But there may be no others. Earth has many unusual properties that are essential for life, and that may never be found elsewhere.
The NY Times reports:
As you mark the longest day of the year, consider the debate among astronomers over whether Earth’s tilt toward the sun helps make life on our world and others possible. ...That's right, the Earth has a tilt that is stabilized by the Moon, giving regular seasons over billions of years. Having a single large moon is probably very unusual.
The solstice occurs because Earth does not spin upright but leans 23.5 degrees on a tilted axis. Such a slouch, or obliquity, has long caused astronomers to wonder whether Earth’s tilt — which you could argue is in a sweet spot between more extreme obliquities — helped create the conditions necessary for life. ...
Is life only possible on an exoplanet with a tilt similar to ours? ...
Mars’s slouch, for example, is currently akin to Earth’s at 25.19 degrees, but it shifts back and forth between 10 degrees and 60 degrees over millions of years. That means that the seasons and climate of the red planet — which is currently experiencing an extreme dust storm — vary wildly. That could create conditions that make life impossible.
Take Earth as an example. Although our planet’s obliquity is relatively constant, it does change by a mere few degrees. Such slight variations have sent vast sheets of glaciers from the poles to the tropics and entombed Earth within a frozen skin of solid ice. Luckily, Earth has managed to escape these so-called snowball states. But scientists are not sure whether the same will be true for planets like Mars with larger variations in their tilts. ...
As such, a stable tilt just might be a necessary ingredient for life. It’s an interesting finding given that the Earth’s tilt never changes drastically thanks to the Moon. And yet astronomers don’t know how common such moons are within the galaxy, said John Armstrong, an astronomer at Weber State University in Utah. If they turn out to be uncommon across the galaxy, it could mean that such stability — and therefore life — is hard to come by.
Earth also has large amounts of surface water, geologic activity (volcanoes and plate tectonics), a large variety of minerals, etc. It also has a huge outer planet, Jupiter, to further stabilize the orbit. There are probably many other such factors that I don't even know about.
We don't have the ability to detect whether distant planets have these features. But common sense would indicate that they would be very unlikely. I think it is likely that Earth has the only intelligent life of this galaxy.