The search for planets outside our solar system – exoplanets – is one of the fastest growing areas of astronomy.
More than 5,000 exoplanets have been discovered in the last few decades, and astronomers now estimate that there is on average at least one planet per star in our galaxy.
Many current research efforts aim to discover Earth-like planets suitable for life. These efforts are focused on so-called “main sequence stars” like our Sun – stars that are powered by the fusion of hydrogen atoms into helium at their cores and remain stable for billions of years. More than 90 percent of all exoplanets known to date have been discovered around main sequence stars.
As part of an international team of astronomers, we’ve studied a star that will be very similar to our Sun billions of years from now and found it has a planet it should have been devouring. In a study published today in Nature, We unravel the mystery of this planet’s existence – and suggest some possible solutions.
A glimpse into our future: red giant stars
Just like humans, stars undergo changes as they age. Once a star has used up all of its hydrogen in the core, the star’s core shrinks and the 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 expect it to grow so large that it will engulf Mercury, Venus, and possibly Earth.
Eventually, the core gets hot enough for the star to begin helium fusion. At this stage, the star shrinks back to about ten times its original size and continues to burn stably for several million years.
We know of hundreds of planets orbiting red giant stars. One of these is called 8 Ursae Minoris b, a planet about the mass of Jupiter in an orbit that keeps it about half as far from its star as 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 planet’s gravitational pull on the star. In 2019, the International Astronomical Union named the star Baekdu and the planet Halla, after the tallest mountains on the Korean Peninsula.
A planet that shouldn’t be there
Analysis of new data on Baekdu collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) space telescope has led to a surprising discovery. Unlike other red giants we’ve found that host exoplanets in nearby orbits, Baekdu has already begun to fuse helium in its core.
Using the techniques of asteroseismology, which studies waves inside stars, we can determine what material a star is burning up. For Baekdu, the frequencies of the waves clearly showed that helium had begun to be burned in its core.
The discovery was puzzling: if Baekdu burns helium, it should have been much larger in the past – so big that it should have engulfed the planet Halla. How is it possible that Halla survived?
As is often the case in scientific research, the first course of action was to rule out the most trivial explanation: that Halla never really existed.
In fact, some apparent discoveries of planets orbiting red giants using the Doppler wobble technique later turned out to be illusions created by long-term fluctuations in the behavior of the star itself.
However, subsequent observations ruled out such a false-positive scenario for Halla. Baekdu’s Doppler signal has remained stable for the past 13 years, and a close examination of other indicators revealed no other possible explanation for the signal. Halla is real – which brings us back to the question of how it survived the devour.
Two stars become one: a possible survival scenario
After confirming the planet’s existence, we came up with two scenarios that could explain the situation we see at Baekdu and Halla.
At least half of all the stars in our galaxy did not form in isolation like our sun, but are part of binary star systems. If Baekdu was once a binary star, Halla might never have been in danger of being engulfed.
A merger of these two stars may have prevented the expansion of both stars to a size large enough to engulf the planet Halla. Had a star become a red giant on its own, it would have engulfed Halla. However, if it had merged with a companion star, it would go straight into the helium burning phase 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 created a cloud of gas and dust from which the planet could have formed. In other words, the planet 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 is finding ways to make exoplanets appear in places where we would least expect them. 
Daniel Huber, Astronomer, University of Sydney
This article has been republished by The Conversation under a Creative Commons license. Read the original article.