The sole planet in our solar system's habitable zone is Earth. Mercury and Venus are not livable because they are too close to the sun to support liquid water. Evidence implies, however, that the Sun used to be much brighter. If it were, then these two planets would have been destroyed by heat long ago.
Earth is the only known planet in the universe that can support life as we know it. No other world has been found to contain oxygen, water, basic chemicals needed for life as we know it, although there may be others out there undiscovered. Scientists think it likely that if other worlds exist they are mostly made of hydrogen and helium with maybe a little bit of rock and ice. The only evidence of this comes from searching for chemical signatures on Jupiter's moon Europa and Saturn's moon Titan.
It's also possible that there are other planets in our galaxy that are like Earth. They might have environments similar to that of Mars or Venus today, except without the severe pressure changes caused by distance from the sun. Or they could have oceans of hot liquid rock, like Io. We just don't know about them because we can't see them from across the galaxy.
In addition to being the only known planet in its star's habitable zone, Earth also happens to be the only one with an atmosphere that allows life as we know it.
The habitable zone of a planet is the distance from a star at which liquid water may survive on its surface as long as the planet has a sufficient atmosphere. Earth is comfortably located within the sun's habitable zone in our solar system. Other planets including Mars, Venus and Ganymede find themselves outside the zone.
Based on these definitions, we can say that Mars is not in the habitable zone of our sun because the temperature on Mars is too high for life as we know it. Also, the only other planet in our solar system inside the habitable zone is Earth, so we can conclude that the habitable zone is where Earth lives.
The image above shows you how close Earth comes to being in the habitable zone of our sun. If Earth were any closer, the heat would be too much for liquid water, but if we were farther away, there wouldn't be enough sunlight for liquid water to exist on Earth now or anytime in the past.
This means that there must be something special about Earth that makes it suitable for life. Scientists think they might be able to figure out what this something is by looking at other planets in our solar system.
The habitable zone is the region of space around a star that is neither too hot nor too cold for liquid water to exist on the surfaces of nearby planets. Consider the possibility that Earth is located in the same location as Pluto. The Sun would be scarcely visible (about the size of a pea), and much of the Earth's seas and atmosphere would freeze. But if Earth were instead located where Mars is today, the Sun would be a huge bright ball burning with over 90% of its current energy output. Life as we know it would be impossible to survive such intense heat or darkness.
Based on this information, what can you say about the potential for life on other planets? Scientists think they know of at least one planet that fits our definition of being in the habitable zone: Our own Earth. There are also many candidates that fall just outside the zone but still provide a warm enough climate for water to exist in some form. These include Venus, Mars, and Saturn's moon Enceladus.
Of course, there are also many planets that are far from any source of light and heat that could possibly sustain life. We just don't know about them yet because they're so distant from us. In fact, scientists think there might be hundreds of billions of planets in the galaxy alone, so it's very likely that there are many more worlds like Earth waiting to be discovered.
Our knowledge about other planets is limited because observations have only been possible from within our solar system.
Habitability of planets
Mars is the most livable planet in our solar system after Earth for a variety of reasons.