The Jupiter-like planet was among the spoils of war after the Milky Way clashed with another galaxy billions of years ago.
The first planet of extra-galactic origin in the Milky Way has been found.
The planet is located about 2,000 light-years from Earth in the constellation Fornax.
The finding also challenges theories of planet formation, which hold that stars need more than hydrogen and helium to produce planets.
Between six billion and nine billion years ago, the Milky Way and another smaller galaxy found themselves at approximately the same place at approximately the same time.
Our galaxy emerged the victor, taking with it some spoils of war -- stars and material from the crushed galaxy -- which even to this day remain not quite meshed into the Milky Way's overall churn and flow. Scientists now discover there was a tag-along as well: a Jupiter-like planet known as HIP 13044b.
Like most of the 500 or so planets that have been discovered beyond our solar system, not much is know about the adoptee, which was found by measuring the slight gravitational tugs it exerts on its parent star.
But astronomers suspect it's a survivor. Not only did HIP 13044b successfully migrate from its galaxy of origin to ours, the planet also apparently survived the brutal ballooning of its parent, which has transitioned from a hydrogen-burning middle-aged star into a helium-fueled senior citizen known as a red giant.
Our own sun will undergo a similar change of life in another five billion years or so.
HIP 13044b's existence is bit of a mystery too, since it is the first planet of any origin found around a star lacking resources beyond hydrogen and helium. Such so-called "metal-poor" stars weren't believed to have the right stuff to produce planets.
"More statistics are needed to really find out how likely it is for metal-poor stars to form planets," study co-author Rainer Klement, with the Max-Planck-Institut für Astronomie, told Discovery News. "I think this will be the biggest impact of the research."
"They did an extremely careful job," added exoplanet-hunter Sara Seager, with the Massachusetts Institute of Technology. "They seemed to have gone through the scenarios very carefully."
The transplanted planet, which is at least 1.25 times the mass of Jupiter, lies about 2000 light-years from Earth in the southern constellation of Fornax, also known as the Furnance.
Lead researcher Johny Setiawan, also from Max Planck, and colleagues remain on the hunt for any HIP 13044b sibling planets -- those that are still around anyway. From the spin rate of the parent star, which is faster than it should be, scientists think the red giant already has consumed some of HIP 13044b's sisters.
Setiawan and colleagues' research appears in this week's Science.