As researchers learn more and discover new environments in which life can sustain itself, the requirements for life on other planets may be redefined. Different types of planets may drive processes that help or hinder habitability in different ways. For example, planets orbiting low-mass stars in the habitable zone may be tidally locked, with only one hemisphere facing the star at all times. Some planets may be limited to only periodic or local habitable regions on the surface if, e.
The sizes of the 17 new planet candidates, seen here in orange, are compared to colorized representations of Mars, Earth and Neptune. Every planet is differnet.
But Earth is still special. Log in. In general, gas giants tend to clump up near their home stars. When astrobiologists evaluate a planet, they look not just at its current conditions, but for signs that it could have been habitable in the past. Liquid water leaves behind a kind of footprint. Mars, for example, has what looks like dry river and lake beds, making astrobiologists wonder if the cold, dry planet could have once been covered with life.
If it was, there may be enough remaining liquid water to sustain microbial life deep underground or under the frozen ice caps. Living things change their environment, leaving behind certain physical and chemical signatures. Even a planet that is lifeless today could have fossil evidence of ancient life forms.
Fossils and chemical remnants can tell us a lot about what extinct life might have looked like. Conditions That Support Life. Your browser doesn't support HTML5. Please upgrade. What makes a planet habitable? The Earth is 4. Dinosaurs are even more recent inhabitants, coming onto the stage between and 65 million years ago. The numbers involved are too vast for us to comprehend, but we can think of them as the time on a hour clock.
Life requires some specific conditions to survive, especially in terms of temperature. Therefore, a big question is why the Earth is unique in having maintained a temperature suitable for life for most of its existence. The existence and endurance of life on Earth becomes even more remarkable when we consider all the dramatic changes that the Earth has undergone over the millennia: the formation and breakup of huge supercontinents, massive volcanic eruptions, meteorite impacts, and the increase of atmospheric oxygen content from virtually nothing to an atmosphere that is breathable for us today.
So what makes the Earth so uniquely suitable for life? Unlike Venus or Mars, the Earth has an active plate tectonic system, which continuously produces and destroys the crust.
However, while tectonics give the Earth active water and gas cycles, the tectonic system itself moves too slowly to be the process that actively maintains the climate. Some theories, such as the Gaia Theory, suggest that life itself has been modifying the climate, so as to keep the Earth habitable.
While it is indisputable that life has shaped the planet — for example, by oxygen formation through photosynthesis — it is much less clear if life can actively moderate the climate to keep a continuously habitable planet.
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