Planet from another galaxy
The
first planet has been found around a star that seems to be an interloper from
another galaxy. Curiously, the star also contains fewer heavy elements –
thought to be needed to build planets – than any other planet-hosting star yet
discovered.
The planet, which is 1.25 times as massive as
Jupiter, lies 2300 light years from Earth and orbits a bloated, ageing star
slightly less massive than the sun. Johny
Setiawan of the Max Planck Institute for Astronomy in
Heidelberg, Germany, and colleagues found the planet by the way its gravity
caused its host star to wobble.
The host star, called HIP 13044, is a member of
a group of stars called the Helmi stream that have unusual, elongated orbits
that bring them far above and below the disc of the galaxy, where the sun and
most other Milky Way stars reside. The Helmi stars are thought to be remnants of a small galaxy
torn apart by the Milky Way 6 billion to 9 billion years ago.
Dearth of
metals
Astronomers announced a possible planet in the nearby Andromeda
galaxy in 2009, but its presence has not yet been confirmed. So this makes the
newly found planet, called HIP 13044 b, the first to be discovered around a
star apparently from another galaxy.
"This
cosmic merger has brought an extragalactic planet within our reach," says
team member Rainer Klement, also of the Max Planck Institute for Astronomy.
In
addition to its unusual origins, the host star is puzzling because it has fewer
elements heavier than hydrogen and helium than any other star known to host a
planet. Its light spectrum suggests it has just 10 per cent as much iron as the
previous record holder, and only 1 per cent as much as the sun.
Planets
are thought to form from discs of gas and dust left over from the formation of
the parent star. In the prevailing theory of planet formation, called core
accretion, dust grains stick together to form rocky worlds, and some of these
rocky bodies then grow massive enough to attract surrounding gas, becoming gas
giants like Jupiter.
Alternative
scenario
Dust
is made up of heavy elements, so stars depleted in these elements would have a
hard time making planets in this scenario.
This suggests the planet formed another way,
says Alan Boss of the Carnegie Institution of
Washington, DC, who was not a member of the team.
He proposes an alternative mechanism
that he
has long championed, in which dense regions of the planet-forming disc simply
collapse under their own gravity to form planets. In this scenario, planets
could form mainly from gas, without first forming a rocky core.
that he
has long championed, in which dense regions of the planet-forming disc simply
collapse under their own gravity to form planets. In this scenario, planets
could form mainly from gas, without first forming a rocky core.
He
adds: "The fact that the star is also likely to have come from somewhere
other than the disc of our galaxy makes it even more remarkable, and supports
the suspicion that planetary systems are rife in the universe.
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