These days, the discovery of yet another planet orbiting yet another distant star in our galaxy doesn’t seem out of the ordinary. Unless, of course, the planet and star came from a galaxy far, far away.
HIP13044b is the first planet ever detected in the Milky Way that was born outside our galaxy.
Thanks to improvements in telescope technology, astronomers have found evidence for almost 500 extrasolar planets in the past 15 years, heavenly bodies of different sizes orbiting distant stars. But all of those confirmed by scientists have originated within the Milky Way.
The newly discovered planet has a mass at least 1.25 times that of Jupiter and orbits a star called HIP13044, a giant near the end of its life that is part of the “Helmi stream”. This is a group of stars that once belonged to a dwarf galaxy before it was cannibalised by the Milky Way between six and nine billion years ago.
“This discovery is very exciting,” said Rainer Klement of the Max Planck Institute for Astronomy (MPIA) and author of a paper published on Thursday in Science Express. “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.” HIP13044 is around 2,000 light years from Earth and appears in the southern constellation Fornax. Astronomers detected the planet by looking out for the small, gravitationally induced movements of its star as the planet orbits. The researchers measured the wobbles using a spectrograph connected to a 2.2-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile.
“This discovery is part of a study where we are systematically searching for exoplanets that orbit stars nearing the end of their lives,” says Johny Setiawan, also from MPIA and a co-author of the Science paper. “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.” HIP13044b is relatively close to its star, say the scientists, at its closest approach reaching less than 0.055 times the distance between the Earth and the Sun, and taking just over 16 days to complete an orbit. The star itself has already passed the red giant phase, where it would have expanded to several times its original diameter as it ran out of hydrogen fuel — a fate that will befall our own Sun in a few billion years.
“The star is rotating relatively quickly,” said Setiawan. “One explanation is that HIP13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly.” He added that there are unanswered questions about how the planet, which orbits a star containing very few chemical elements other than hydrogen and helium, was formed when there was apparently such a small range of material available. Until now, very few planets have been discovered orbiting stars such as this.
“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.”
Copyright: Guardian News & Media 2010