World of the White Dwarf

No, not the account of the life of a pale-skinned person of restricted growth but thoughts on the possibilities raised by the concept of life on the planet of a white dwarf star as discussed in Ken Croswell’s article in the 2 July 2011 issue of New Scientist.

An argument in favour of this idea is that white dwarves are very stable. No nuclear reactions are taking place; they are just slowly cooling lumps of degenerate matter. They will spend billions of years at a temperature approximately that of our Sun, so a planet at the right distance will be at a stable temperature long enough for life to evolve.

How likely is it that such a star will have planets at all? The innermost planets will have been swallowed up when the star left the main sequence, becoming a red giant. Outer planets would have been vapourised in the giant’s death throes. So it seemed reasonable to assume that white dwarves are planetless.

Then, twenty years ago, planets were discovered orbitting a pulsar. Since pulsars are the remnants of supernova explosions, all planets of the star should have been obliterated. It seems likely that these planets were formed from the disc of dust and gas that surrounded the pulsar after the supernova.

Could the same thing happen around a white dwarf?

Giant stars which end in supernova explosions produce large quantities of elements up to iron in fusion reactions during their lifetime – and elements beyond iron in the supernova explosion so the resultant gas cloud will contain plenty of heavy elements that could condense into rocky planets. Stars like the sun do not get beyond fusing helium into carbon – and most of that will remain in the core to form the white dwarf when its outer layers are shed so the gas cloud will be largely hydrogen and helium plus whatever trace lements had existed in the gas cloud that formed the original star plus material from the original, swallowed up, planets.

The next potential problem is that any planet that forms at a distance that will be suitable for life when the stellar temperature is Sunlike will be superheated in its formative years – at this stage the white dwarf will have a surface temperature of ~20,000K (and in fact will be glowing blue) so such a world will wind up like Venus or perhaps lose its volatiles altogether.

However, it is now believed that planets can migrate inwards from their formation orbit – this accounts for the existance of ‘hot Jupiters’ so perhaps our world could do the same – the remnants of the gas cloud in the early stages of planetary formation might cause the orbit to decay by friction.

So, possibly against the odds, we have an Earth-like world orbitting 0.01 AU from a ‘yellow’ white dwarf (white dwarves are typically one tenthousandth the brightness of the Sun). What will this world be like?

At only 1.5 million kilometers from its star, the planet will be tidally locked, so life is most likely to be found in its twilight zone, where the star is close to the horizon. It would then seem permanently reddenned, just as our Sun seems redder at sunrise and sunset. This permanently red light may have some effect on plant evolution – but that may be to life’s long term advantage as we shall see later.

Furthermore, the planet would whizz round a solar mass white dwarf in a shade under 9 hours. Unless there were particularly bright stars in the vicinity of the planet, this motion would be undetectable to intelligent observers on the planetary surface.

Croswell points out the the closeness of the star would mean that it’s gravity would ensure that the planet had no axial tilt and a perfectly circular orbit – which would mean no libration and the sun would remain perfectly static. Hence places in shadow (in a mountain range say) would be permanently so. There would thus be a fascinating range of microclimates in quite small areas.

The planet would not be able to retain a moon but if the dwarf has more than one planet, the others would be visible as discs. A planet only a little larger than our own Moon orbitting at about 0.007 A.U. would be large enough to generate solar eclipses and if they orbitted in the same plane, would do so every conjunction – every 13 hours or so. If the inner world were as large as Mercury the eclipses woild last several minutes – long enough for observers to become aware of the stars. This 13 hour period between eclipses would give intelligent observers a measure of the passage of time and I am going to refer to it as a ‘xenoday’. Post-eclipse, the planet would wax – but show a decreasing angular diamiter until when it disappears behind the sun it will be close to full but only about a quarter the diamiter of the Moon as seen from Earth.

A planet the size of our Moon but orbitting at 0.015 A.U. would also appear as a disc – about half the apparant diamiter of the Moon seen from Earth at opposition, shrinking to about a tenth the size as it dissapears behind the sun. It will be eclipsed by our posited habitable planet every opposition if it orbits in the same plane. I think, as well as giving intelligent observers a measure of the passage of time, it will be clear that their world is the second of three orbitting their sun.

Measuring the distances of these other planets will not be possible unless observatories are established in the nightlands so that stars provide a background for the measurement of parallexes. When this has been done, if the astronomers have determined the relationship between distance from the primary and orbital period, they will be able to calculate their own distance from their sun.

Here on Earth, stellar distances were first measured using the diamiter of Earth’s orbit as a basis for parallex measurements. By this means, stellar distances as far as 150 terrestrial lightyears can be directly measured. Because of their world’s much smaller orbit, our alien astronomers will only be able to measure parallexes of stars within 1.5 terrestrial light years. Unless they are in a particularly densely populated region of the galaxy, they will not be able to measure the distances of any stars at all. It will not be until they establish telescopes in space that they will be able to measure stellar distances at all. Knowing that stars are at least 9 million times as far away as their own sun, yet are visible to the unaided eye, will tell them that all the stars they can see without telescopes are thousands of times brighter than their own sun. It will not be until they discover other white dwarves that they will find ‘sunlike’ stars. They will likely not search main sequence stars for planets at all – they are very difficult to find. An Earth-szed planet eclipsing a Sun-like star will reduced its apparant bightness by 0.01% once every 700 xenodays or so whereas such a planet orbitting a white dwarf will reduce it’s brightness by 100% once per xenoday or so if they are directly in line. Furthermore, such astronomers might well conclude that main sequence stars, with their variable ouput and flares, cannot possibly support life-bearing planets.

As a final thought – how long will life last on such a world? As the white dwarf cools and reddens, the twilight zone will become too cold for life. However this will be a very slow process and the life zone will move into the daylands. Over tens if not hundres of billions of years, the life zone will be a slowly contracting circle centred on the equatorial noonday point. The last and hardiest organisms will freeze here beneath a dully-glowing, barely red ember.

Tags: alien civilizations, extra-solar planets, white dwarf stars