death of a star

Stars like the Sun have a limited lifespan that results in a series of cataclysms. In about five billion years, the Sun will run out of hydrogen fuel, causing its core to compress and its temperature to increase.

This would cause fusion in the outer layers, causing the Sun to swell into a red giant. At this stage, the expanding Sun will likely engulf Mercury, Venus, and possibly Earth.

Fate of outer planets

While inner planets are at risk of extinction, this distance, five to six times Earth’s distance from the Sun, could gain enough heat from the dying star to melt ice, potentially creating conditions suitable for surface oceans and life.

In our solar system, this scenario can be applied to Jupiter’s icy moons Europa and Ganymede.

However, this habitability range is very narrow. If these planets or moons are too close, their water will evaporate. As the star expands, the optimal habitability zone shifts outward.

The emergence of white dwarfs

After the red giant phase, the star sheds its outer layers, leaving behind a dense core known as a white dwarf. White dwarfs, although initially hot and bright, are incredibly small (about the size of Earth) and have limited thermal output.

For a planet to remain hot enough for liquid water to exist, it must orbit very close to the white dwarf, at a distance of about 1,496,689.92 kilometers, or about 1% of the distance from the Earth to the Sun.

Any nearby planets must have been destroyed during the star’s expansion, and any distant icy bodies that have now melted must be too far away to remain warm. The question arises: How can these planets migrate to the new habitable zone around the white dwarf?

Speaking at the 244th meeting of the American Astronomical Society, Juliette Becker of the University of Wisconsin-Madison highlighted the mechanism of tidal migration. This process involves a dynamic instability that pushes the planet into a highly eccentric, comet-like orbit, bringing it closer to the white dwarf. Over time, gravitational forces rotate around this orbit, keeping the planet at a constant, habitable distance.

Discovery of habitable planets

Detecting these planets involves observing transits as a planet passes in front of its star from our perspective. Although white dwarfs are not known to host many exoplanets, the James Webb Space Telescope (JWST) recently discovered two potential candidates. However, none of these planets passes through their own white dwarf, making direct observations difficult.

If the planet passes near the white dwarf, astronomers will be able to use transit spectroscopy to analyze the planet’s atmosphere. This method, which measures the absorption of starlight by a planet’s atmosphere, can detect the presence of water. Because of the small size and brazenness of white dwarfs, this method may be more effective than for ordinary stars.

Although water alone does not guarantee life, the possibility that previously frozen worlds around white dwarfs may become habitable opens new horizons for astrobiology. These worlds could provide an example of life after the death of stars, inspiring new research.