Planet-eating stars hint at Earth’s ultimate fate
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One illustration shows a planet being eaten by its host star. | Credit: Robert Lea (created with Canva)
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Our sun is about halfway through its life, which means Earth is too. Once a star has exhausted its hydrogen-based nuclear fuel, its diameter increases more than a hundredfold, swallowing up any unlucky planets in close orbit. That day is at least 5 billion years away for our solar system, but scientists have spotted a possible insight into the fate of our world.
Using data from TESS Observatory (Transiting Exoplanet Survey Satellite)astronomers Edward Bryant from the University of Warwick and Vincent Van Eylen from University College London compared systems with stars in the main sequence of their lives – fusing hydrogen, like the sun – with post-main sequence stars closer to the end of their lives, with or without planets.
“We have seen that these planets are becoming more and more rare [as stars age]” said Bryant. In other words, planets disappear as their host stars age. Comparing planetary systems with younger and older stars clearly shows that the gap is not because the planets weren’t there to begin with: the older stars are simply hungry.
“We’re pretty confident that this is not due to a training effect,” Bryant explained, “because we don’t see big differences in the mass and [chemical composition] of these stars compared to the main sequence star populations.
Complete engulfment isn’t the only way giant stars can wipe out planets. As they grow, giant stars also exert greater and greater tidal forces on their satellites, which cause their orbits to disintegrate, strip them of their atmosphere, and can even tear them apart completely. The aspect of orbital decay is potentially measurable, and this is the effect that Bryant and Van Eylen considered in their model of how planets die.
“We study the frequency of planets around different types of stars, with the number of planets per star,” Bryant said. Bryant and Van Eylen identified 456,941 post-main sequence stars in the TESS data and, from these, found 130 planets and planet candidates with close orbits. “The fraction [of stars with planets] becomes significantly fainter for all shorter-period stars and planets, which is entirely consistent with the predictions of the theory that tidal decay becomes very strong as these stars evolve.
Astronomers use TESS to find exoplanets by looking for the dimming of light as they pass in front of their host stars, a miniature eclipse known as a miniature eclipse. transit. As with any method of detecting exoplanets, transits are best suited to large Jupiter-sized planets in relatively small orbits lasting less than half an Earth year, sometimes a lot less. These solar systems are therefore not very similar to ours in this respect. Studying planets orbiting post-main sequence stars poses additional challenges.
“If you have the same size planet but a larger star, your transit is smaller,” Bryant said. “This makes it more difficult to find these systems because the signals are much shallower.”
However, even though the stars in the data samples have a much larger surface area, their mass is comparable to that of the sun, and that’s what matters most, the researchers said. A star with the same mass as the sun will go through the same stages of life and die in the same way, and this similarity is what helps reveal the future of our solar system.
“The processes that take place once the star evolves [past main sequence] can tell us about the interaction between the planets and the host star,” said Sabine Réffertan astronomer from the Universität Heidelberg who was not involved in the study. “We’ve never seen this kind of difference in planet occurrence rates between [main sequence] and giants before because we didn’t have enough planets to statistically see this difference before. It’s a very promising approach.”
Planets: integral part of a balanced stellar breakfast
Exoplanet science is one of the greatest successes in astronomy of the modern era: since the first discovery of exoplanets 30 years ago, astronomers have confirmed more than 6,000 planets and identified numerous other candidates for follow-up observations. At the same time, the job can be difficult when dealing with planets orbiting post-main-sequence stars.
A delicate aspect of this work is linked to the age of the stars, which formed billions of years before our Sun. Older stars have a lower abundance of chemical elements heavier than helium, a measure astronomers call “metallicity“Observations found a correlation between high metallicity and exoplanet abundance.
“A small difference in metallicity…could potentially double the occurrence rate,” Refffert said, emphasizing that the paper’s general conclusions would be valid but that the details would need to be refined with better metallicity data.
Future observations to measure metallicity using spectra, as well as the masses of stars and planets, would improve the model. Additionally, the European Space AgencyIt is Mission Platoscheduled for launch in December 2026, will add more sensitive data to TESS observations.
Earth’s fiery doom is still far in the future, but researchers have taken a big step forward in understanding how dying stars might eat their planets. With more TESS and Plato data, we might even glimpse the tiny orbital changes that indicate a planet is heading toward its doom—a bleak end for this world but a wonderful discovery for our understanding of the coevolution of planets and their host stars.
