Mars it geologically dead (a solid core, no volcanic activity, no plate tectonics) from more than a billion years ago. Including it, we have a negligible greenhouse effect in Mars, a significative greenhouse effect in Earth and a super-greenhouse effect in Venus. Without the greenhouse efect, all three would be frozen worlds. Given the range of intensity of the greenhouse effect, I guess Mars, Earth and Venus are all 3 in the habitable zone. The new findings, however, mean that more work will need to be done to determine which ones really are life-friendly and which ones are not, at least for “life-as-we-know-it” anyway. This includes smaller, dimmer stars as well as ones more like our own Sun. This makes these moons still potentially habitable even though they are far outside of the habitable zone around the Sun.īy design, the first exoplanets being found by Kepler are those that orbit closer in to their stars as they are easier to detect. The same may be true for Saturn’s moon Enceladus. The tidal forces exerted by Jupiter on its moon Europa, for example, are thought to create enough heat to allow a liquid water ocean to exist beneath its outer ice crust. In some cases, tidal heating can be a good thing though. Even Venus now is not substantially heated by tides, and neither is Mercury.” Planetary scientist Norman Sleep at Stanford University adds: “We’ll have to be careful when assessing objects that are very near dim stars, where the tides are much stronger than we feel on present-day Earth. While technically still within the habitable zone, they would have effectively been sterilized by the tidal heating process. They would then be more difficult to distinguish from other planets in those solar systems which may still be habitable. What’s problematic is that these planets could subsequently actually have their orbits altered by the tidal heating so that they are no longer affected by it. So even though they are within the habitable zone, they would lack oceans or lakes. The planets would then be subjected to greater tidal heating from the star, enough perhaps to cause them to lose all of their water, similar to what is thought to have happened with Venus in our own solar system (ie. This effect could cause planets to become “tidal Venuses.” In these cases, the planets orbit smaller, dimmer stars, where in order to be in that star’s habitable zone, they would need to orbit much closer in to the star than Earth does with the Sun. We figured out you can actually limit a planet’s habitability with an energy source other than starlight.” Simple enough, but tidal heating adds a new wrinkle to the equation. According to Rory Barnes, a planetary scientist and astrobiologist at the University of Washington, “This has fundamentally changed the concept of a habitable zone. The closer a planet is to its star, the hotter it will be, and the farther out it is, the cooler it will be. The habitability factor is determined primarily by the amount of heat coming from the planet’s star.
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