Ageing stars may be destroying the giant planets orbiting closest to them, according to a new study by astronomers at UCL and the University of Warwick.
Once stars like the Sun run out of hydrogen fuel, they cool down and expand to become red giants. In the Sun's case this will happen in about five billion years.
In the new study, published in the Monthly Notices of the Royal Astronomical Society, researchers looked at nearly half a million stars that had just entered this "post-main sequence" phase of their lives.
The team identified 130 planets and planet candidates (i.e., that still need to be confirmed), including 33 that were previously unknown, orbiting closely around these stars.
They found such planets were less likely to occur around stars that had expanded and cooled enough to be classed as red giants (i.e. that were further on in their post-main sequence evolution), suggesting many of these planets may already have been destroyed.
Lead author Dr Edward Bryant (Mullard Space Science Laboratory at UCL and the University of Warwick) 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 causing its orbit to shrink, making it spiral inwards until it either breaks apart or falls into the star."
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."
For their study, the researchers used data from NASA's Transiting Exoplanet Survey Satellite (TESS). They used a computer algorithm 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 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.
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).
