An exoplanet from another galaxy
Astronomers have discovered the first exoplanet that originated in another galaxy. The planet's host star belongs to a dwarf galaxy which was swallowed up by our home galaxy, the Milky Way, billions of years ago. Remarkably, the Jupiter-like planet orbits a star nearing the end of its life. It appears to have survived the star's "Red giant" stage, which offers a tantalizing glimpse of one possible fate of our own Solar System in the distance future. The results are being published on November 18 in Science Express.
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Künstlerische Darstellung des gelblichen Sterns HIP 13044 und, rechts unten, seines Planeten HIP 13044 b. HIP 13044 gehört zum Helmi-Sternstrom, einem Überrest einer Zwerggalaxie, die vor Milliarden von Jahren von der Milchstraße geschluckt wurde.
Over the last 15 years, astronomers have detected nearly 500 exoplanets orbiting ordinary stars in our cosmic neighborhood. Now, for the first time, astronomers have detected an exoplanet whose origin appears to lie outside our own galaxy.
The planet, which has been designated HIP 13044 b, has a minimum mass of 1.25 times the mass of Jupiter. The star system is located about 2000 light-years from Earth in the southern constellation Fornax ("the chemical furnace").
The planet was discovered with the radial velocity method, which measures tiny wobbles of a star caused by a planet's gravitational pull. HIP 13044's wobbles were detected with the high-resolution spectrograph FEROS at the 2.2 m MPG/ESO telescope at ESO's La Silla observatory in Chile.
The planet and its host star appear to have originated in a dwarf galaxy that was swallowed by the Milky Way galaxy between six and nine billion years ago. Such galactic cannibalism is an ordinary occurrence in galactic evolution. Typically, remnants of swallowed-up dwarf galaxies can be detected as ribbon-like arrangements of stars known as "stellar streams". In this case, HIP 13044 is part of the so-called "Helmi stream".
"This is an exciting discovery," says Rainer Klement of the Max Planck Institute for Astronomy (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."[1]
The newly discovered system has a number of unusual properties. "We found HIP 13044 b as part of a systematic search for exoplanets around stars that are nearing the end of their life," says MPIA's Johny Setiawan, who led the research. While the host star HIP 13044 was probably rather similar to our own Sun earlier on, it has since gone through the "Red Giant" phase, in which a star cools and expands to hundreds of times the radius of the Sun. It has now settled down into another quiet phase powered by the nuclear fusion of Helium, which is expected to last a few million years in total.
The fact that the exoplanet survived the red giant stage provides an intriguing glimpse of one possible fate of our own planetary system: our Sun is expected to become a Red Giant in around five billion years. Setiawan and his colleagues hypothesize that HIP 13044 b's current close orbit – its present average distance to its host star amounts to a mere 12 per cent of the distance between the Sun and the Earth, with an orbital period of only 16.2 days – was initially much larger, and that the planet migrated inwards during the star's Red Giant phase.
There is some evidence that some closer-in planets did likewise, and did not survive: "HIP 13044 is rotating relatively quickly for a star of this particular type," says Setiawan. "One explanation is that HIP 13044 swallowed its inner planets during the Red Giant phase, which would make the star spin more quickly." HIP 13044 b's survival might be in jeopardy, though. In the next stage of its evolution, the star is headed for renewed expansion, and may engulf the planet.
With only this single data point, it is impossible to tell how common this particular evolution is. More definite conclusions – and an understanding of how much HIP 13044 tells us about our own planetary system's future –will only be possible once significantly more planets orbiting similar stars – stars that have reached the later stages of stellar evolution - have been found. This is the aim of an ongoing search by Setiawan and his colleagues.
One final puzzle is that the new planet's host star HIP 13044 appears to contain very few elements heavier than hydrogen and helium (in technical terms, it is "extremely metal-poor") – fewer than any other star with planets. "It is a puzzle for the widely accepted model of planet formation how such a star, which contains hardly any heavy elements at all, could have formed a planet," adds Setiawan.
Background information
The work described in this release is slated for publication in the journal Science. An electronic version will be published in advance on November 18, 2010 in Science Express as Setiawan et al., "A Giant Planet Around a Metal-poor Star of Extragalactic Origin". The members of the team are Johny Setiawan, Rainer J. Klement, Thomas Henning, Hans-Walter Rix, Boyke Rochau and Tim Schulze-Hartung (all from the Max Planck Institute for Astronomy) and Jens Rodmann (European Space Agency).
