These White Dwarfs Shred Their Planets Apart. But Wait, Where Do the Planets Even Come From?

 Planets orbiting close to a red giant are destroyed once the star dies, yet we have evidence of planets orbiting stellar remains. How is that even possible?

Artist's concept of a Jupiter-like planet orbiting a white dwarf; Source: Wikipedia

Dying stars are weird. Every star normally resides on the main sequence, where they happily fuse hydrogen to helium for millions or billions of years — just like our Sun is doing at the moment and for five billion years to come. However, when there’s no hydrogen left, the star throws a tantrum, growing to sizes that engulf any planet in its way. 

At this red giant stage, the star fuses elements heavier than helium until it reaches a point where it can no longer burn heavier elements — then, as if someone pulled the plug, the star sheds its outer layers into space, leaving only an exposed Earth-sized core behind. At least, that’s how our Sun’s going to end its life. 

The core that remains is called a white dwarf, as dead as can be, shining its remaining energy into space until it cools to a point that it becomes a black dwarf. 

Like every star, even zombie white dwarfs have habitable zones, where water could exist in liquid form. However, most planets orbiting the progenitor are destroyed in the star’s death throes. But you’d be fooled to think that white dwarfs’ orbits are vacant and dark — we now have evidence of planets around white dwarfs. 

476 light-years from Earth lies WD 1145+017, a white dwarf in the constellation Virgo. The progenitor was a blue star 2.48 times as massive as our Sun, residing on the main sequence for about 550 million years. 

If WD 1145+017 had any planets before, they’re all dead now. Nevertheless, the zombie star is the first white dwarf confirmed to be orbited by a dwarf planet! Discovered with the Kepler space telescope in 2015, the small planet is quite a place for extremes. 

At just 400km across, it’s much smaller than the dwarf planet Pluto measuring 2’300km. WD 1145+017 b, its designation, therefore might be closer to being an exoasteroid than a dwarf planet. 

It orbits its white dwarf at a distance of just 750’000km, nothing compared to the Earth’s 150 million km from the Sun. At this close distance, the dwarf planet takes just four and a half hours to orbit the star. 

That makes WD 1145+017 b quite the paradox — had the planet been there before the star died, it would surely have been engulfed. If we do want to find out more about it, then we should hurry, because the dwarf planet is currently vaporizing in the star’s heat; another by-product of the extreme proximity.

Another tiny exoplanet orbits the star WD 1054-226, 117 light years away in the constellation Crater. The white dwarf is surrounded by planetary debris, some pieces the size of the Moon, whizzing around the star in just 25 hours. Because the debris is evenly distributed, it has been suggested that an Earth-sized planet influences the disk. 

Interestingly enough, the planet would also orbit the white dwarf in its habitable zone.  

Wherever the debris comes from, it can only be a stellar postmortem product. If the material had been there when the progenitor star became a red giant, it would have been swallowed by the star’s layers. 

564 light years away, another white dwarf is orbited by planetary debris. Another product that came after the star’s death? 

The Helix Nebula. Every young white dwarf is surrounded by a planetary nebula of ejected material; Source: Wikipedia

Zombie planets are rare, but they exist. They are a challenge to our understanding of planetary formation. The consensus in the scientific community is that planets form in protoplanetary disks around young stars when material clumps together, a process that would theoretically take several million years to form perfect planets. 

However, if we speed up that process, wouldn’t it be possible for debris disks left in the wake of a star’s death to develop new planets? Speaking of “zombie” planets… 

Take pulsar planets. As the name suggests, these orbit pulsars, rapidly spinning neutron stars that have died in brutal supernova explosions. The first exoplanets we ever discovered orbit a pulsar — the Lich system. 

Now, scientists don’t think that the planets in the Lich system have been there before the star died. One theory is that material expelled during the supernova could form planets, as I mentioned above.

However, our nebular hypothesis states that it takes about 10 million years until planets fully form. A supernova remnant, or planetary nebula* in the case of a white dwarf, would dissipate too fast for planets to form if our theories are correct. 

And yet, when we look at protoplanetary disks of young stars, we see that some already have planets at a young age. 

Take HL Tauri, a protostar not more than 100’000 years old that is actively developing planets. In a triple star system designated GW Orionis, the three Sun-like stars are not older than one million years and we have evidence for a giant planet in the making. 

If planets are developing that quickly around young stars, then they should also be able to form from the debris of dying stars, unleashing a new generation of planets carrying the ashes of their precedents. In the case of supernova remnants, the material has a strong magnetic field, which could even enhance the formation of planets. 

It would be a big coincidence if all the planets we observe around white dwarfs and neutron stars are just captured from interstellar space. But one thing is clear: wherever these planets come from, they defy our perception of stellar corpses being dead — they offer a new beginning. 

*not to be confused with a protoplanetary disk. Planetary nebulae are the ejecta of Sun-like stars after their deaths. 

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