As though planets from outside our Solar System were not exciting enough, astronomers have recently discovered a planet orbiting a star from outside our galaxy Johny Setiawan reports.
For two decades, astronomers have known that there are planets beyond our Solar System (Wolszczan & Frail, 1992). Orbiting other stars, they are known as extrasolar planets or exoplanets. So far, more than 500 exoplanets have been detected, the majority of which orbit stars with characteristics similar to those of the Sun (as described in Jørgensen, 2006). In particular, more than 90% of the stars hosting exoplanets are in the same evolutionary phase as the Sun – the main-sequence phase, during which stars burn hydrogen (see image below).
After several billion years, their fuel is almost exhausted and they start swelling, pushing the outer layers away from what has turned into a small and very hot core. These ‘middle-aged’ stars become enormous, and thus cool and red, and are known as red giants.
After the red giant phase, the star enters the horizontal-branch phase, during which the energy source is helium fusion in the core, and hydrogen fusion in the shell surrounding the core. This is the stage in which the star HIP 13044 is now.
Unlike much more massive stars, these Sun-like stars do not end their existence in dramatic explosions, but die peacefully as planetary nebulae, blowing out everything but a tiny remnant, known as a white dwarf.
To learn more about the evolution of stars, see Boffin & Pierce-Price, 2007 (small stars), Székely & Benedekfi, 2007 (massive stars) and the Langton Star Centre websitew1
Image courtesy of ESO / S Steinhöfel
Our research group at the Max Planck Institute for Astronomy in Heidelberg, Germany, however, concentrates on the search for planetary companions around stars that are not in the main-sequence phase, but in later evolutionary stages. These include what is known as the red giant phase, during which the star expands to hundreds of times its original diameter. The detection of planets around such giant stars is important for the study of the evolution of planetary systems. In particular, it allows us to predict the future of our own Solar System.
Recently, our team have successfully detected a planet around the star HIP 13044, which has left the red giant phase.
The star HIP 13044, which is about 2000 light years away from our Solar System in the southern constellation of Fornax (‘the furnace’), is significantly different from other known stars with planets. In particular, it has a very low abundance of the metal iron – less than 1% of what the Sun has. High metal abundance (stellar metallicity) is important in the core accretion model of planet formation: the more metal there is in the star system, the higher the probability of forming a planet. Given these low iron levels, we had not expected to find a planet around HIP 13044.
What makes this star particularly interesting, however, is the fact that HIP 13044 is one of a group of stars crossing our galaxy, the Milky Way, and orbiting the centre of the galaxy on similar orbits; such a group is known as a stellar stream. The Helmi stream, to which HIP 13044 belongs, is known to have its originoutside our galaxy (Helmi et al., 1999). It is assumed that the gravitational pull of the Milky Way drew these stars into our galaxy.
This is the first time that 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.
Although the star HIP 13044 and its attendant planet HIP 13044b are now within the Milky Way, they are still 2000 light years from Earth; whereas the star can be seen with a telescope, the planet itself is far too small to observe directly. How, then, did we detect it?
Using a technique known as radial velocity, we looked for tiny telltale wobbles of the star caused by the gravitational tug of its orbiting companion. By examining the stellar spectral lines at intervals, we detected changes to those lines (see diagram below). These indicate changes in the velocity of the star along the line of sight and can reveal the presence of an unseen low-mass companion, such as a planet. Although there are other techniques for detecting exoplanets (for example microlensing, as described in Jørgensen, 2006), the radial velocity method has proved the most successful. For these precise observations, we used the high-resolution spectrograph FEROS attached to the 2.2 m MPG / ESO telescope at the European Southern Observatory’s La Silla facility in Chilew2. This observatory is equipped with world-class instruments for detecting extrasolar planets.
HIP 13044 b is one of the few exoplanets known to have survived the red giant phase of its host star, during which the star expands massively after exhausting the hydrogen fuel supply in its core. The host star HIP 13044 has now contracted again and entered the horizontal branch, burning helium in its core. This is the first exoplanet detected around a horizontal-branch star. The discovery of HIP 13044 b, therefore, is particularly intriguing when we consider the distant future of our own planetary system; the Sun is already halfway through its life and is expected to become a red giant in about five billion years.
Not only the existence of the newly discovered planet is interesting; its characteristics are also unusual. HIP 13044 b has a mass at least 1.3 times that of Jupiter, the biggest planet in our Solar System, and orbits at a distance 0.12 that of the distance between the Sun and Earth (0.12 astronomical unit). Because it is so much closer to its host star than we are to the Sun, HIP 13044 b orbits its host star in only 16.2 days rather than the year it takes Earth. Such a small planetary orbit is common for stars in the main sequence, like the Sun, but is unusual for stars in late evolutionary phase like giant stars.
Our team hypothesises that the planet’s orbit might initially have been much larger, but that it moved inwards during the red giant phase. If the planet had been closer to the star, it may not have been so lucky: the star is rotating relatively quickly for a horizontal branch star, and one explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly (for an explanation of why this is, see Carlberg et al., 2009)
Although HIP 13044 b has so far escaped the fate of these inner planets, the star will expand again in the next stage of its evolution. HIP 13044 b, having survived this long, may nonetheless be about to be engulfed by its star. This could also foretell the demise of even our outer planets – such as Jupiter – when the Sun approaches the end of its life.
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