A new study suggests that as stars age, they may be killing off the giant planets they orbit closest to. The study, led by astronomers from University College London and the University of Warwick, provides new evidence that these planets can be pulled inward and destroyed as their host stars evolve.
Stars like our Sun will eventually run out of hydrogen fuel. When that happens, they begin to cool and expand, entering a stage known as a red giant. Scientists estimate that the Sun will reach this stage in about 5 billion years.
The new discovery is Royal Astronomical Society Monthly Noticesis based on observations of nearly half a million stars that have recently entered this “post-main sequence” stage of their life cycles.
Lost planets around red giant stars
The researchers identified 130 planets and planet candidates (that is, they still need to be confirmed) orbiting close to these stars, including 33 that had not been detected before.
But a clear pattern emerged. Planets in tight orbits were much rarer around stars that had expanded enough to become red giants (i.e., stars that had evolved further after the main sequence). This suggests that many of these nearby planets may have already been destroyed.
Lead author Dr Edward Bryant (UCL’s Mullard Space Science Institute and the University of Warwick) explained: “This is strong evidence that planets can quickly spiral out and be destroyed as stars evolve out of the main sequence. This has been the subject of debate and theory for some time, but now we can see this effect directly and measure it at the level of large populations of stars.”
“While we expected to see this effect, we were still surprised by how efficiently these stars swallow nearby planets.”
Gravity destroys planets
The researchers believe this process is caused by a gravitational tug of war between the star and its planet, known as tidal interaction. This effect becomes stronger as the star gets larger.
Dr Bryant said: “This disruption is thought to occur due to a gravitational tug of war between the planet and the star, called tidal interaction. As the star evolves and expands, this interaction becomes stronger. “Just as planets create tides, they also pull on stars. These interactions slow the planets down, shrinking their orbits and causing them to spiral inward, eventually breaking apart or collapsing into the star.”
What this means for our solar system
The discovery also raises questions about our solar system’s distant future.
Co-author Dr Vincent van Eylen, from UCL’s Mullard Institute for Space Science, said: “In a few billion years, our sun will expand to become a red giant. Will the planets in our solar system survive when that happens? We are finding that in some cases they cannot.”
“Earth is certainly safer than the giant planets we study, which are much closer to the star. But we only looked at the first part of the post-main-sequence phase, the first million or two million years. The star still has a lot of room to evolve.”
“Unlike the giant planets lost in our study, Earth itself could survive the Sun’s red giant phase. But life on Earth probably won’t.”
How did scientists find planets?
To conduct the study, the researchers used data from NASA’s Transiting Exoplanet Survey Satellite (TESS). They used a computer algorithm to detect the small, repeated dips in starlight that occur when a planet passes in front of a star. The analysis focused on giant planets with short orbits (that is, they orbit their stars in less than 12 days).
The team started with more than 15,000 possible signals. After applying rigorous checks to eliminate false positives, they narrowed the list to 130 planets and planet candidates. Of these, 48 were already confirmed, 49 were previously identified as planet candidates (i.e. still need to be confirmed), and 33 were newly discovered candidates.
As stars evolve, planets decrease
The results showed that as the star ages, the number of approaching giant planets clearly decreases. Overall, such planets were found in only 0.28% of the stars studied. Young stars after the main sequence had a high proportion of 0.35%, similar to stars still in the main sequence. In contrast, more evolved red giants showed a much lower proportion of only 0.11%. (For this analysis, the researchers excluded the smallest 12 of the 130 planets identified.)
Using TESS data, scientists can estimate the size (radius) of each planet. To confirm whether these objects are truly planets rather than planetary candidates, astronomers must determine their mass and rule out alternative stars such as low-mass stars and brown dwarfs (“failed stars” whose cores are not under high enough pressure to start fusion).
This is done by tracking the subtle movements of the host star and measuring the gravitational force exerted by the orbiting object.
Dr. Bryant added: “Knowing the masses of these planets will help us understand exactly what causes these planets to spiral and destroy.”
The researchers received funding from the UK Science and Technology Facilities Council (STFC).
