2023-06-30 12:56:00
8 Ursa Minoris b
30 June 2023 14:56 Robert Klatt
Exoplanet Halla (8 Ursae Minoris b) in der Nähe des Roten Riesen Baekdu (8 UMi) )oknerakaM madA/yrotavresbO kceK .M .W(Photo: ©
The exoplanet Halla (8 Ursae Minoris b) orbits its red giant Baekdu (8 UMi) so closely that it should not actually exist. How the exoplanet was able to survive is still unclear.
Honolulu (USA). Until now, astronomy has assumed that stars grow to many times their original size at the end of their lives and devour their planets. researchers of University of Hawaii around Marc Hon have now discovered an exoplanet that shows that this fate is not always inevitable. According to their publication in the specialist magazine Nature This is the exoplanet 8 Ursae Minoris b, which is also officially known as Halla.
Halla has similar properties to Jupiter and orbits its star Baekdu (8 UMi) at a distance of only 0.5 astronomical units (AU). Observations with the two Maunakea Observatories on Hawaiʻi Island, the WM Keck Observatory and the Canada-France-Hawaiʻi Telescope (CFHT), indicate that the exoplanet continues to exist despite Baekdu’s normally dangerous development.
Star of the exoplanet is a red giant
Observations of Baekdu’s stellar oscillations, made with NASA’s Transiting Exoplanet Survey Satellite, also show that the star is burning helium at its core. This is an indication that it has swollen into an enormous red giant before. It can be assumed that the star underwent an expansion of up to 1.5 times the distance of the planetary orbit, which would have resulted in the envelopment of the planet. After that, it reduced to its current size, which is just a tenth of that distance.
“The engulfment of a planet has catastrophic consequences for either the planet itself or the star, or even both. The fact that Halla has managed to survive in close proximity to a giant star that would otherwise have devoured it highlights the planet as an exceptional survivor.”
Radial velocity reveals exoplanet Halla
Astronomers from Korea have discovered the exoplanet Halla using radial velocity in 2015. The radial velocity makes it possible to measure the regular motion of a star due to the gravitational effect of the planet orbiting it. The University of Hawaii scientists made additional observations in 2021 and 2022, following finding that the star must have been larger than the planet’s orbit at an earlier date.
They used the Keck Observatory’s high-resolution echelle spectrometer and the CFHT’s ESPaDOnS device. According to Daniel Huber, the resulting findings confirmed that the almost circular orbit of the planet with a period of 93 days had remained constant for over a decade and that the oscillating movement must indicate the presence of a planet.
“Taken together, these observations confirmed the existence of the planet and raise the intriguing question of how the planet actually survived. The observations from multiple telescopes on Maunakea have been critical in this process.”
Several theory for survival
At a distance of 0.46 astronomical units from its star, the planet Halla resembles hot Jupiter-like planets that are thought to have formed in larger orbits before moving close to their stars. However, given a rapidly evolving host star, such formation becomes an extremely unlikely avenue for survival for the planet Halla.
Another theory for the planet’s survival is that it was never in danger of being engulfed. Similar to the famous planet Tatooine from Star Wars, which orbits two suns, the host star Baekdu may originally have consisted of two stars, the team says. A merger of these two stars may have prevented either of them from growing large enough to engulf the planet.
A third possibility is that Halla is a newborn planet and the violent collision between the two stars created a cloud of gas from which the planet formed. In other words, the planet Halla might be a recently born second generation planet.
Nature, doi: 10.1038/s41586-023-06029-0
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