Being a planet can be a dangerous business, particularly at the start and end of a star's life. We have reason to believe stars often devour their own planets, and have even pinpointed cases where it seems to have happened long ago, but now we’re getting to watch the effects in real time.
RW Aur A is a young star 450 light-years away. Eighty years ago astronomers noticed a dip in brightness lasting around a month, something that has been repeated every few decades. Recently these dips have become more frequent, with one lasting six months. These weren’t subtle changes, like when a planet passes in front of a star – the most recent saw light reduced by a factor of six. Meanwhile, the stellar companion RW Aur B has remained unchanged.
Given RW Aur A's relative youth, these changes are not as far outside the norms of stellar behavior as those of Tabby's Star, so we haven't had to resort to theories like alien megastructures, but something definitely needed explaining.
Dr Hans Moritz Guenther of MIT has used the Chandra X-Ray Observatory to investigate RW Aur A during both bright and dim phases. He thinks two objects orbiting the star collided, creating a cloud of debris. The dimming is a result of the dust released by the collision passing between us and RW Aur A, blocking some of its light. Follow-up collisions between remnants have renewed this occasionally. Some of this dust has been absorbed into the star's corona, changing its composition and the spectrum we see.
We can't tell a lot about the collision’s components, but Guenther says at least one must have been large enough to be considered a planet. "If our interpretation of the data is correct, this would be the first time that we directly observe a young star devouring a planet or planets,” he said in a statement.
By studying the changes in brightness at both X-ray and visible light wavelengths, Guenther and co-authors of a paper published in The Astronomical Journal were able to determine that there is a lot of iron in the dust cloud, and in the coronal absorption, with 10 times as much present during the dim phase in 2017 as when RW Aur A was at its brightest in 2013.
We know from our own Solar System that planets experience many collisions, some with very large objects, in the early years after formation, so it’s not surprising such an event would occur. We've previously found two stars, thought to be born from the same cloud, with very different metal contents. The likeliest explanation is that the more metal-rich star consumed rocky material equivalent to 15 times the mass of the Earth. However, the cannibalistic ways of that Sun-aged star were probably long in the past.
We've also seen less-certain examples where the process seems to have been more recent, but these were still millions of years ago, rather than occurring as we watch.
RW Aur A is thought to be less than 10 million years old, exceptionally young by star standards. We expect most cases of planet consumption to happen at times like this, before larger objects on collision orbits are weeded out, or towards the end of a star's life as it puffs up to become a red giant. The gravitational effects of RW Aur B probably disturb planetary orbits, making such collisions particularly likely.
Even aside from the dimming, RW Aur A is an interesting star because it has jets of material visible in X-ray wavelengths coming from its poles.