When the Sun reaches the end of its main sequence in about 5 billion years, it will enter what is known as the Red Giant Branch (RGB) phase, during which it will expand, potentially consuming Mercury, Venus and possibly Earth. Not long after, it will undergo gravitational collapse and blow off its outer layers, leaving behind a dense remnant known as a white dwarf. Although planet Earth will eventually meet its end in this way, it does not mean the end of the solar system, as it is the white dwarf surrounded by clouds of trace elements.
This is the nature of the universe, where the only constant is change and nothing is wasted. Still, an international team of astronomers was surprised when they examined an ancient white dwarf that was actively accumulating material from its former planetary system. Using the WM Keck Observatory on Maunakea, Hawaii, the team obtained spectroscopic evidence for 13 chemical elements commonly associated with rocky bodies. This discovery challenges our current understanding of the late stages of stellar evolution.
The team included astronomers from the Trottier Institute for Research on Exoplanets and the Department of Physics at the Université de Montréal, the Center de Recherche en Astrophysique du Québec (CRAQ), the Earth and Planets Laboratory at the Carnegie Institution for Science, the Gemini Observatory/NOIRLab, the Space Telescope Science Institute (STScI), and several universities. Their findings were presented in a paper published October 22 in *The Astrophysical Journal Letters*.
*LSPM J0207+3331 is a particularly old white dwarf located 145 light-years from Earth in the constellation Triangulum. Photo credit: Université de Montréal*
The system in question was LSPM J0207+3331, a white dwarf system located 145 light-years from Earth. This stellar remnant is extremely old, estimated to be 3 billion years old, and surrounded by a hydrogen-rich shell. Based on the spectra obtained with the High Resolution Echelle Spectrometer (HIRES) on the Keck I telescope, the team discovered a variety of minerals, including sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca), titanium (Ti), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) and strontium (Sr).
According to the team’s analysis, these minerals were once part of a differentiated body of rock at least 200 km (120 miles) in diameter that was torn apart by the star’s gravity. The resulting debris disk is the oldest and most metal-rich ever observed around a hydrogen-rich white dwarf. “This discovery challenges our understanding of the evolution of the planetary system,” said lead author Érika Le Bourdais from the iREX Institute. “The ongoing accretion at this stage suggests that white dwarfs may also contain planetary remnants that are still undergoing dynamic changes.”
The system is an example of a “polluted white dwarf,” which refers to stellar remnants surrounded by clouds of matter. Almost half of those observed showed signs of accumulation of heavy elements, but these are usually obscured by their hydrogen-rich atmosphere, making this detection particularly significant. The presence of heavy elements suggests that these stars still had planetary systems that were dynamically perturbed (perhaps by a passing star or alien planet) at some point in their long history.
In the case of LSPM J0207+3331, the team thinks the disturbance was likely recent (within the last few million years) and likely caused the rocky body to spiral inward toward its star. This is based in part on the amount of rocky material they discovered, which is unusually high for a 3 billion-year-old white dwarf. From this, the team concluded that the system could be an example of delayed instability, in which interactions between multiple planets occur that gradually destabilize their orbits over billions of years. Co-investigator John Debes from STScI explained:
Something clearly disturbed this system long after the star’s death. Even after billions of years, there is still a reservoir of material that can contaminate the white dwarf. This suggests that tidal disruption and accretion mechanisms remain active long after the main sequence phase of a star’s life. Mass losses during stellar evolution can destabilize orbits and impact planets, comets, and asteroids. Future observations could help distinguish between a planetary shock or the gravitational effect of a stellar encounter with the white dwarf.
Artist’s impression of a planet orbiting a white dwarf star. Photo credit: WM Keck Observatory.
The next step will be to investigate what may have disturbed the system, which could include a Jupiter-sized planet still orbiting LSPM J0207+3331. Such a planet will be difficult to detect visually, but could be found by measuring the gravitational influence it has on its star. In this regard, ESA’s Gaia Observatory could be sensitive enough to indirectly detect the presence of outer planets. Infrared observations from NASA’s James Webb Space Telescope (JWST) could also help search for outer planets in this system.
These results not only challenge current assumptions about late stellar evolution, but also provide a clearer picture of what the solar system will one day become.
Further reading: STScI, The Astrophysical Journal
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