets will likely be nearby (in proximity) to Earth," said study co-author Dorian Abbot, a geoscientist at the University of Chicago.

For instance, the closest known star to the Sun, Proxima Centauri, is a red dwarf, and it remains indeterminate whether or not it has a planet. The probability that red dwarf planets might be comparatively near to Earth "means that anything geoscientists can tell astrophysicists about habitability of these planets will be vital for planning future missions."
The scientists simulated planets of Earth's size and gravity that experienced between 63% and 77% as much sunlight as Earth. They also modelled a super-Earth planet 50% wider than Earth with 38 percent stronger gravity, because astrophysicists have found super-Earth worlds about red dwarfs. For instance, Gliese 667Cb, a super-Earth at least 4.5 times the mass of Earth, orbits Gliese 667C, a red dwarf about 22 light years from Earth. They set this super-Earth on a course where it get about two-thirds as much as sunlight as Earth.

"This is because surface winds transport sea ice to the day side where it is melted easily," said lead study author Jun Yang at the University of Chicago.
Furthermore, ocean currents transport heat from the day side to the night side on these planets.

 "Ocean heat transport powerfully affects the climate and sea ice thickness on our Earth," Yang said. "We found this may also work on exoplanets."

If a super-Earth has very big continents covering most of its night side, the researchers found ice sheets of at least 3,300 feet (1,000 meters) thick could cultivate on its night side. Though, the day sides of these super-Earths would dry out totally only if they got less geothermal heat from volcanic activity than Earth, and had 10% of the amount of water on Earth's surface or less. same outcomes were observed with Earth-sized planets.

"The significant implication is that it may be simpler than previously believed to keep liquid water on the dayside of a tidally locked planet, where photosynthesis is probable," Abbot said. "There are many concerns that will affect the habitability of M-star planets, but our outcomes propose at least that water-trapping on the night side will only be an issue for comparatively dry planets with big landmasses on their nightside and comparatively low geothermal heat flux."

Based on current and near-future technology, Yang said it would be very problematic for astrophysicists to gauge how thick the sea ice or the ice sheets are on the night sides of red dwarf planets and test if their models are precise. Still, using current and upcoming technology "it may be probable to know whether the day sides are dry or not," Yang said.

The scientists modelled three dissimilar arrangements of continents for all these planets. One was a water world with no landmasses and global oceans of varying depths. Another involved a supercontinent covering the night side and an ocean covering the day side. The last imitated Earth's configuration of continents. The planets also had atmospheres alike to Earth's, but the scientists also tried lower levels of the greenhouse gas, carbon dioxide, which traps heat and helps keep planets warm.

When it came to super-Earths covered completely in water, and super-Earths with continental configuration like Earth's, the scientists discovered it was unlikely that all their water would get stuck on their night sides.

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