Ageing stars likely destroy their closest planets

Ageing stars look to be destroying the giant planets orbiting closest to them, according to a new study by astronomers at University of Warwick and UCL.

Once a star like the Sun runs out of hydrogen, it cools down and expands to become red giant. In the Sun’s case this will happen in about five billion years and scientists think this expansion will cause the destruction of Mercury, Venus and perhaps Earth, but lack evidence on how or whether this will definitely happen.

In a new study published in the Monthly Notices of the Royal Astronomical Society, researchers from Warwick and UCL have looked at nearly half a million nearby star systems to get more clarity on the matter by finding out how common it is for a nearby planet to survive their host star becoming a red giant.

Across these star systems, they found that planets are much less likely to be found orbiting close-by to red giant stars, indicating that many of the planets were likely already destroyed when their host stars expanded.

Lead author Dr Edward Bryant, Warwick Astrophysics Prize Fellow, University of Warwick, who completed most of this work while at the Mullard Space Science Laboratory at UCL said: “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.

“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 cause its orbit to shrink, making it spiral inwards until it either breaks apart or falls into the star.”

The researchers focused their investigation on stars that had just entered the “post-main sequence” phase of their lives (after running out of hydrogen) and only found 130 planets and planet candidates (including 33 we didn’t know about before) orbiting closely around these ageing stars.

When limiting their investigation to just the stars that had progressed to the stage of cooling and expanding (and hence classed as red giants), they found that the chance of a red giant hosting a nearby planet was only 0.11%, about three times lower than the percentage of a main-sequence star hosting a close giant planet.

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.”

While this study has found that rate at which giant planets occur decreases with how old the star is, there is much to learn from the small number of planets that are still found closely orbiting a red giant star. But more data is needed to get to the bottom of why some, but not all planets fall victim to ageing stars.

Dr Bryant concluded by saying: “Once we have these planets’ masses, that will help us understand exactly what is causing these planets to spiral in and be destroyed.”

ENDS

The paper, 'Determining the impact of post-main sequence stellar evolution on the transiting giant planet population' is published in MNRAS. DOI: https://doi.org/10.1093/mnras/staf1771

The researchers received funding from the UK Science and Technology Facilities Council (STFC).

Notes to Editors

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Matt Higgs, PhD | Media & Communications Officer (Press Office)

Email: Matt.Higgs@warwick.ac.uk | Phone: +44(0)7880 175403

Research Methodology

This study was performed used data from NASA’s Transiting Exoplanet Survey Satellite (TESS). A computer algorithm allowed the researchers to search for the repeated dips in brightness that indicate an orbiting planet is passing in front of the star, focusing on giant planets with short orbital periods (i.e., that took no more than 12 days to orbit their star).

The team began with more than 15,000 possible planet signals, and applied rigorous tests to rule out false signals, eventually whittling this number down to 130 planets and planet candidates. Of these, 48 were already known, 49 were already identified as planet candidates (i.e., they still need to be confirmed), and 33 were new candidates detected for the first time.

The team found that the more advanced a star’s evolution, the less likely it was to host a nearby giant planet. The overall occurrence rate of such planets was measured at just 0.28%, with the youngest post-main sequence stars showing a higher rate (0.35%) similar to that of main sequence stars, and the most evolved stars, which had cooled and swelled enough to be classed as red giants, dropping to 0.11%. (For this analysis, the researchers excluded the smallest 12 of the 130 identified planets.)

From the TESS data, researchers can estimate the size (radius) of these possible planets. To confirm them as planets rather than planet candidates, astronomers must rule out the possibility of these bodies being low-mass stars or brown dwarfs (“failed stars” whose core pressure is not high enough to start nuclear fusion) by calculating their mass.

This can be done by precisely measuring the movements of their host stars and inferring the gravitational tug of the planets (and therefore their mass) from wobbles in these movements.

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5th November 2025