AFP

by Jean-Louis Santini

WASHINGTON (AFP) – A hot, gaseous and fast-spinning planet has been found orbiting a dying star on the edge of the Milky Way, in the first such discovery of a planet from outside our galaxy, scientists said Thursday.

Slightly larger than the size of Jupiter, the largest in our solar system, the newly discovered exoplanet is orbiting a star 2,000 light years from Earth that has found its way into the Milky Way.

The pair are believed to be part of the Helmi stream, a group of stars that remains after its mini-galaxy was devoured by the Milky Way some six to nine billion years ago, said the study in Science Express.

"This discovery is very exciting," said Rainer Klement of the Max Planck Institute for Astronomy.

"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."

Astronomers were able to locate the planet, coined HIP 13044 b, by focusing on the "tiny telltale wobbles of the star caused by the gravitational tug of an orbiting companion," the study said.

They used a powerful telescope owned by the European Southern Laboratory at La Silla Observatory in Chile, located at an altitude of 2,400 meters (7,800 feet) some 600 kilometers (375 miles) north of the capital, Santiago.

The planet is quite close to the star it is orbiting, and survived a phase in which its host star went through a massive growth after it depleted its core hydrogen fuel supply, a phase known as the "red giant" stage of stellar evolution.

"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 lead researcher Johny Setiawan of the Max Planck Institute for Astronomy.

The exoplanet is likely to be quite hot because it is orbiting so close to its star, completing each orbit in just over 16 days, and is probably near the end of its life, astronomers said.

The star may have already swallowed other planets in its orbit, making the star spin more quickly and meaning that time is running out for the surviving exoplanet.

Astronomers were mystified as to how the planet might have formed, since the star contained few elements heavier than hydrogen and helium and planets typically form out of a complex cloud of spinning space rubble.

"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," said Setiawan.

"Planets around stars like this must probably form in a different way."