As stars age, they expand. That’s bad news for planets orbiting close to their stars, according to a new study published in the Monthly Notices of the Royal Astronomical Society this month. The study suggests that planets closest to their stars, especially those that orbit their stars in just 12 days or less, are at a higher risk of being sent to their doom by their aging suns.
The idea that a dying star might engulf or destroy planets is not new, but there have not been many detailed surveys done to examine the exact process by which this happens, or at what stage of a star’s evolution planets are most at risk. This latest study surveyed a sample of more than 400,000 post-main sequence stars to determine if they could detect a decrease in the population of planets around these older planets. They found it.
Using data from the Transiting Exoplanet Survey Satellite (TESS), they detected 130 planets close to their stars, thirty-three of which are newly discovered candidate planets.
The team found that gas giants orbiting close to an aged star occur at a rate of about 0.28%. For stars that have only just entered their post-main sequence phase, that rate is about 0.35, but it drops to about 0.11% for the oldest stars in the population that have reached the red giant phase. In other words, the process of stellar aging is killing off planets.
“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,” says lead author Edward Brant from University College London and the University of Warwick. “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.”
The data shows that the shorter the orbital period of a planet, the more likely it is to be destroyed. Tidal forces between the star and the gas giant, similar to those between the Earth and Moon, cause the planet’s orbit to decay, ultimately spiralling inward to its destruction. Alternatively, these tidal forces could rip gas giants apart – an equally dramatic end for these planets.
Our own Sun is expected to reach its post-main sequence stage in about 5 billion years. The outlook for Earth’s survival is better than planets orbiting close in like, Mercury and Venus, but it will still be a rough ride.
“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,” says co-author Vincent Van Eylen from University College London. “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.”
The team hopes to improve our understanding of planetary destruction around aging stars in future using the PLATO mission, set to launch in late 2026, whose planet-finding capabilities will allow them to examine even older stars in the red giant phase than those observed by TESS.