Caution! Do not cross this line... (NASA/ESA, G. Bacon STScI)

"What a deep voice you have," said the little girl in surprise.

"The better to greet you with," said the wolf.

"Goodness, what big eyes you have."

"The better to see you with."

"And what big hands you have!" exclaimed Little Red Riding Hood, stepping over to the bed.

"The better to hug you with," said the wolf.

"What a big mouth you have," the little girl murmured in a weak voice.

"The better to eat you with!" growled the wolf...

Stars and planets have what might be the most dysfunctional relationship in astrophysics. Planet-sized objects seem to form contemporaneously with stars, coalescing and coagulating out of the great disk-like platters of gas and dust that orbit increasingly massive and dense central proto-stellar objects. Angular momentum (the product of the distribution of mass with its velocity) is shed from these central blobs into the surrounding disk, helping the star gather and relax into its final state and pushing some disk material into further orbits. But some planets, especially the larger ones, seem to be hell bent on getting closer, not further, from their parent stars. During the early stages of proto-planetary evolution they can migrate inwards, pushing and pulling against the disk of matter to haul themselves into astonishingly close and unexpected orbits - helping create the population of 'hot Jupiters' that we find around a few percent of all stars.

Other mechanisms can also bring planets swooping in perilously close to their parent stars. Planets' gravitational pull on each other can destabilize a system, causing some worlds to be ejected to interstellar space (presumably forming the population of known 'rogue' planets), and others to either come crashing into the central star or to end up on highly elliptical orbits that skim near to the star at their closest approach. Extreme proximity to a star can both strip off planetary atmospheres and volatiles (perhaps transforming a perfectly good gas giant into a more paltry object) and give rise to strong tidal effects between planet and star. These tidal effects, much like here on Earth, can stretch and squeeze a planet and also evolve its orbit. An elliptical orbit can be eroded down to something circular, matching what we see in these close-orbiting planets. But in particularly close configurations a planet can raise tides on the star itself, and one consequence of that is the potential for the planet to spiral inwards over millions to tens of millions of years and end up being shredded and engulfed by the star.

It appears to be all love and hate between planets and their stellar parents. But although the above scenarios are plausible explanations for the configurations we see in exoplanetary systems, we don't know for sure what happens. In particular we don't know exactly how often a planet gets gobbled up by its star.

Come here my pretty... (NASA/GSFC/Reddy)

An intriguing new study by Metzger, Giannios, and Spiegel may shed some light on this question, literally. Their paper on 'Optical and X-ray Transients from Planet-Star Mergers' investigates what the precise consequences are when a massive planet (a hot Jupiter) winds up being destroyed by a star. Exactly how this epic disaster plays out depends in part on the relative size and density of the planet itself. A high density world will plunge deep into the stellar atmosphere before being disrupted, but it will generate a great hot wake of matter that can emit a 'burst', or transient blip of extreme ultraviolet or X-ray radiation. A low density planet will tend to just be 'siphoned' off into the star well before it gets close enough for rapid disruption, being steadily and somewhat discretely consumed. Interestingly, intermediate density planets can be shredded into a great disk of material encircling the star. This rapidly spinning matter will heat up through frictional effects and power a bright flood of radiation that will persist for weeks or months.

Thus, when a star eats a planet it's possible that we would see it quite clearly. In fact these events may be as bright as more standard stellar 'novae' - where a white dwarf ignites a shell of nuclear fusion around itself - but distinguished in flavor and certain physical details. So the big question is how often do we expect this kind of thing to happen? Metzger, Giannios, and Spiegel dig into this and come up with an estimate that a star somewhere in our galaxy may eat a hot Jupiter between once a year and once every ten years. That's a bit of a challenge to find, the sky is a big place, but they also point out that if we monitored our nearby galactic neighbor Andromeda (M31) with optical and X-ray telescopes we'd stand a pretty good chance of spotting this type of astrophysical infanticide. Similarly, the latest generation of all-sky monitoring observatories have a good chance of catching this consumption happening in our own galaxy.

So make sure to watch the heavens, you might just catch a glimpse of a planet being wolfed down...