(dr) University College London
New study suggests that old stars appear to be destroying the giant planets that orbit closest to them.
According to a new study carried out by astronomers at UCL (University College London) and the University of Warwick, old stars appear to be destroying the giant planets that orbit closer to them.
When a star like the Sun runs out of hydrogen, it cools and expands until it becomes a red giant. In the case of the Sun, this will happen in about five billion years, and scientists think this expansion will cause the destruction of Mercury, Venus and perhaps Earth, but there is a lack of evidence as to how or if this will definitely happen.
In a new one published in the journal Monthly Notices of the Royal Astronomical Society, researchers analyzed about half a million stars that had just entered this “post-main sequence” phase of their lives.
The team identified 130 planets and planet candidates (that is, that still need to be confirmed), including 33 that were previously unknown, orbiting around these stars.
They discovered that these planets were less likely to occur around stars that had expanded and cooled enough to be classified as red giants (that is, they were more advanced in their post-main sequence evolution), which suggests that many of these planets may have already been destroyed.
Dr. Edward Bryant, lead author and University of Warwick Fellow, who carried out most of this work while at UCL’s MSSL (Mullard Space Science Laboratory), said: “This is strong evidence that, As stars evolve beyond their main sequence, they can quickly cause planets to spiral and be destroyed. This has been the subject of debate and theory for some time, but now we can see its direct impact and measure it at the level of a large population of stars.
“We already expected to see this effect, but even so we were surprised with how effectively these stars seem to swallow their nearby planets.
“We think that destruction occurs due to gravitational ‘rope game’ between the planet and the star, the so-called tidal force. As the star evolves and expands, this interaction becomes stronger. Just as 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 decrease, causing it to spiral inward until it fragments or falls into the star.”
Co-author Dr Vincent Van Eylen, from UCL’s MSSL, said: “Within a few billion years, our Sun will increase in size and become a red giant. When this happens, will the Will the planets in the Solar System survive? We are discovering that, in some cases, planets do not survive.
“A Terra is certainly safer than the giant planets in our study, which are much closer to their star. But we’ve only looked at the early part of the post-main sequence phase, the first one or two million years – stars have a lot more evolution ahead of them.
“Unlike the missing giant planets in our study, Earth itself Earth could survive the Sun’s red giant phase. But life on Earth probably wouldn’t survive“.
For their study, the researchers used data from the TESS (NASA’s Transiting Exoplanet Survey Satellite). They used a computer algorithm to look 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 have taken no more than 12 days to orbit their star).
The team started with more than 15,000 possible signals and applied rigorous tests to exclude false signals, ultimately reducing this number to 130 planets and planet candidates. Of these, 48 were already known, 49 had already been identified as planet candidates (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 was, the less likely it was to host a nearby giant planet. The overall rate of occurrence of such planets was measured at just 0.28%, with younger post-main sequence stars showing a higher rate (0.35%), similar to that of main sequence stars, and more evolved stars, which have cooled and swollen enough to be classified as red giants, falling to 0.11% (for this analysis, the researchers excluded the 12 smallest planets out of the 130 identified).
From the TESS data, researchers can estimate the size (radius) of these possible planets. To confirm them as planets and not planet candidates, astronomers have to rule out the possibility that these bodies are low-mass stars or brown dwarfs. (“failed stars” whose pressure in the nucleus is not high enough to initiate nuclear fusion), calculating its mass.
This can be done by precisely measuring the motions of their host stars and inferring the planets’ gravitational pull (and therefore their mass) from oscillations in these motions.
Dr. Bryant added: “Once we have the masses of these planets, it will help us understand exactly what causes these planets to spiral and be destroyed.”
Although this study concluded that the rate of occurrence of giant planets decreases as the star ages, there is much to learn from the small number of planets that are still in close orbit around a red giant star. However, more data is needed to understand why some, but not all planets, are victims of aging stars.
