A mysterious planet just 520 light years away should not exist, say astronomers.
The Jupiter-like gas giant has defied logic by escaping engulfment by its host star.
It orbits a kind of decaying star known as K-type – slightly cooler than our Sun and orangeish in colour.
As they balloon outward anything too close is swallowed up. But gas giant 8 Ursae Minoris b has survived.
‘The distance between the Earth and its star, the Sun, is 1 astronomical unit, or 1 au,’ said co-author Dr Dimitri Veras, of Warwick University.
‘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 international team also analysed reasons for the strange phenomenon – and came up with two possible ideas.
‘The first, more plausible argument, is that the star once had a companion that quenched its increase in size, allowing the planet to survive,’ said Dr Veras. ‘This binary star 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.’
The exoplanet lies in the constellation of Ursa Minor – located in the northern hemisphere.
A year lasts only 93 days – about a quarter of one on Earth. After 12 months, the planet will have completed four orbits of its star.
Co-author Dr Marc Hon, of Hawai’i University, 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.’
The findings, published in the journal Nature, are based on information collected by Nasa’s alien hunter TESS (Transiting Exoplanet Survey Satellite).
Co-author Professor Bill Chaplin, of Birmingham University, 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 observed by the space telescope to confirm beyond any doubt the star is a core-helium-burning red giant.’
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