More than 5,000 exoplanets have been detected in the past few decades, and astronomers now estimate that on average there is at least one planet for every star in our galaxy. Many current research efforts are aimed at detecting Earth-like planets suitable for life. These efforts focus on so-called "main sequence" stars like our Sun - stars that are powered by the conversion of hydrogen atoms into helium at their cores and remain stable for billions of years. More than 90 per cent of all exoplanets known so far have been detected around main sequence stars.

Just like humans, stars change as they age. When a star has used up all the hydrogen in its core, its core shrinks and its outer envelope expands as the star cools.
During this "red giant" phase of evolution, stars can grow to more than 100 times their original size. When this happens to our Sun in about 5 billion years, we estimate that it will grow large enough to swallow Mercury, Venus and possibly Earth.
Eventually the core heats up sufficiently for the star to start fusion of helium. At this stage the star shrinks to about 10 times its original size and continues to burn steadily for tens of millions of years.

We know of hundreds of planets orbiting red giant stars. One of these, 8 Ursae Minoris b, is a Jupiter-mass planet, and its orbit puts it only half the distance from its star that the Earth is from the Sun.
The planet was discovered in 2015 by a team of Korean astronomers using the "Doppler wobble" technique, which measures the gravitational pull of the planet on the star. In 2019, the International Astronomical Union named the star Baekdu and the planet Halla after the highest mountains on the Korean peninsula.

An analysis of new data collected by NASA's Transiting Exoplanet Survey Satellite (TESS) space telescope about Baekdu has revealed a surprising discovery. Unlike other red giants that have been found to host exoplanets in close orbits, Baekdu has already started to melt helium in its core.
Using asteroseismological techniques, which study the waves inside stars, we can determine what material a star is burning. For Baekdu, the frequencies of the waves showed conclusively that it had already started burning helium in its core.
This discovery was puzzling: if Baekdu was burning helium, it should have been much more massive in the past - so massive that it should have swallowed the planet Halla. How is it possible that Halla survived?
As is often the case in scientific enquiry, the first thing to do was to eliminate the least important explanation: Halla never really existed.

Indeed, some apparent discoveries of planets orbiting red giants using the Doppler wobble technique were later shown to be illusions created by long-term changes in the behaviour of the star itself.
However, follow-up observations have ruled out such a false-positive scenario for Halla. The Doppler signal from Baekdu has remained constant over the past 13 years, and close examination of other indicators has shown no other possible explanation for the signal. Halla is real - which brings us back to the question of how it survived the engulfment.

After confirming the existence of the planet, we arrived at two scenarios that could explain the situation with Baekdu and Halla. At least half of the stars in our galaxy are part of binary systems, not solitary stars like our Sun. If Baekdu was once a binary star, Halla may never have been in danger of being swallowed up.
The merger of these two stars may have prevented one of them from expanding enough to swallow the planet Halla. If one of the stars had become a red giant on its own, it could have swallowed Halla - but if it merged with a companion star, it could have jumped straight to the helium-burning stage without growing large enough to reach the planet.

Alternatively, Halla could be a relatively newborn planet. The violent collision between the two stars may have produced a cloud of gas and dust from which the planet could have formed. In other words, Halla could be a recently born "second-generation" planet.
Whichever explanation is correct, the discovery of a nearby planet orbiting a helium-burning red giant star shows that nature finds ways for exoplanets to appear where we least expect them.

 

Source: https://www.sciencealert.com/