The Fate of Planets Around Evolving Stars

When stars like our Sun exhaust their hydrogen fuel, they begin to cool and expand, transforming into red giant stars. Our Sun is expected to undergo this dramatic transformation in about five billion years. This research, published in the Monthly Notices of the Royal Astronomical Society, analyzed nearly half a million stars that have recently entered this ‘post-main sequence’ phase of their evolution.

Searching for Planets Around Evolved Stars

The research team identified 130 planets and potential planet candidates (those still needing confirmation) orbiting these aging stars, including 33 new candidates not previously discovered. They found that such planets were less common around stars that had expanded and cooled sufficiently to become red giants. This pattern suggests that many of these planets may have already been destroyed.

Evidence of Planetary Destruction

Lead author Dr. Edward Bryant explained, “This is strong evidence that as stars evolve away from their main sequence, they can quickly cause planets to spiral inward and be destroyed. This topic has been debated and theorized for some time, but now we can see its effects directly and measure it across a large sample of stars.”

The destruction occurs through a powerful gravitational struggle known as tidal interaction. As the star grows and expands, these forces intensify. “Just as the Moon pulls Earth’s oceans to create tides, the planet pulls on the star,” Bryant said. “These interactions slow the planet and cause its orbit to shrink, drawing it inward until it disintegrates or falls into the star.”

Implications for Our Solar System

Co-author Dr. Vincent Van Eylen added, “In a few billion years, our Sun will swell and become a red giant. When this happens, will the planets in our solar system remain? We find that in some cases, those planets do not survive.”

“Earth is safer than the giant planets in our study, which are much closer to their star. But we only looked at the early part of the post-main sequence phase, the first million or two million years of it – stars have much more evolution to undergo.”

“Unlike the giant planets lost in our study, Earth itself may survive the red giant phase of the Sun. However, life on Earth likely will not.”

Using TESS Data in Research

To conduct their research, the team used data from NASA’s Transiting Exoplanet Survey Satellite (TESS). They employed an algorithm to identify small, regular dips in starlight caused by planets passing in front of their stars. Their focus was on giant planets with short orbits (those taking 12 days or less to orbit their star).

The researchers began with over 15,000 potential signals and used rigorous checks to eliminate false positives, ultimately narrowing the list to 130 confirmed or candidate planets. Among these, 48 were already known, 49 were known candidates awaiting confirmation, and 33 were entirely new discoveries.

Fewer Planets Around Older Stars

The researchers found that stars further along in their evolution were significantly less likely to host close-in giant planets. The overall occurrence rate was only 0.28%, with younger post-main sequence stars showing a higher rate (0.35%) similar to main sequence stars. More evolved stars, classified as red giants, showed a sharp decline to 0.11%.

Using TESS data, astronomers can estimate the size (radius) of the planet. To confirm whether these objects are true planets or low-mass stars or brown dwarfs (‘failed stars’ that did not ignite nuclear fusion), their mass must be determined. This is done by measuring small shifts in the host star’s motion caused by the planet’s gravity. These ‘stellar wobbles’ allow scientists to infer the planet’s mass.

Dr. Bryant added, “Once we have the masses of these planets, it will help us understand exactly what causes these planets to decay and be destroyed.”

Conclusion

Research on red giant stars and their surrounding planets offers new insights into stellar evolution and its impact on nearby planets. With increasing evidence of gravitational destruction of planets, we gain a better understanding of the future of our solar system. Although Earth may survive the Sun’s red giant phase, life on the planet may not. This research highlights the importance of studying stars at different stages of their evolution to understand the fate of surrounding planets and the implications for planets in our solar system.