The alternative version is that Halla formed from the gas created by colliding stars.

When stars near the end of life, they swell between 10 and 100 times their current size, engulfing the nearest planets in the process. This fate awaits our Earth too as well as many planets in other solar systems as stars age and expand. Yet, astronomers from the University of Hawaiʻi’s Institute for Astronomy (IfA) have made a groundbreaking discovery: a planet that survived what seemed like certain destruction by its host star.

The findings, according to a study published in the journal Nature by an international team of astronomers, sheds light on a little unknown astronomic event: planetary survival in hostile stellar environments. The planet 8 UMi b - also named Halla - is a Jupiter-sized giant orbiting the red giant star Baekdu (8 UMi) at just half the Earth-Sun distance.

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Observations from Hawaiʻi’s Maunakea Observatories revealed Halla’s improbable survival. The researchers, led by Marc Hon, a NASA Hubble Fellow at IfA, analyzed Baekdu’s oscillations using data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Their findings indicate that Baekdu, now burning helium in its core, had previously expanded to a size large enough to engulf Halla before shrinking to its current dimensions.

“Planetary engulfment typically results in catastrophe for either the planet, the star, or both. The fact that Halla has managed to persist in the immediate vicinity of a giant star that would have otherwise engulfed it highlights the planet as an extraordinary survivor,” Hon explained in a university statement.

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Halla was first discovered in 2015 by Korean astronomers using the radial velocity method, which tracks a star’s movements caused by an orbiting planet’s gravitational pull. The IfA team conducted further observations from 2021 to 2022 using advanced instruments at Keck Observatory and CFHT, confirming the planet’s 93-day orbit.

At 0.46 astronomical units (around 149 million kilometers) from its star, Halla is comparable to "hot" or "warm" Jupiters, which are thought to form at greater distances before migrating closer to their stars. However, the extreme expansion of Baekdu during its red giant phase would have made this migration pathway highly improbable.

The team also explored alternative explanations. One theory posits that Halla was never in danger of engulfment. Baekdu may originally have been part of a binary star system, and the merger of its two components might have prevented it from expanding enough to engulf the planet.

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Another possibility is that Halla is a "second-generation" planet, forming from the gas cloud created by the collision of the two stars.

Binary star systems are common, but scientists are only beginning to understand how planets can form around them; it’s possible that more planets exist around evolved stars than they previously believed, said Hon.

Halla’s discovery not only challenges assumptions about planetary survival but also opens up new avenues for understanding the complex interactions between stars and their planets. Simple put – we still know very little about the cosmos.

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