Scientists have discovered two ancient planets orbiting a strange old star right in our stellar backyard.
One of the newly found planets is perhaps five times the mass of Earth, and astronomers believe it is the right distance from the host star to potentially sustain liquid water on its surface. That means it could be suited for life.
The second planet is even bigger -- a super-Earth, but it is probably too cold to support liquid water.
And researchers say there may be more planets in the system as well.
"The star is very quiet, and more detailed observations may reveal more planets," said Guillem Anglada Escude of Queen Mary University of London. "Most likely they will be even smaller.... True Earth-like candidates are possible in the system."
Anglada Escude is the lead author of a paper describing the discovery published in the journal Monthly Notices of the Royal Astronomical Society.
The two newly found planets orbit Kapteyn's star, a red dwarf star that lies a relatively close 13 light-years away. It is currently the 25th closest star to our planet, although that has and will change over time.
It is small as stars go. It has about one-third the mass of the sun and it is 30 times less luminous.
It may also be almost as old as universe itself.
Kapteyn's star is known as a "halo" star, part of a group of stars that follow a strange orbit through the Milky Way galaxy.
The sun and most of the other stars in the galaxy are in a disk, but halo stars are distributed in a sphere. While Kapteyn's star may be close to the sun now, in 100 million years it will be on the other side of the galaxy.
Astronomers think stars like Kapteyn's were born in a dwarf galaxy that was absorbed by the Milky Way long ago. The remaining core of this galaxy is probably Omega Centauri, a group of hundreds of thousands of old stars about 16,000 light-years from Earth. If Kapteyn's star was once part of this group, that would make it about 11.5 billion years old, or slightly more than 2 billion years younger than the universe.
"Stars like Kapteyn's are boiling much more quietly than the sun, and they can last tens of hundreds of billions years," said Anglada Escude.
He added that the newly discovered planets were probably just as old as their host star -- ancient worlds orbiting an ancient sun.
The planets were discovered using new data from several land-based telescopes that can detect tiny changes in the motion of the star.
Anglada Escude said he was interested in finding planets around nearby stars for two reasons: "Scientifically speaking, these are the easiest ones to try to characterize further," he said. "Beyond that my personal motivation is exploration. The same way we are exploring the solar system, we might be able to explore these worlds one day."