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 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 focused on finding Earth-like planets suitable for life. These efforts focus on so-called “main-sequence” stars such as our Sun, stars that are powered by the fusion of hydrogen atoms into helium in their cores and remain stable for billions of years. More than 90 percent of all known exoplanets to date have been found around main-sequence stars.
However, recent research has led to a discovery that raises many questions for astronomers. An international team of astronomers studied a star very similar to our Sun billions of years later and found that it has a planet that by all rules should have been consumed by it. A study published today in Nature reveals the mystery of this planet’s existence and offers some possible solutions.
A glimpse into our future: the red giants
Like humans, stars change with age. When a star has used up all its hydrogen in its core, the star’s core shrinks and its outer shell expands as the star cools. At this stage of evolution, stars, called “red giants,” can grow more than 100 times their original size. When this happens to our Sun, in about 5 billion years, it is expected to become so large that it will swallow Mercury, Venus, and possibly Earth.
However, red giants are not uncommon in our galaxy. We know of hundreds of planets orbiting these stars. One of them is called 8 Ursae Minoris b, a planet with a mass about the same as Jupiter in orbit, which 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” method, which measures a planet’s gravitational pull toward a star. In 2019, the International Astronomical Union named Baekdu star and planet Halla after the highest mountains on the Korean peninsula.
A planet that shouldn’t be
Analysis of new data on the Baekdu star collected by NASA’s Transiting Exoplanet Survey Satellite (TESS) space telescope has led to an unexpected discovery. Unlike other red giants that contain exoplanets in close orbits, Baekdu has already begun synthesizing helium in its core.
Using the methods of astroseismology, which studies waves inside stars, astronomers determined that Baekdu had begun fusing helium in its core. However, this discovery raised many questions: if Baekdu is burning helium, it must have been much larger in the past-so large that it should have consumed the planet Hulla. How is it possible that Hulla survived?
It’s a mystery that astronomers can’t yet solve. They have proposed several possible solutions, but none of them offer a complete explanation. One hypothesis is that Hulla could have formed in a wider orbit and avoided being absorbed by a star. Another hypothesis suggests that Hulla could have been a planet of another star that was captured by Baekdu.
However, further research and more data are needed to definitively solve this mystery. Astronomers hope that future missions, such as the James Webb Space Telescope, will help solve this mystery.
