A study by astronomers at University College London and the University of Warwick reveals that aging stars destroy giant planets orbiting nearby as they expand into red giants. Using NASA's TESS telescope, researchers analyzed nearly half a million stars and found far fewer close-orbiting planets around more evolved stars. This provides direct evidence of planetary destruction through tidal interactions.
Aging stars, upon exhausting their hydrogen fuel, cool and expand into red giants, a phase our Sun is expected to enter in about five billion years. A new study published in the Monthly Notices of the Royal Astronomical Society examined nearly half a million stars in the early post-main sequence stage of evolution. The research team identified 130 planets and potential candidates with short orbits of no more than 12 days, including 33 new discoveries, by analyzing dips in starlight from NASA's Transiting Exoplanet Survey Satellite (TESS).
The findings show a stark pattern: close-orbiting giant planets are significantly less common around stars that have further evolved into red giants. The overall occurrence rate among post-main sequence stars is 0.28%, dropping to 0.35% for younger ones and just 0.11% for the most evolved red giants. This decline suggests many planets have been engulfed by their host stars.
Lead author Dr. Edward Bryant from the Mullard Space Science Laboratory at UCL and the University of Warwick stated: "This is strong evidence that as stars evolve off their main sequence they can quickly cause planets to spiral into them and be destroyed. This has been the subject of debate and theory for some time but now we can see the impact of this directly and measure it at the level of a large population of stars."
The destruction mechanism involves tidal interactions, where the expanding star's gravity causes the planet's orbit to shrink, leading it to spiral inward. As Dr. Bryant explained: "Just like the Moon pulls on Earth's oceans to create tides, the planet pulls on the star. These interactions slow the planet down and cause its orbit to shrink, making it spiral inwards until it either breaks apart or falls into the star."
Co-author Dr. Vincent Van Eylen added: "In a few billion years, our own Sun will enlarge and become a red giant. When this happens, will the solar system planets survive? We are finding that in some cases planets do not. Earth is certainly safer than the giant planets in our study, which are much closer to their star. But we only looked at the earliest part of the post-main sequence phase... Earth itself might survive the Sun's red giant phase. But life on Earth probably would not."
The study focused on the first one or two million years of post-main sequence evolution. Future mass measurements of these planets, via stellar wobbles, will clarify the destruction process. The research was supported by the UK Science and Technology Facilities Council.