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Some planets have a permanent day side and a permanent night side, akin to how the moon, from the vantage of earth, always shows the same side despite its rotations and revolutions. The ‘terminator’ is the dividing line between the ‘day’ and ‘night’ sides of the planet and terminator zones are the regions that could exist in that sweet spot between too hot and too cold.
Such planets are quite common because they exist around stars that make up about 70 percent of the stars seen in the night sky -- so-called M-dwarf stars, which are relatively dimmer than our sun.
The dark sides of terminator planets, would mean a perpetual night and freezing temperatures, whereas the side facing the star could be too hot for water to remain liquid. Only recently have astronomers been able to show that such planets can sustain habitable climates confined to this terminator region because researchers have mostly studied ocean-covered exoplanets in their search for candidates for habitability. How large and wide such zones are remains to be known.
Recognising terminator zones as potential harbours for life also means that astronomers will need to adjust the way they study exoplanet climates for signs of life, because the bio-signatures life creates may only be present in specific parts of the planet’s atmosphere. It could also mean more creative use of space telescopes such as the James Webb Space Telescope to look for terminator zones. It is believed that such an approach would improve the chances of discovering habitable planets.
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