A new study suggests that aging stars may be wiping out the giant planets that orbit closest to them. The research, led by astronomers at UCL (University College London) and the University of Warwick, provides fresh evidence that these planets can be pulled inward and destroyed as their host stars evolve.
Stars like our Sun eventually run out of hydrogen fuel. When that happens, they begin to cool and expand, entering a phase known as a red giant. Scientists estimate that the Sun will reach this stage in about five billion years.
The new findings, published in the Monthly Notices of the Royal Astronomical Society, are based on observations of nearly half a million stars that have recently entered this "post-main sequence" phase of their life cycle.
Missing Planets Around Red Giants
Researchers identified 130 planets and planet candidates (i.e., that still need to be confirmed) orbiting close to these stars, including 33 that had not been detected before.
However, a clear pattern emerged. Planets in tight orbits were much less common around stars that had expanded enough to become red giants (i.e. that were further on in their post-main sequence evolution). This suggests that many of these close-in planets may have already been destroyed.
Lead author Dr. Edward Bryant (Mullard Space Science Laboratory at UCL and the University of Warwick) explained: "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.
"We expected to see this effect but we were still surprised by just how efficient these stars seem to be at engulfing their close planets."
The Gravitational Pull That Dooms Planets
The team believes the process is driven by a gravitational tug-of-war between the star and its planet, known as tidal interaction. As a star grows larger, this effect becomes stronger.
Dr. Bryant said: "We think the destruction happens because of the gravitational tug-of-war between the planet and the star, called tidal interaction. As the star evolves and expands, this interaction becomes stronger. 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 causing its orbit to shrink, making it spiral inwards until it either breaks apart or falls into the star."
What This Means for Our Solar System
The findings also raise questions about the distant future of our own solar system.
Co-author Dr. Vincent Van Eylen (Mullard Space Science Laboratory at UCL) said: "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, the first one or two million years of it -- the stars have a lot more evolution to go.
"Unlike the missing giant planets in our study, Earth itself might survive the Sun's red giant phase. But life on Earth probably would not."
How Scientists Found the Planets
To carry out the study, researchers used data from NASA's Transiting Exoplanet Survey Satellite (TESS). They relied on a computer algorithm to detect tiny, repeated dips in starlight that occur when a planet passes in front of its star. The analysis focused on giant planets with short orbital periods (i.e., that took no more than 12 days to orbit their star).
The team started with more than 15,000 possible signals. After applying strict checks to eliminate false positives, they narrowed the list to 130 planets and planet candidates. Among these, 48 were already confirmed, 49 were previously identified as planet candidates (i.e., they still need to be confirmed), and 33 were newly discovered candidates.
Fewer Planets as Stars Evolve
The results show a clear decline in the number of close-in giant planets as stars age. Overall, only 0.28% of the studied stars hosted such planets. Younger post-main sequence stars had a higher rate of 0.35%, similar to stars still on the main sequence. In contrast, more evolved red giants showed a much lower rate of just 0.11%. (For this analysis, the researchers excluded the smallest 12 of the 130 identified planets.)
Using TESS data, scientists can estimate each planet's size (radius). To confirm whether these objects are truly planets rather than planet candidates, astronomers must determine their mass and rule out alternatives such as low-mass stars or brown dwarfs ("failed stars" whose core pressure is not high enough to start nuclear fusion).
This is done by tracking subtle movements in the host star and measuring the gravitational pull exerted by the orbiting object.
Dr. Bryant added: "Once we have these planets' masses, that will help us understand exactly what is causing these planets to spiral in and be destroyed."
The researchers received funding from the UK Science and Technology Facilities Council (STFC).
