Planets don't sit still. The seemingly stable orbits of our Solar System could easily give the impression that once a planet forms, it tends to stay in orbit where it started. But evidence has piled up that our Solar System probably isn't as stable as we'd like to think, and many of the exosolar systems we've now seen can't possibly have formed in their current state. In a few cases, we've spotted stars that contain elements that were probably delivered by a planet spiraling in.

Now, scientists may have caught the process while it was happening. A star that dimmed for a couple years has somehow ended up with 15 times the iron it had in earlier observations, suggesting it ran into a planet or a few smaller planet-forming bodies.

Not so stable

If you were to take the current configuration of the Solar System and run it forward a million years, nothing much would change—all the planets would be in the same orbits they started in. But run it forward a few billion years and strange things can happen. The orbital setup is chaotic, and future changes are very sensitive to the starting conditions. In addition, many of the features of the Solar System are hard to explain using planetary formation models, leading to the proposal of the Grand Tack, in which a much younger Jupiter migrated inward toward the Sun before being dragged out to its current position by Saturn.

All of this information provided evidence that our Solar System may not be as stable as we think it is. But the characterization of exosolar systems has clearly indicated that many are unstable. Gas giants have been found far too close to their host stars to have possibly formed there and are clearly in the process of having their atmospheres boiled off. Smaller planets are found in such tightly packed orbits that there wouldn't have been enough material to make them all in that little space; orbital mechanics indicates that they probably migrated inward before stabilizing each other's orbits.

Since many of these examples involve inward migration, they raise the question of how often planets don't settle down and instead migrate right into the star they had been orbiting. We've spotted a few cases where white dwarf stars seem to carry lots of elements they shouldn't. The simplest explanation is that the star that formed the white dwarf had previously engulfed a planet. But we haven't found clear examples of young stars that have eaten a planet early in the exosolar system's history—a period before these systems should establish some sort of orbital stability.

But that's what scientists think they've now seen while observing a binary star system called RW Aur A.

Taking the plunge

RW Aur contains two stars with masses similar to that of our Sun. But they're far, far younger, estimated to be only 10 million years old. At this stage, they are still surrounded by large disks of gas and dust, which can serve as the birth place of planets. The planet formation process involves the coalescing of this material into larger and larger bodies, ultimately concluding in the production of planetesimals, bodies that range in size from large asteroids up to nearly the size of Mars and Mercury. Catastrophic collisions among these bodies ultimately produce planets.

While we can't image these small bodies directly at RW Aur, it's clear that interesting things are going on in the system. In 2011, and again in 2014, RW Aur A experienced a dramatic dimming. Imaging of the star revealed streams of gas being drawn off the disk by its companion star, RW Aur B. These streams could easily account for the dimming of the star as they passed across the line of sight from the star to Earth.

But then came a 2017 event that the streams don't seem to work for. Within the general dimming at the time, there was a lot of short-term variability that is best explained by material close to the star. That was enough to prompt additional imaging using other hardware, and that's when things got even stranger.

The Chandra X-ray Observatory got good images of RW Aur A during the dimming event, and they showed some unusual features at high energies. The features are best explained as arising from iron that has lost a lot of electrons (in the neighborhood of 23 of its normal 26). These high energies shouldn't be present in the disk, suggesting the iron is residing in the star's corona.

There's one problem with that idea, however: previous imaging had indicated that RW Aur A contained less iron than the Sun. The new observations would require that it had 15 times more iron than the Sun. That much iron could be supplied if a large planetesimal (or small group of them) had migrated inward until the gravity of RW Aur A tore it apart. This would also handily explain why the star experienced a dimming at the same time.

Most of the other scenarios the researchers consider can't explain either the dimming or the surge in iron. The only one that can, an odd failure for the star's magnetic field to couple with material in the disk, should cause additional changes in the star over the next couple of years. We'll have to wait to sort this out definitively, but for now the planetesimal is the leading candidate.

If that theory holds up, this work could be some of the earliest direct evidence we have that planetary bodies really do end up colliding with the star they formed around. While it's obviously implied by a lot of the other data we have, direct evidence always gives us more confidence that we really know what's going on.

The Astronomical Journal, 2018. DOI: 10.3847/1538-3881/aac9bd (About DOIs).