Are Mars and Venus considered to be in the habitable zone?
Venus: no. Mars: yes.This can be shown quite simply. Venus has a run away greenhouse effect and is similar in size and shape to our own beloved planet. If Venus was ok the habitable zone, this run away effect would have never happened.Mars on the other hand has lost all its atmosphere. Because Mars is a smaller planet, it has lost much if not all of the heat in its core, meaning no magnetic field and no protection from the solar wind. Over time Mars’ atmosphere was stripped away. However there is strong evidence that in early history Mars once had flowing, liquid water. It probably had a more pressurised atmosphere to to allow for this water to form.Also think of it this way. There are always science fiction stories of terraforming Mars into a second earth, yet few concern Venus, despite its physical similarities. Mars is theoretically habitable, Venus is less so.
If a planet is in the habitable zone, does it mean that humans could survive there?
This is Venus, closet planet to our own, named after the Roman goddess of love & beauty, and often called sister planet of planet Earth.Scientist have reasons to believe that Earth and Venus were very similar at one stage but took completely different evolutionary path.Recent data modelling by NASA’s Goddard Institute for Space Studies (GISS) in New York has suggested that Venus may have had a shallow liquid-water ocean and habitable surface temperatures for up to 2 billion years of its early history.Venus today is a hellish world with runaway green house effect and carbon dioxide atmosphere 90 times as thick as Earth’s. Temperatures reaches 864 degrees and it has very slow clockwise rotation.Depending on whom you ask Mars is considered just barely in the habitable zone while Venus is considered outside because Venus is classified as a hostile planet whereas Mars not that much. Earth is considered perfectly suitable to sustain life.However, Earth started as very different planet and for much of history was not suitable for terrestrial life.To conclude, Earth is in Habitable Zone while Venus is not and that is because we know for sure that Life exists on Earth whereas its highly unlikely it exist on Venus.
Why is earth the only planet with water on it?
You would be more correct in asking why is earth the only planet with a copious quantity of liquid water. Isaac Asimov describes this situation with Goldilocks. Venus is too hot; Mars is too cold; but earth is just right. He is explaining temperature and relative distance to the sun in comparison. Water is everywhere in the solar system and the universe as a whole. The trick for having it in big enough quantities and in liquid form is the planet must have enough gravity to hold on to it, and must be at the right distance range from its sun.
If kepler is an habitable planet for humans why there is no life there ?
I'm assuming you mean Kepler 438b 'the most Earth-like planet yet discovered'. It is 470 light years away, too far for us to be able to detect life for certain. By 'life' you seem to mean 'technologically advanced civilisation', judging by your question. There is a much higher chance of simple life-forms than very advanced complex life-forms. Although we know that Kepler 438b is in the 'goldilocks zone' (not too hot, not too cold) and the right size to be a rocky planet, crucially, we don't know if it has a liquid core and therefore a strong magnetic field to protect evolving life from radiation, and to prevent ablation of the atmosphere by the solar wind from its sun.
Atheists, how do you explain Earth's perfect positioning in our solar system? Not too close to the Sun, but not too far away to freeze.
Your answer assumes that the Earth is in the “perfect location” and that it was placed there because we are on it.For more than half the year, when near aphelion and perihelion, the Earth is most decidedly not at the”perfect” distance from the sun, as nasty hot summers and annoying frozen winters prove. Believe it or not, North/South Hemisphere axial tilt rotation differences aren’t actually the only thing to affect the seasons.And it was here long before us. The only reason it ever seems at all “perfect” for us is because we evolved to take advantage of the conditions of the Earth as we found it.Had the Earth had different average temperatures or biochemical makeup, but still managed to evolve life, we would be drastically different from what we are, and would consider those different conditions to be more or less “perfect” for life.In fact, the Earth has changed greatly over the eons.Several times.Free atmospheric and oceanic oxygen caused the first known mass extinction event… all the microorganisms adapted to the early O2-free environment, to which oxygen was deadly poison, were wiped out when cyanobacteria learned to make it as a part of photosynthesis to extract energy from sunlight.The reason we haven’t got gigantic land animals like the dinosaurs is because, today, there is much less free oxygen in the air than there was a hundred million years ago and more. There just isn’t enough to support that kind of body mass. The only reason whales are still big is they support a lot of themselves through buoyancy rather than muscle.Those are just the two major changes in environment that have led to major changes in what kind of life can survive on Earth that I can recall off the top of my presently insomniac head. I’m sure an expert could name many more.
If it is possible that life exists on earth might have came from space rocks, what are other possibilities?
Actually this is not a possibility, this is a speculation. The fact that our earth is positioned perfectly in the habitable zone is a major increment to the idea. And the possibility of a space rock bearing life as we know it today, hitting the right planet in the habitable zone at the right time at the right place is so infinitesimally small that it's almost zero.The habitable zone is responsible for the existence of liquid water and oxygen and the required temperature for existence of life on earth .So as it turns out some of the complex inorganic chemical compounds already existing in the hostile atmosphere of our prehistoric earth attracted organic compounds around them in order to gain stability, thus forming lipids. Some of these compounds were phobic or addictive towards light, gravity, heat, moisture etc. and thus branched into different forms of lives. That's the evolution by Darwin.As for extraterrestrial biological origin, intelligent civilisation may have started out life on earth, but that result can simply be rejected by our old Fermi paradox.
Why is Earth the only planet in the solar system with life in it?
Earth as a planet is situated in the habitable zone. This a region around a star which is at such a distance that the conditions are right enough for life to sustain. Earth’s orbit is perfectly alligned in the habitable zone. If earth would have been any closer, the sun's heat would incinerate every life form on earth, if it was any further, everything would freeze and life again wouldn't exist. Since earth is the only planet in the solar system which is in the habitable zone, it is the only planet with intelligent life on it.Other planets also have life forms on them, but that is yet to confirm completely. Scientists believe that microscopic organisms might exist on planets like mars and some planetary satellites like titan, europa and enceladus etc.But macroscopic organisms are thought to exist only on our planet.We're a lucky “miracle”. Although I doubt we're mere miracles.