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Artists impression of the eventual merger between two stars that allowed Halla to escape engulfment
The planet Halla may have once orbited two stars that interacted with one another by mass transfer as depicted. The eventual merger between the stars allowed Halla to escape engulfment and persist around a helium-burning giant star. Credit: Julian Baum

A giant planet has escaped engulfment by its host star, to the surprise of scientists around the world.

The very unusual planet, 8 Ursae Minoris b, survives in its current location, 520 light years from Earth, in the Milky Way, for reasons very hard to explain. In normal circumstances, it should have been engulfed by its host star – which expands significantly as it decays. But 8 Ursae Minoris b remains in existence.

The discovery was made by a collaboration of more than 40 academics around the world – including Professor Bill Chaplin at the University of Birmingham, Marc Hon at the University of Hawai’i, and Dr Dimitri Veras at the University of Warwick, which hosts leading research into white dwarf stars.

This is a great example of the detailed, forensic studies we can now perform thanks to the latest data, including using the natural oscillations of the host star (asteroseismology) observed by the TESS space telescope to confirm beyond any doubt that the star is a core-helium-burning red giant.

Professor Bill Chaplin - University of Birmingham

Dr Veras said: “The distance between the Earth and its star, the sun, is 1 astronomical unit, or 1 au. 8 Ursae Minoris b is orbiting its star at 0.5 au. While the distance is not unusual in itself, what is strange is that the star is in the process of dying, having already spent some of its fuel.

“When a star spends fuel, it expands in size significantly. This star would have already expanded its size to 0.7 au, which should have engulfed and destroyed the planet. So it’s very unusual that the planet still exists.”

The scientists then further investigated reasons as to why the planet remains unscathed - publishing their findings in Nature.

“We came up with two possible explanations”, added Dr Veras. “The first, more plausible argument, is that the star once had a companion star (a "binary star"), that quenched its increase in size, allowing the planet to survive. This binary companion eventually merged with the main star, and now just appears as a single star.

“The second theory, which is less widely explored, again invokes a binary companion. This time the merger of the two stars produced a disc from which this planet was generated – known as a second-generation planet.

“The wider impact is a better understanding of the evolution of stars like our Sun and the evolution of planets like those we see in the solar system, and the need to look out for additional unusual cases like the observed system. Sometimes the rarest cases can reveal the most about stellar and planetary physics.

“Understanding these rare planetary systems enables us to learn new ways in which planets and stars evolve, and motivates us to discover even more rare examples.”

Marc Hon, University of Hawai’i, said: “Most stars are in binary systems, but we don’t yet fully grasp how planets may form around them. It is plausible that many more peculiar planetary systems may exist due to the influence of binary companions.”

Professor Bill Chaplin added: “This is a great example of the detailed, forensic studies we can now perform thanks to the latest data, including using the natural oscillations of the host star (asteroseismology) observed by the TESS space telescope to confirm beyond any doubt that the star is a core-helium-burning red giant.”