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The exoplanet, which should not exist, has been re-examined

The exoplanet, which should not exist, has been re-examined

New research once again addresses the mystery of 8 Ursae Minoris b (8 UMi b), an exoplanet that should not exist.

Illustration of an exoplanet being swallowed by its star. 8 UMi b somehow avoided this fate. (Gemini International Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani; CC BY 4.0)

The exoplanet 8 UMi b (also known as Halla) surprised astronomers when it was discovered. When its star swelled into a red giant, it should have been swallowed, but the planet survived, yielding to the star's resolute force during its 93-day orbit.

Previously, this mystery was explained by the fact that the star, 8 UMi, was previously less massive and had a companion star. As it began to expand into a red giant, it swallowed the companion. As a result, the development path of 8 UMi's interior was changed, and its expansion was halted, so the planet avoided the bitter fate that awaited it.

We can test the theory if we can determine the age of 8 UMi. If the star is old, say 9 billion years, then this is true, but if it is young, the binary star theory is unlikely, and the mystery of 8 UMi b has not been solved.

A group of astronomers led by Huiling Chen (Peking University) determined the age of the star. The researchers analyzed positional and photometric data recorded by the Gaia space telescope, in addition to a detailed spectrum recorded by the 1.93-meter diameter telescope at the Haute-Provence Observatory in France. Thanks to the data, the star's temperature, surface gravity and chemical composition were determined.

Using the data, the researchers estimated the age of 8 UMi using three different methods: using isochorism (theoretical relationships between the brightness and temperature of stars with different masses but the same age), kinematic analysis, and chemical composition analysis. Using all three methods, the age of the star was estimated at between 1.9 and 3.5 billion years, which is much younger than the 9 billion years estimated by binary star theory.

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Based on the latest calculations regarding the age of 8 UMi, it is very unlikely that a merger with a companion star would have saved 8 UMi bt from destruction. But how can it still exist?

Age estimates based on chemical tests. Based on the two methods, the age of the star can be estimated between 3.3 and 3.5 billion years. (Chen et al. 2024)

The researchers stress that more studies are needed before the mystery is solved, but a recently discovered star property offers an answer: The researchers estimate the mass of 8 UMi at 1.7 solar masses, which is 13% larger than previous estimates. This means that 8 UMi b is slightly denser than expected, and that the orbital period of 8 UMi b has a slightly larger orbital distance: perhaps large enough for the planet to remain close to its star.

source: AAS Nova

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