An exoplanet almost "next-door" to Earth in cosmological distances could harbor life, scientists say, because it's possible liquid water may exist on its surface.

The "super-Earth" planet orbiting Kapteyn's star just 13 light years from us is located in the star's "Goldilocks zone," the "just right" distance to allow for liquid water on its surface, an international team of astronomers is reporting.

The planet, dubbed Kapteyn b, is five times more massive than Earth. It orbits is star in 48 hours and could be hot enough for water to exist in liquid form, they say.

Another planet discovered in orbit around the star, dubbed Kapteyn c, is too chilly for that to be occurring, but its possibly habitable neighbor is an exciting find, researchers say.

"Finding a stable planetary system with a potentially habitable planet orbiting one of the very nearest stars in the sky is mind blowing," says study co-author Pamela Arriagada from the Carnegie Institution.

"This is one more piece of evidence that nearly all stars have planets, and that potentially habitable planets in our galaxy are as common as grains of sand on a beach."

An intriguing aspect of Kapteyn b is that it is believed to have originated outside our galaxy, and was pulled into our Milky Way along with its parent star when its original galactic home was torn apart during a gravitational encounter with our galaxy.

Kapteyn's star, which is one-third the mass of our sun, was discovered in 1898 by Dutch astronomer Jacobus Kapteyn.

One of the fastest-moving stars ever observed, it is situated in what is known as the galactic halo, a far-flung cloud of stars circling the Milky Way.

A red dwarf, it is easily observed with any amateur telescope in the southern constellation of Pictor, and can be seen from the Southern Hemisphere. Red dwarf stars shine in the cooler part of the spectrum.

Kapteyn's star and its planets are very old, astronomers say, perhaps more than 11 billion years, which suggests they formed just 2 billion years after the birth of the universe itself.

That makes them twice as old as the Earth.

While astronomers can only theorize the existence of water on Kapteyn b based on it its distance from its star, next-generation instruments currently being developed may some day be able to probe its atmosphere in an effort to confirm it, the researchers say.

The discovery of the two planets was made using instruments at the La Silla observatory in Chile, operated by the European Southern Observatory.

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