These large atmospheres increase both the absorption and radiation of heat, allowing the planet to better maintain a stable temperature. However, something important happens when planets decrease in size: as they warm, their atmospheres expand outward, becoming larger and larger relative to the size of the planet. The runaway greenhouse effect occurs when the atmosphere absorbs more heat that it can radiate back out into space, preventing the planet from cooling and eventually leading to unstoppable warming until its oceans turn to steam in the atmosphere. But, as Arnscheidt and his colleagues demonstrated, this definition doesn't hold for small, low gravity planets. The inner-edge of the habitable zone is defined by how close a planet can be to a star before a runaway greenhouse effect leads to the evaporation of all the surface water. Astronomers hunt for these habitable planets within specific distances of certain types of stars - stars that are smaller, cooler and lower mass than our Sun have a habitable zone much closer than larger, hotter stars. Generally, planets are considered habitable if they can maintain surface liquid water long enough to allow for the evolution of life, conservatively about one billion years. Setting a lower bound for habitability in terms of planet size gives us an important constraint in our ongoing hunt for habitable exoplanets and exomoons." "But actually, there are many other variables to habitability, including mass. "When people think about the inner and outer edges of the habitable zone, they tend to only think about it spatially, meaning how close the planet is to the star," said Constantin Arnscheidt, A.B. The research was published in the Astrophysical Journal. This research expands the search area for life in the universe and sheds light on the important process of atmospheric evolution on small planets. In a recent paper, Harvard University researchers described a new, lower size limit for planets to maintain surface liquid water for long periods of time, extending the so-called Habitable or "Goldilocks'' Zone for small, low-gravity planets. But how small is too small for habitability? Of course, in real life, celestial objects that small can't support life because they don't have enough gravity to maintain an atmosphere.
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