For the first time, NASA has found a planet most like Earth

There's no replacing Earth, but scientists have found a planet that may come close.

Using the Kepler space telescope, an international team of astronomers has discovered a rocky planet about the size of Earth in what's called "the habitable zone" of a distant star in the Milky Way Galaxy.

Related: Scientists look at icy moons in search of alien life

That means the planet is just the right distance from its star to keep water liquid. Too close to a star and water evaporates, too far and it freezes.

Liquid water is a fundamental building block of life as we know it.

It's unclear if this new planet, dubbed Kepler-186f, actually has water, but scientists think there is a good chance it does since H2O is so common in our own solar system.

Related: Just how many Earth-like planets are out there?

The size of the newly discovered planet is also a crucial factor in its potential for life, says Stephen Kane, a researcher on the project and an assistant professor of physics and astronomy at San Francisco Sate University.

"This is the smallest planet that we have found in the habitable zone," he said.

He says if a planet is too big, as most "exo-planets" discovered so far have been, gravitational forces will be so strong that they will pull in large amounts of hydrogen and helium as the planet forms. This creates an atmosphere that isn't conducive to life as we know it.

However, if a planet is too small, it will lack the gravity needed to retain an atmosphere, something crucial for protecting the surface from harmful cosmic rays.

This new planet, however, is just the right size to avoid either scenario.

“It is only 10 percent larger than the Earth,” Kane explained.

Blocking the sun

Kelper-186f orbits a small sun called a dwarf star. This star is much cooler than our sun, allowing Kepler-186f to orbit closer to the star while still being in a habitable zone.

Scientists detected this new planet by watching this dwarf star for signs of dimming. When it dims, they can infer that a mass has passed in front of it, briefly blocking the light it sends toward Earth. The researchers were able to determine the size of the planet by the amount of light it blocked as it passed. They also calculated its distance from its home star by how long it took to complete one orbit around the star. 

How Kepler finds new planets

Habitable planets

Kepler-186f is the latest in a growing list of potentially habitable planets.

Kane maintains a website chronicling the best candidates for life on these so-called exo-planets. By his count there are currently 54.

Still, we know very little about most of these planets other than their size and distance from a sun.

Kane points out, the Kepler telescope mission wasn't designed to go in-depth on any particular planet, but rather to identify as many planets as possible.

He says so far, it's done a wonderful job locating places where life may exists.

"The frustrating part about is is that... we can't easily get any closer to them," he said.

Kepler-186f for instance, is about 500 light years away. It would take hundreds of thousands of years to travel there with current technology.

However, future telescopes, like the James Webb Space Telescope, may be able to give us a better look at some of these fascinating planets.