One of the wonderful things of living in a galaxy populated by hundreds of billions of stars is that it provides us with the chance to understand the latter’s entire evolutionary path from birth to death, just by observing the galaxy’s many different stellar populations. This fact has just become even clearer by the Kepler space telescope’s newest and somewhat coincidental discovery of a white dwarf star caught in the act of devouring the left-over remains of its planetary system. Besides being fascinating in their own right, these first-ever observations of planetary cannibalism around a white dwarf also provide a glimpse of our own Solar System’s future five billion years from now, when the Sun will have been reduced to an inauspicious stellar remnant after destroying most of the planets, including Earth.
The Kepler space telescope is no stranger to unique and unusual discoveries. Even though it was crippled in 2013 following the failure of its second reaction wheel, NASA’s prolific planet-hunting observatory was ultimately given a new lease on life when it was given a new extended mission one year later, called K2. Since that time, Kepler has made several notable exoplanet discoveries that have grabbed the headlines, including the detection of what has been characterised as “Earth’s bigger cousin,” a Neptune-sized alien world with a comet-like tail, and more recently with the detection of a mass of objects of unknown origin around the star KIC 8462852. Now, added to this list of strange and fascinating discoveries comes the first-ever planet-like transit in front of a white dwarf star. The latter are what’s left behind after medium-sized stars like the Sun have ended their lives. When a star with a mass up to eight times that of the Sun can no longer fuse hydrogen into helium in its core during the end of its life it inflates its outer atmosphere and swells up to become a red giant, while at the same time the core starts to implode due to its own gravity. Eventually, the star will dissipate its outer layers into space forming a planetary nebula while leaving behind an exposed dense Earth-sized core—a white dwarf.
The detection of exoplanets around dead stellar remnants isn’t anything new. The first exoplanet ever to be discovered in the early 1990s was found orbiting a rapidly rotating neutron star, better known as pulsar. Since then astronomers have also made similar discoveries around white dwarf stars. In addition, many white dwarfs have been found to be surrounded by circumstellar dust disks with many of them rich in heavier elements like oxygen, magnesium, iron, and silicon. Scientists had been long puzzled about the abundance of such heavy elements in the vicinity of white dwarfs, since the outer layers of these stellar corpses are made up entirely of hydrogen and helium—any heavier elements would be buried due to their weight deep inside where they couldn’t be spectroscopically detected. It had been hypothesized that these detections of heavier elements around white dwarfs were due to the presence of neighboring asteroid or cometary debris disks, or even planet-sized bodies that were vaporised after falling onto the white dwarfs themselves, yet the lack of more detailed observations kept the origin of these spectroscopic signals unknown.
The mystery seems to have been finally solved, thanks to a set of observations that were made during the Kepler space telescope’s first observing K2 campaign between 30 May and 21 Aug. 2014. While scanning stars in the direction of the constellation Virgo for the telltale dip in brightness that would indicate the presence of a transiting exoplanet, Kepler detected a strange transit signal in front of the white dwarf star WD 1145+017, located 570 light-years away, which happened to be in the same star field that Kepler was studying. While analysing WD 1145+017’s transit curve, a team of astronomers led by Andrew Vanderburg, a graduate student at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., were surprised to see that it was quite asymmetric. When an extrasolar planet happens to cross the face of its star, it causes a characteristic U-shaped transit curve which is well-defined. Yet the transit curve of WD 1145+017 was unusually elongated. Furthermore, while the main transit itself covered the white dwarf by more than 40 percent, it also exhibited varying depths with a period of 4.5-5 hours. Follow-up observations with ground-based telescopes further strengthened the case that what the researchers were seeing were due to the presence of one and possibly even more small planet-like bodies with a cometary tail that were located very close to the white dwarf.
“The eureka moment of discovery came on the last night of observation with a sudden realization of what was going around the white dwarf,” says Vanderburg. “The shape and changing depth of the transit were undeniable signatures. We are for the first time witnessing a miniature ‘planet’ ripped apart by intense gravity, being vaporized by starlight and raining rocky material onto its star.”
According to the researchers’, whose study is being published in the Oct. 22 issue of the journal Nature, the planetary debris material that falls onto WD 1145+017 most probably is the result of previous collisions between planets whose orbits were disrupted as their host star turned into a red giant. The disruption of the previously stable planetary system would have led to planets slamming onto one another, eventually creating a circumstellar dust disk that would be populated with countless planetesimals with sizes as large as Ceres—our Solar System’s biggest asteroid. The evaporation of all this material by the white dwarf would be enough to explain the latter’s abundance in heavy elements like iron, silicon and magnesium, which are the characteristic elements of rocky planetary bodies. “For the last decade we’ve suspected that white dwarf stars were feeding on the remains of rocky objects, and this result may be the smoking gun we’re looking for,” says Fergal Mullally, staff scientist at NASA’s Ames Research Center in California. “However, there’s still a lot more work to be done figuring out the history of this system.”
The processes that astronomers are witnessing around WD 1145+017 might also be similar to what will happen to the planets in our own Solar System, after the Sun will have exhausted all of its fuel and turn into a white dwarf, several billions of years from now. “This is something no human has seen before,” comments Vanderburg on Kepler’s observations. “We’re watching a solar system get destroyed.” Ultimately, all this destruction may be just one step in the never-ending cycle of planetary birth and death in the Cosmos with many scientists speculating that out of all this debris inside such circumstellatr disks, new generations of planets could eventually form, illuminated and heated by the light of the dead and inconspicuous white dwarfs. With expected lifespans in the order of a few hundred billions of years, white dwarfs could theoretically provide more than enough time for planets around them to emerge and even become habitable.
One has to wonder if that hasn’t already come to pass somewhere out there in the cosmic infinity.