ESO finds weird doomed planet from another galaxy

Could HIP 13044 b be the strangest exoplanet yet found? We are getting used to the discoveries of bizarre exoplanets but newly-discovered world HIP 13044 b not only survived the cataclysm of its star swelling into a red giant, but it also originally came from another galaxy!


It came from another galaxy: This artist’s impression shows HIP 13044 b, an exoplanet orbiting a star that entered our galaxy from beyond. (Image credit: ESO/L. Calçada)


If you should be travelling about 2000 light years (about 600 parsecs) from Earth in the direction of the southern sky constellation of Fornax (the Laboratory Furnace), you might come across a red giant, a dying star that has exhausted its hydrogen fuel and now is burning helium in fits and starts. This star, designed by the astronomers of distant Earth as HIP 13044, is accompanied by at least one planet, a bit larger than our system’s giant Jupiter.

HIP 13044 is one of the Universe’s survivors. The transition from main sequence star to red giant is not is a gentle process, undoubtedly HIP 13044 b had sister planets which were engulfed as their parent star expanded. HIP 13044 b may yet share their fate, as it is perilously close to its bloated star. Its orbit is elliptical, at its closest point it is less than one stellar diameter from the star’s surface (the actual distance is about 0.055 AU). HIP 13044 b circles its star once every 16.2 days. Almost certainly the planet’s orbit might initially have been much larger, but it has moved inwards during the star’s expansion to its red giant phase. Currently about six times the diameter of our Sun, the star will expand further as it ages and the planet could very well be swallowed by it one day. Until then, HIP 13044 is most likely on the way to becoming what some astronomers call a chthonian planet, a gas giant world that has lost its dense atmosphere leaving just its core as a large sun-baked rocky sphere. The HIP 13044 system is a glimpse of what may lie in store for our own Solar System.

Yet more starting still is the fact that this dying system was born in a stellar nursery in another galaxy. HIP 13044 is one of tens of millions of stars in the “Helmi stream”, stars that belonged to a dwarf galaxy consumed by our own Milky Way galaxy, some six to nine billion years ago. Several such streams, all evidence of our galaxy’s cannibalistic tendencies, have been found, familiar fiery Arcturus being a member of one such stellar stream. HIP 13044 has survived both the death of its star and its home galaxy.

The stars of the Helmi stream are old and deficient in heavy elements. This scarcity is a symptom of their age, for they were born in the Universe’s youth when elements other than hydrogen and helium were rare. This means the very existence of planets orbiting one of these stars is surprising. Yet again HIP 13044 b shows itself to be an enigma. No other such planets have been found around other old and deficient stars. Perhaps it is time to revisit theories of planetary formation.

This bizarre alien solar system was discovered by a team of astronomers using the FEROS high-resolution spectrograph fitted to the MPG/ESO 2.2m telescope at the European Southern Observatory’s La Silla Observatory in Chile. The planet was detected by looking for the tiny wobbles of the star caused by the gravitational pull of an unseen orbiting companion.

A wise man once said “The universe is not only stranger than we imagine, it is stranger than we can imagine.” Discoveries like this suggest that he was right.

(The paper announcing this discovery is available from the journal Science.)

Article by Colin Johnston,  Science Education Director