A Planet From Another Galaxy…and That’s Not The Most Interesting Part


Astronomers have discovered the first extrasolar planet that was born in another galaxy.  And it gets even better…

HIP 13044 is an interloper.  There was a time when it belonged to another galaxy.  Around 6 to 9 billion years ago, its host “dwarf” galaxy was torn apart, and slowly subsumed into ours.  We can see that the star belonged to another galaxy because we can trace the remains of the dwarf as it was swallowed by our galaxy – the Helmi Stream.

Even as the dwarf galaxy met its end, HIP 13044 was not alone.  It possessed a Jupiter of its own (HIP 13044b), which  astronomers at La Silla in Chile found using the Doppler Wobble technique.  This is the first confirmed detection of a planet from another galaxy.  We can now say with much more certainty that planets are not a fluke of the Milky Way, and that the Universe has quintillions of planets (and that’s not hyperbole, that’s our best estimate, although so far it only applies to the observable universe).  That’s a huge number – to simply list the names of these planets would require about a million terabytes of data (or one exabyte), roughly 20,000 times the content of all the books ever written.

While an extragalactic planet is an obvious first, astronomers will be interested for many other reasons.  Firstly, HIP 13044 is a metal-poor star.  This is somewhat bizarre.  Planet formation theory (especially core-accretion theory, which expects planets to form by growing dust grains) generally expects stars with planets to be metal-rich – after all, you can’t make planets without the raw materials.  We could maybe ascribe this to some metal loss over time, but its companions in the Helmi stream are also metal-poor.

Also, this is an old star.  It’s on what astronomers call the Horizontal Branch – a late stage of stellar evolution where the hydrogen in the star’s core has been exhausted.  When a star stops burning hydrogen, it starts to expand and enters the Red Giant Branch.  HIP 13044b orbits so close to its parent star (about a tenth of Earth’s orbit) that during that tumultuous period the star would have engulfed the planet.  While the star’s outer layers expand, the core collapses until it becomes dense enough to begin helium burning and join the Horizontal Branch. Once helium burning begins, the expansion ends and the star returns to a more modest size, allowing the planet to emerge.

This planet has survived the worst that HIP 13044 can throw at it and it’s still here – this can tell us quite a lot about the fate of planets like ours when their star exhausts itself.  And it has almost no raw materials for planet-building – how do you make something when you have almost no ingredients?  So, while HIP 13044b is interesting as an extragalactic planet (there may be more of them we can find, looking for microlensing events in Andromeda), it may have a much bigger effect on the astronomical community as a metal-poor survivor of stellar absorption.

All it takes is one discovery – and suddenly exoplanet science starts to look a whole lot different.

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2 thoughts on “A Planet From Another Galaxy…and That’s Not The Most Interesting Part

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