In our solar system there aren’t any other planets like earth, at least with regard to water. Is it our particular orbit/distance from the sun? The other planets, as far as we know, are devoid of water. How/why is it just earth with water?
Liquid water is a function of our distance from the sun. Much closer, and water boils away. Much farther, and it freezes forever.
Mars has frozen water in relative abundance, as do many other objects in the Solar System, most notably Jupiter’s moon Europa. Europa could possibly even have a liquid ocean beneath its surface, kept from freezing due to tidal forces.
I question the premise. Each planet is pretty different from the others once you get past the basics.
Distance from the Sun *must *be pretty different for each planet, lest they have collided long ago. That alone drives a lot of the individuality.
Pluto has ice mountains 11,000 ft high. Heard that on the radio just today!
Our solar system is a pretty small sample size. Earth is in what’s known as the Goldilocks zone: not too hot, not too cold, just right.
The universe is a pretty big place.
The bolded part is just plain wrong. The solar system is awash in water. Many of the other bodies have more water than Earth does, it is just that most of it is ice.
Click on image to enlarge.
We have the liquid water, but we also have ocean currents and a thick atmosphere that help distribute heat. We have plate tectonics, not sure there is evidence of that anywhere else. We have a moon that interacts with the oceans in the form of tides. We have some dry land and oceans in decent proportions. We have seasons, so most of the surface is not too hot or cold all the time.
Most of all, we have life. Indeed, this is a pretty unique place.
I used to wonder where the oceans came from. When I posed the question to my ten-year old son a few years ago, he said the water came from space. He was right.
There are competing hypotheses, but Mars and Venus may have had oceans in the distant past and may have looked quite similar to early Earth. Venus suffered from a runaway greenhouse effect. Mars lost its magnetosphere and its atmosphere was stripped away by the solar wind.
One of the unfortunate truths in astronomy is that no matter how much can be inferred from what we can see with our current telescopes, we’ve never seen what a single extrasolar planet looks like. We couldn’t even see what pluto actually looked like until we sent a probe close to it.
Stating that planets like earth can only occur within a goldilocks zone is one of those things for which we have exactly one concrete example and a lot of extropolation. Heck, earth is the largest rocky planet in our solar system, we know that there are larger rocky planets elsewhere but we don’t have a single example close enough to probe from which to draw conclusions. We can’t say with any real certainty how their atmospheres work, or whether they’re likely to have the rotating iron core which is so important for earth life.
Always be aware of the limitations to your knowledge. Many of our theories and predictions could easily be right- but we won’t know for sure until our evidence gets much, much better.
Nitpick: compared with most worlds that have atmospheres, Earth’s is rather thin. Only Mars and Pluto have thinner ones. Otherwise, Venus, Jupiter, Saturn, Titan, Uranus, and Neptune all have thicker atmospheres.
Earth could retain a much thicker atmosphere. After all, compare Venus, which is somewhat smaller, yet has 90 times as much atmosphere. One of the main reasons why Earth’s is so thin is the liquid water. Carbon dioxide combines with calcium oxide in the oceans to make calcium carbonate (CaCO3), which is the main component of limestone. It’s estimated that there’s about 60 atmospheres of CO2 bound up in rocks on Earth.
Whoa, whoa. This thread is about other planets in the solar system. 
To the OP, I think the anthropic principle is relevant to your question. Why is our planet seemingly uniquely able to support life like ours? Because if it weren’t, we would not be here to ponder such questions.
The other planets have their own unique things, too, but most of them are still unknown to us because, well, they’re way out there and inhospitable to us.
So is the moon. As well as Venus. Mars eliptical orbit causes it to leave the zone periodically
The more proper answer is that we are designed for Earth, or more accurately certain environments on Earth.
Their are places on Earth where life exists which is very different from the type of life we had come to assume, bottom of the Ocean near vents for instance, they have evolbed to thrive on SO2. Its possible that the other Planets are teeming with life, but we don’t recognise them as life.
It seems that Earth is very fortunate in that another planet collided with it, which gave us a relatively large moon that performs useful work on the planet.
Pretty much.
All the shows I’ve watched on the Science channel go on and on about how the Earth is in the “goldilocks zone,” the region around a star within which planetary-mass objects with sufficient atmospheric pressure can support liquid water at their surfaces.
And we have to have the atmosphere to protect the water from being destroyed by radiation from the sun. Or something like that. Mars, for example, couldn’t hold onto its atmosphere and without that, the water left too.
Our magnetic field shields us from the solar wind, which keeps our atmosphere from blowing away as it did on Mars.
Which makes me wonder: Since Venus lacks an internal magnetic field, how long will it take for its atmosphere to be blown away to the point where the atmospheric pressure is similar to Earth’s? What would the temperatures be there?
[nitpick from That Guy Who Hates to Be Him]
“unique” in OP hed takes no modifier by definition
Luck, really. The Earth is certainly in the Goldilocks butter zone…not too hot, not too cold. Also, early on the Earth was hit by a wandering planetesimal a bit less than the size of Mars, which gave us a larger core than is thought to be the norm (thus more dynamic tectonics and magnetic field to prevent the atmosphere and water being stripped off, and without having to have a huge radiating gas giant to do it for us) and the Moon, which has stabilized our spin somewhat and made the weather less extreme. Additionally, though the early Earth lacked water on the surface due to how it formed, we lucked out with a bunch of water ice bearing asteroids and such hitting the Earth late enough in the game that we could retain the water and building up an atmosphere to keep it.
We basically won the cosmic lottery…like a bunch of times. The reason the Earth is so unique is that all those things (plus a bunch more like our Jupiter not going all the way into a close orbit around the sun and taking us out in the process…which would have really sucked, if anyone was here to notice) had to come together in just the right way at just the right time to make it possible. Plus, there is the fact that we are here to marvel at how unique it is and actually know that it is so. 
ETA: Wouldn’t rule out liquid water on other bodies in our solar system btw…just not on the surface.
How so? The Earth is certainly a lot more unique than any other planet we know of, so it’s perfectly reasonable to ask why it’s so unique.
I think they mean it in a grammatical way, i.e. you don’t need the word ‘so’, it should read ‘Why is Earth unique?’