Endnote
[1]
Because of the great distances involved, current telescopes are not nearly powerful enough to systematically observe exoplanets in other galaxies. There have been tentative claims of the detection of extragalactic exoplanets through "gravitational microlensing" events: During such events, a star A passing in front of an even more distant star B leads to a subtle, but detectable "flash". Some features of that flash indicate that the star A is accompanied by a planet. However, this method relies singular events – the chance alignment of a distant light source, planetary system, and observers on Earth – making it inherently unlikely that such a detection of an extragalactic planet can ever be confirmed.
Questions and Answers
How was the planet detected
The planet was detected indirectly, using the so-called radial velocity method. As a planet orbits it host star, its gravitational pull will make the star change its position ever so slightly. In fact, if you look closely, you will find that the planet is not the only object that is orbiting: both the planet and its host star orbit the system's center of mass. When its orbit makes the star move towards the Earth, the stars light will be shifted slightly towards smaller wave-lengths ("blue-shift"); when it moves away from us, its light will be shifted towards longer wave-lengths ("red-shift") in what is known as the Doppler Effect. Sensitive studies of the star's light (in particular, its "spectral lines") can detect these periodic shifts, deduce the stars motion, and use this information to infer the presence, and some of the properties, of the planet.
Which instruments and telescopes were used in the observations?
The observations were made using the high-resolution spectrograph FEROS (short for "Fibre-fed Extended Range Optical Spectrograph") at the 2.2 m MPG/ESO telescope, which is located at the European Southern Observatory's La Silla observatory in Chile. The 2.2 m telescope has been in service 1984, and is on loan from the Max Planck Society to ESO.
What does this discovery tell us about planets in other galaxies?
To be frank, little we did not know. There is no reason to think anything other than that there are planets orbiting stars in other galaxies – similar to the way that, before 1995, there was no reason to think anything other than that there are planets orbiting Sun-like stars other than our Sun. Still, it's very exciting to have confirmation, if only for the special case of a dwarf galaxy that has been swallowed by the Milky Way Galaxy.
How did this planet end up in our own galaxy?
Spiral galaxies like our own Milky Way grow by swallowing smaller galaxies, so-called "dwarf galaxies" (cf. this MPIA press release). During these acts of cannibalism, the dwarf galaxy is distorted severely. One typical outcome are "stellar streams", longish structure made of stars which wrap around the larger galaxy. Over the course of a few billion years, these stellar streams grow ever more distinct as their stars mix with the larger galaxy's own. In the case of HIP 13044, the stellar stream is clearly detectable – it is the "Helmi stream", which is extremely well studied, and HIP 13044 is clearly a part of that stream. This means that HIP 13044 used to be part of the earlier dwarf galaxy and, going by age estimates based on models of stellar evolution, should definitely have been part of the dwarf galaxy before it was swallowed by the Milky Way. The same should hold for the planet HIP 13044 b, making it indeed an exoplanet from another galaxy.
What does this discovery tell us about the future of our Solar System?
More than 90% of known exoplanets orbit stars that, like our Sun, are in the "main sequence phase" of stellar life – a quiet adulthood lasting for billions of years. In contrast, the new planet's host star, HIP 13044, is in a very late stage of stellar evolution; it is a so-called "horizontal branch" star that, just previously, was a Red Giant star. Thus, we are looking at a planet that has survived the Red Giant phase of stellar evolution. This is an interesting data point when it comes to predicting how our own Solar System's planets are likely to fare in about 5 billion years, when the Sun becomes a Red Giant star. The Jupiter-like planet's close orbit could be an indication that our Solar System's giant planets, too, might end up closer to a post-Red giant Sun. However, for any more solid predictions, more data points are needed. The present discovery was made in the course of a project that is systematically searching out planets orbiting stars in the late stages of stellar evolution, and is eagerly searching for exactly those additional data points.
What is the significance of the lack of heavy elements?
In astronomical parlance, all elements heavier than hydrogen and helium are called "metals". HIP 13044 is extremely poor in metal – it contains less than 1% as many metals as the Sun. In the most widely accepted model of planet formation ("core accretion"), this is very unusual. The prediction of these models is: The higher the metal abundance in the system, the higher the probability to form a planet. This is evidence for alternative mechanisms of planet formation (e.g. those involving "gravitational instabilities"), which allow for the formation of planets around extremely metal-poor stars.
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