This alien intruder from another galaxy is in many ways different from other exoplanets observed by astronomers.
Located about 2000 light-years from Earth in the southern constellation of Fornax (the Furnace), the Jupiter-like planet orbits a dying star of
extragalactic origin and risks to be engulfed by it.
It is the first time astronomers have discovered an exoplanet orbiting a star that entered our Milky Way from another galaxy.
"This discovery is very exciting," says Rainer Klement of the Max-Planck-Institut für Astronomie (MPIA), who was responsible for
the selection of the target stars for this study.
"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.
The star, known as HIP 13044, was discovered by a European team of astronomers using the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory in Chile.
The exoplanet HIP 13044 b, orbits a star of extragalactic origin, even though the star now finds itself in 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.
HIP 13044 b orbits very near its star.
At the closest point in its elliptical orbit, it is less than one stellar diameter from the surface of the star
(or 0.055 times the Sun-Earth distance). It completes an orbit in only 16.2 days.
Scientists have considered the possibility that the planet's orbit was much larger before, but that it moved inwards during the red giant phase.
"The star is rotating relatively quickly for an horizontal branch star," says says Johny Setiawan, from MPIA, who led the research.
"One explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly."
This artist's impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy.
This planet of extragalactic origin was detected by a European team of astronomers using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile.
The Jupiter-like planet is particularly unusual, as it is orbiting a star nearing the end of its life and could be about to be engulfed by it,
giving clues about the fate of our own planetary system in the distant future.
Credit: ESO/L. Calçada
Astronomers are not familiar with many exoplanets like HIP 13044 b
Artistic impression of HIP 13044 b. Credit: Teleskopdatenbank.de
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.
Until now, these so-called horizontal branch stars have remained largely uncharted territory for planet-hunters.
"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."
So far HIP 13044 b has survived and manage to escape the fate of the star's inner planets, but it might be doomed after all.
The star will
expand once again in the next stage of its evolution and HIP 13044 b might be engulfed then.
Video Credit: ESO
By studying HIP 13044 b and its star, scientists can gain knowledge about the demise of our outer planets, such as Jupiter,
when the Sun approaches the end of its life. In addition, the star also offers information 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," adds Setiawan.
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