Washington: A planet with a minimum mass 1.25 times that of Jupiter has been discovered orbiting a star of extragalactic origin, even though the star now finds itself within our own galaxy.
It is part of the so-called Helmi stream — a group of stars that originally belonged to a dwarf galaxy that was devoured by our galaxy, the Milky Way, in an act of galactic cannibalism about six to nine billion years ago.
"This discovery is very exciting. For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin. Because of the great distances involved, there are no confirmed detections of planets in other galaxies. But this cosmic merger has brought an extragalactic planet within our reach," said Rainer Klement of the Max-Planck-Institut für Astronomie (MPIA), who was responsible for the selection of the target stars for this study.
The star is known as HIP 13044, and it lies about 2000 light-years from Earth in the southern constellation of Fornax (the Furnace).
Adding to its claim to fame, HIP 13044 b is also one of the few exoplanets known to have survived the period when its host star expanded massively after exhausting the hydrogen fuel supply in its core — the red giant phase of stellar evolution. The star has now contracted again and is burning helium in its core.
"This discovery is part of a study where we are systematically searching for exoplanets that orbit stars nearing the end of their lives. This discovery is particularly intriguing when we consider the distant future of our own planetary system, as the Sun is also expected to become a red giant in about five billion years," said Johny Setiawan, also from MPIA, who led the research. "
HIP 13044 b is near to its host star. It completes an orbit in only 16.2 days. Setiawan and his colleagues hypothesize that the planet``s orbit might initially have been much larger, but that it moved inwards during the red giant phase.
The star also poses interesting questions about how giant planets form, as it appears to contain very few elements heavier than hydrogen and helium — fewer than any other star known to host planets.
"It is a puzzle for the widely accepted model of planet formation to explain how such a star, which contains hardly any heavy elements at all, could have formed a planet. Planets around stars like this must probably form in a different way," added Setiawan.
The results are published in Science Express.