By Jonny Lupsha, Wondrium Staff Writer
When our Sun dies, it will flame-broil our planet and become a white dwarf. Most planets orbiting white dwarfs are so destroyed by their stars’ deaths that they only exist as space debris. An intact planet orbiting a white dwarf has just been discovered.
Some 6,500 light years away, an intact planet roughly the size of Jupiter has been discovered orbiting a dead star classified as a “white dwarf.” In most circumstances, the death of a star results in an enormous physical and chemical reaction that causes its innermost orbiting planets to be incinerated and blown to bits. Such a fate awaits Earth in 5 billion years or so.
But what about planets further away from the star in question? For example, what would happen to Jupiter and Saturn in such an event? Scientists may have just found the answer in a planet known as MOA-2010-BLG-477Lb, which survived its star’s death and remains intact after such solar devastation. Study of the planet will likely continue for years to better predict the posthumous existence of the outer planets of our own solar system.
What are white dwarfs? In his video series Introduction to Astrophysics, Dr. Joshua N. Winn, Professor of Astrophysical Sciences at Princeton University, explained their existence.
“White dwarfs are not fusing hydrogen,” Dr. Winn said. “They’re not fusing anything—there’s no internal power source. They’re held up against the force of gravity by Heisenberg’s uncertainty principle. I kid you not—the electrons in a white dwarf do not collapse into a black hole for the same reason that electrons in atoms do not occupy the same orbits around the nucleus.”
According to Dr. Winn, a white dwarf is a giant, planet-sized atom that has as much mass as a star.
In ordinary stars, the thing that prevents a gravitational collapse is gas pressure. The higher temperature and density toward the center of a star lead to increasing pressure with depth, and the resulting outward force opposes gravity’s inward pull, which allows the star to reach a sort of balance in terms of forces.
“When a star runs out of nuclear fuel, the radiated energy isn’t being replenished anymore, so the pressure drops,” Dr. Winn said. “When the core is compressed to a high enough density, quantum theory becomes relevant. The star comes up against the Pauli exclusion principle: No two electrons can occupy the same exact quantum state—the same position, energy, and angular momentum.”
In a white dwarf, the Pauli exclusion principle creates a pressure that pushes the electrons apart and keeps gravity at bay. So where do white dwarfs come from?
After a period of about 10 billion years, a star like the Sun has converted its core entirely into helium. Fusion stops, gas pressure plummets, and the core contracts, which makes the star’s density rise unless it causes the core to heat up.
“This is a remarkable and seemingly paradoxical property of all stars,” Dr. Winn said. “When they lose energy, their cores heat up. Eventually, the core gets dense and hot enough to ignite helium fusion.”
Through a series of unstable nuclear reactions, the outside of the star swells and it becomes a giant. The outer layers become unbound from the core and expand throughout the universe, while the exposed core eventually becomes an inert ball of carbon and oxygen.
“That’s a white dwarf,” stated Dr. winn.