Blog 33: Free-form blog post

Last spring, during one very long night in the eighth floor of the science center, Barra, Sean, April and I remotely operated the Minerva telescope, loudly sang disney songs, and ate way too much junk food in order to obtain three light curves. Two of those light curves were of exoplanets transiting other stars (we wanted to be very sure of our grade in the class). The third exoplanet was not transiting a star at all, but a white dwarf. One of our TFs last semester, Andrew Vanderburg, had asked us to take a light curve of this curious system for a project he was working on. Fast forward about half a year, and that project is now been published in Nature, Sky and Telescope, and a few other astronomical publications. In light if its recent publicity, I figured I would actually read the Nature paper so I can gain a deeper understanding of what exactly we were observing that night atop the science center.

The object in question is a white dwarf, one of the most common fates for a star, in the Virgo region, about 570 light years away from Earth. White dwarf atmospheres are generally composed of lighter elements, with a carbon and oxygen core and a helium and hydrogen outer shell. However, a quarter to a half of all white dwarfs observed contain heavier elements in their atmospheric spectrum, a puzzling fact knowing heavy elements should sink to the core. This discrepancy caused astronomers to suspect that outside materials had been introduced to the white dwarf's atmosphere, either through surrounding dust or disrupted asteroids, but no evidence had been found.

This white dwarf in question, WD 1145 + 017, was being observed for a different reason. The object exhibited a telltale, periodic, dip in its light curve. This occurred about once every 4.5 hours and could obscure up to 40% of the white dwarf's light. This behavior is a telling sign of a transiting exoplanet, though no exoplanets had ever been discovered around a white dwarf before.

An artist's rendering of a disintegrating rock planet around a white dwarf.

However, the periodic dip in the light curve was not a typical one either. Unlike a consistently transiting exoplanet around a star, this light curve was more erratic. The results were puzzling, until researchers saw two asymmetric light curves that are indicative of a disintegrating planet, which had been observed on main sequence stars before. The asymmetry was evidence for a comet-like tail trailing the planet and the variable transit depths indicated a disintegrating system. Combining this with data showing evidence of heavy elements, like nickel and iron, indicated to researchers that the disintegrating planet was rocky.

Some of the main periodic light curves of WD 1145 + 017.

The planet is about twice the distance from the Earth to the Moon away from the white dwarf, and about the mass of Ceres. The origin of the planet is unclear; it was most likely disrupted from a previous orbit by the gravity of the white dwarf. There is still a lot to learn about this unique system. It is currently the best evidence we have for external white dwarf "pollution" and exoplanets transiting white dwarfs. This is definitely something I will keep an eye on in the future.

Sources:
https://www.cfa.harvard.edu/~avanderb/page1.html
http://www.nature.com/articles/nature15527.epdf?referrer_access_token=JQgS2CFJ63QHiqwEPtPBJNRgN0jAjWel9jnR3ZoTv0OabmFzXPDJ8_WF8JNyTjQSuCoJ78UVkpUoy_k_5w1_QoSdPQRTejjUKTX0cRCWjHcKxRGYqDGUtTqkJgFEKN4xBCrjmjgKIdaoaOP99aALkSQAZ5OmnFbTUeiSGH9Dk_EhDjR0a7Z65xPZ34YmCVHX7DtNfxjg2G-X9-H-TMLmdg%3D%3D&tracking_referrer=www.nature.com