'Burnt-out star forms nursery life'

At first glance, the solar systems of white dwarfs, the white-hot remnants of sun-like stars, do not appear to be the most suitable place to harbor an Earth-like planet. However, appearances can be deceiving, says astronomer Eric Agol.

What are White Dwarfs?
Over their long life, stars like the sun gradually convert their hydrogen into helium, releasing gigantic amounts of energy - enough per gram of hydrogen to power a small family's electricity consumption for about a century.

A white dwarf (center) is much smaller than the sun (right) but much hotter.

After a few billions of years (with the sun: another five billion years, so don't join a doomsday cult yet) the hydrogen is gone and a huge red giant is formed (imagine: the sun that swallows the earth), which after millions of years by one great explosion repels its red-hot mantle.

It forms an often spectacular planetary nebula, while the white-hot core remains.

The matter in this nucleus is extremely dense and is called electron liquid, because due to the enormous gravity atoms no longer exist and atomic nuclei and electrons roam together. A teaspoon of electron liquid has a mass of a thousand kilograms: white dwarfs are about the size of Earth but contain the mass of a star. They are very hot: tens of thousands of degrees, but have a very small surface in relation to their mass, so that they radiate relatively little energy.

As far as we know, the red giant phase followed by the great explosion that blows away the star mantle leaves little to nothing about planetary systems. Planets close to the star are gobbled up or boiled dry by the red giant. Planets that survive this will be hit hard by the final explosion. In short: not exactly a pleasant environment for life.

The habitable zone of a white dwarf is very small, but it remains comfortable for billions of years.

However, once the white dwarf has formed, it creates a stable zone in which life can develop and which, according to calculations by astronomer Eric Agol of the University of Washington, remains capable of sustaining life for about three billion years.

These are planets that are very close to their star: 0.3 to 1.5 million kilometers, one hundredth AU (an AU is the distance Earth-Sun) or one to four times the distance Earth-Moon.

Due to the enormous tidal forces, such a planet quickly loses its rotation and has an eternal day, with the sun always in the same place in the sky. The night side will be covered with thick layers of ice. A year would last a day or even a few hours.

So astronomers in this world should brave the bitter cold of the night side and build good tracking scopes.

The planet would remain habitable for about three billion years, according to Agol's calculations. Life on Earth originated much faster, so in principle such a planet should be able to develop life. As the white dwarf cools, the planet will gradually freeze.

Agol thinks that planets around a white dwarf are fairly easy to find, because white dwarfs are the size of an Earth-like planet, so covering a planet will quickly reduce the light intensity. However, the existence of a planet so close to the heart of a former star (to give an impression: if the earth were at that distance from the sun, the sun would occupy a quarter of the sky) the migration of a distant planet that survived the red giant phase.

An extremely rare event, astrophysically speaking, but not impossible. In our own solar system, some say the migration of planets explains the curious rotation of ice giant Uranus.

Sources
ArXiv Blog
ArXiv

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