Life after stellar death

Occasionally one hears of someone proposing what looks at first sight like a really improbable project. The latest one is an experiment to detect signs of life on planets orbiting white dwarf stars.

The instrument making such a project possible is the soon-to-be-launched James Webb Space Telescope. Optimistic is the word.

Long ago, astrophysicist Subrahmanyan Chandrasekhar calculated that in later life, if the core of a star exceeds 1.4 times the mass of the Sun, it would end its life in a huge explosion, a supernova. On the other hand, smaller stars, like the Sun would have a much more drawn out end; they would become white dwarf stars. This number is now known as "Chandrasekhar's Limit.” This means the Sun will end up as a white dwarf star.

Stars get their energy through nuclear fusion, where very hot, extremely compressed hydrogen becomes helium and other elements, liberating a lot of energy in the process. This is the origin of the heat and light that renders our planet inhabitable. For almost all stars, all the fuel they will ever have was acquired when they formed. This means that at some point in its life a star is going to start running short of fuel. Paradoxically, when the fuel shortage starts to bite, the star swells enormously into a red giant. Its brightness increases by hundreds or even thousands of times. Such squandering of the last remaining fuel means this stage is not going to last long.

When the Sun reaches that point, Mercury and Venus will end up engulfed and Earth might be. In any case, it will be incinerated. The frozen planets and moons in the outer Solar System will become warmer and maybe suitable for life. However, the inevitable end of the Sun's fuel supply comes soon.

The outer parts of the Sun get sneezed off into space leaving the naked core, a white hot lump of nuclear waste products, mainly helium, about the size of the Earth, where a teaspoonful of this core material would weigh more than a tonne. It has become a white dwarf.

However, despite being white hot, the small size means the rate at which energy is radiated into space becomes fractions of a percent of what the original star produced. In the Solar System, the outer planets would become more frozen than before, and the inner planets, incinerated cinders, will now freeze.

Intriguingly, although the white dwarf has no fuel, it is so miserly with its energy radiation that it will take billions or tens of billions of years to cool off.

This raises an interesting possibility. A planet orbiting a white dwarf star would not suffer from space weather - disruption of technical infrastructure due to their star's bad behaviour.

A white dwarf star also shines with a fairly steady brightness for billions of years, providing in some ways a better environment than our ancestors had on Earth. The Sun has brightened steadily by about 30% since life first appeared on Earth and our terrestrial ecosystems had to adapt to it.

However, the problem is the size of a white dwarf's "Goldilocks Zone". This is the distance from a star where the heat would be just right for liquid water to exist on a planet's surface. Bright stars have quite deep Goldilocks Zones.

Very dim stars have very narrow ones, which lie close to the star. Our Solar System has no planets that will be just right when the Sun becomes a white dwarf. Even Mercury is too far. One would think that any planets close enough to become comfortable when their star becomes a white dwarf will have been fried by the star during its maturity and then incinerated and engulfed when the star became a red giant.

Prospects aren't good. However, there is only one way to know the sort of planets that orbit white dwarfs, and that is to look for them.

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After dark, Saturn and brilliant Jupiter lie low in the south, with Mars rising in the east. Venus, which is even brighter, rises in the early hours. The Moon will be Full on the 1st of October.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory in Penticton. E-mail: ken.tapping@nrccnrc.gc.ca. This is a regular column which appears every Friday in The Herald.