Most Milky Way stars are orbited by planets, and more than 97% are expected to end their
lives as white dwarfs. The evolution of host stars after the main sequence will
change the properties of the original planetary systems. While planets closer than approximately 5 astronomical units will most likely not survive the post-main-sequence lifetime of their parent star, any planet
further away will survive.
Some planets in previously stable orbits around a star undergoing
mass loss will become dynamically unstable, and may get sufficiently close to the white dwarf
that tidal forces will be capable of disrupting them. The indirect evidence of such disrupting
events is the presence of debris discs around at least 4% of white dwarfs, and metal lines
in the spectra of around 25-50 % of white dwarfs.
So far the most compelling confirmation of a remnant planetary system has been the
detection of asymmetric transits in the light curve of the white dwarf WD 1145+017, caused by material
from possibly disintegrating planetesimal orbiting near the Roche limit.
WD 1145+017 also
shows an infra-red excess characteristic for a dust disc, and metal photospheric lines
in its spectrum. In addition, a circumstellar gas disc was detected in the system.
Transits of
WD 1145+017 showed several distinct periods in the range of 4.5-4.9 h and were variable
in depth. The system evolved quickly, with the overall dip activity increasing from 2014
until mid-2017, with only a small activity decrease in mid-2016.
An international team of astronomers led by Marie Karjalainen (Isaac Newton Group of Telescopes,
Spain), used the QUCAM CCDs mounted on ISIS at the William Herschel
Telescope to obtain time resolved spectroscopy of dust and gas from the extrasolar planetesimals
orbiting WD 1145+017.
The QUCAM detectors are electron-multiplying frame-transfer CCDs,
capable of performing high-speed or faint-target spectroscopy, with
minimal dead-time and essentially zero readout noise.
Observations were taken both in the blue arm, covering 3800-4025 Å, and in the red arm, covering 7000-7430 Å. When comparing the transits in both arms,
observations show significant colour difference between the in- and out-of-transit data
of the order of 0.05-0.1 mag (deeper in the red arm). This 'bluing' is
surprising and not usual in dusty environments, which typically show reddening.