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, Research Associate, 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."
