LSPM J0207+3331 was discovered in 2019 by a participating scientist in the Backyard Worlds: Planet 9 citizen science project. Astronomers quickly noted that it was very cold for a white dwarf star, evidence of its advanced age, as these stars gradually cool over time.
The team of John Debes, co-author of the study and researcher at the Space Telescope Science Institute in Baltimore, also detected an infrared excess around the star, indicating the likely presence of a debris disk. However, the presence of such a disk is often associated with a “polluted” atmosphere, containing traces of planetary material falling on the star. The team from the University of Montreal therefore analyzed the spectroscopic observations of LSPM J0207+3331 in search of these signatures.
A rocky planet?
“White dwarfs offer us one of the only ways to directly measure the composition of exoplanets,” emphasizes Patrick Dufour, co-author of the study and full professor at the University of Montreal. When planetary debris gets too close, it is torn apart by the star’s gravity and ends up staying there, polluting its atmosphere and leaving a detailed chemical fingerprint of its composition.
In the case of LSPM J0207+3331, the observations turned out to be even more spectacular than expected.
“It is very difficult to detect planetary remains in the atmosphere of cold, hydrogen-rich white dwarfs like this one,” says Érika Le Bourdais. Their atmosphere is more opaque and the heavy elements sink quickly towards the center of the star. We expected to see only a few chemical elements, but we found several!”
The analyzes revealed the presence of 13 elements: sodium, magnesium, aluminum, silicon, calcium, titanium, chromium, manganese, iron, cobalt, nickel, copper and strontium.
“Discovering such a diversity of elements is exceptional,” continues the researcher. And the amount of rock material present is unusually high for such an old white dwarf.”
Chemical analysis helps paint the picture of the disintegrated body: an object large enough to have distinct layers, for example a metallic core and a rocky mantle, much like Earth or like the asteroid Vesta in the solar system. The ratio of the elements also indicates that it was therefore a rocky world with little ice, closer to an asteroid than a comet.
Patrick Dufour adds: “Hydrogen-rich white dwarfs represent the vast majority of white dwarfs and the coldest systems among them are among the oldest stars in our galaxy. We weren’t used to looking at them for signs of accretion. This unique case motivates us to extend our search to more of these stars.”
A mystery disturbance
If the chemical portrait of the disintegrated body is now less mysterious, the story of its fall towards the white dwarf star remains difficult to explain. How did an object get deflected to be swallowed up by the star so late in the system’s history?
One of the team of scientists’ hypotheses would be that more distant giant planets, interacting gravitationally over billions of years, would have gradually destabilized the system and sent this object into its fatal orbit. These planets, also very old and therefore not very luminous, however remain difficult to spot with current instruments. The team also mentions the close passage of another star whose gravity would have disrupted the trajectories of debris orbiting the white dwarf.
“Future observations with the space telescope James-Webb or archive data from the space telescope Gaia, of the European Space Agency, could make it possible to distinguish between a planetary reorganization or the gravitational effect of a close encounter with another star,” suggests John Debes.
These scenarios illustrate the complexity and longevity of planetary systems, billions of years after the death of their star, and open new perspectives for research on planetary evolution.
