White Dwarfs + Compact Objects

Our Institute’s researchers study more than just exoplanets. In addition to planets and the stars they orbit, our scientists also look at “dead” stars. Sometimes called compact objects or stellar remnantsthese are the objects left over at the end of a star’s life, once it has used up all of its fuel.

Death of a star

The most common type of stellar remnant is a white dwarf, the final fate of more than 97% of the stars in our Galaxy. White dwarfs are created when a small-to-moderately-large sized star burns all of its nuclear fusion fuel, and its core contracts. The outer layers are blown away to create a beautiful planetary nebula, revealing the dense core which has become a white dwarf. Imagine an object with mass similar to our Sun, but compressed down to the size of the Earth. A teaspoon of white dwarf material would weigh as much as three African elephants! This will be the ultimate fate of our own star, the Sun.

An artist’s rendering of an exoplanet orbiting the pulsar PSR B1257+12. (Credit: NASA/JPL-Caltech)

Neutron stars are even more extreme stellar remnants. They are formed when very large stars run out of fuel. The process begins the same way as for white dwarfs, but in this case, there is too much mass and the contracting core squeezes past the white dwarf stage. The core squeezes down into a ball of pure neutrons. They can have several times the mass of our Sun, but are only about as large as a city. These are the densest objects we know of, and a small handful of neutron star material on Earth would weigh as much as a mountain!

For the most massive stars, the collapse crushes even the ultra-dense ball of neutrons. These huge stars can turn into black holes when they exhaust their fuel. Black holes are extreme objects that have so much matter packed into such a tiny space that not even light goes fast enough to escape if it wanders too close.

Planets around dead stars?

What do these compact objects have to do with exoplanets? Since stellar remnants were once stars, they may have had planetary systems. They may have even retained their planets after their deaths. In fact, the first exoplanets ever discovered were not found around a regular star, but rather around a neutron star.

There is a special category of neutron stars called pulsars. They get this name because they rotate and pulse radio waves with incredible regularity, making pulsars some of the most stable clocks in the Universe. This regular pulse pattern can wobble slightly if other objects orbit the pulsar. It was exactly this kind of pulsar wobble that revealed the very first confirmed exoplanet in 1992 around the pulsar PSR 1257+12. This discovery was made by Canadian astronomer Dale Frail (DRAO) and Polish astronomer Aleksander Wolszczan.

Polluted white dwarfs

An artist’s rendering of a destroyed comet around the white dwarf G29-38. (Credit: NASA/JPL-Caltech/GSFC)

White dwarfs provide a very different way of studying exoplanets. One important way that white dwarfs are different from regular stars is their high surface gravity. The gravity on the surface of a white dwarf would make a human weigh millions of kilograms! This causes heavier elements to sink rapidly, leaving a very clean and pure surface of mostly hydrogen and helium. As astronomers studied more white dwarfs, they discovered that some are “polluted” with heavier elements. The only way these heavy elements could be at the surface is if they were recently or continuously deposited there.

How could white dwarfs get polluted atmospheres? As a regular star nears the end of its life and eventually turns into a white dwarf, it can cause gravitational instabilities in its system. Objects such as planets, asteroids, and comets that may have been on stable orbits before the star died may now become unstable. If one of them falls too close to the white dwarf, it can easily be torn apart and form a disc of material that gets accreted onto the surface. Such polluted white dwarfs show us what these shredded objects were made of. This is different from measuring the composition of an exoplanet’s atmosphere, because polluted white dwarfs also reveal the planet’s crust and core materials, not just the atmospheric gases.

No planets are yet known to exist around black holes, but that doesn’t mean it’s not possible. Researchers are currently looking for signs of planets in systems called x-ray binaries, where a black hole feeds off a stellar companion and emits strong x-rays.

White dwarfs and compact objects at iREx

Many of our iREx researchers are experts on these stellar remnants. Their expertise in these strange objects helps us to study exoplanets in unique ways. To learn more, we invite you to read their profiles:

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