Feb 14 2022

Possible Planet Around White Dwarf

Published by under Astronomy
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Astronomers may have discovered a planet around a white dwarf, and more importantly the planet (if it is real) exists in the habitable zone of the system. This is not the first planet found around a white dwarf – the first was a Jupiter-sized planet found around a white dwarf 6,500 light years away. The find is being touted as indicating the possibility of life, because the planet is in the so-called Goldilocks zone, but that is actually not plausible for several reasons.

White dwarfs are the remnant cores of smaller suns, like the size of our own. When stars die they blow off their outer layers, leaving a hot core behind. If they are large enough that core collapses into a neutron star. And for the largest stars the core will collapse into a black hole. But for sun-sized stars the core remains a hot ember, about the size of a planet like Earth but with 60% or so of its original mass, so extremely dense. White dwarfs will burn hot for billions of years – so why is it improbable that any such planets close to a white dwarf will harbor life?

First, in the process of dying stars will first expand. Our own sun will expand, becoming a red giant, with the surface extending beyond the current orbit of Mars. Any planets in the inner solar system, therefore, will be consumed. Planets just outside this range will likely have any atmospheres stripped away. It is therefore very likely that any life in such a system would be destroyed in the process. It’s possible that a distant Jovian planet with an icy moon harboring life in a deep ocean may survive, but no planet with life living on the surface.

It is, however, possible that after the star becomes a white dwarf a planet in the outer reaches of the system may spiral in to a closer orbit. That is likely what happened here. If an Earth-like rocky planet settles into the habitable zone after all the fireworks, could life then evolve on that planet? I suppose it’s theoretically possible, but they would not have much time. White dwarfs are not generating heat, they are just glowing with the heat left over from their days undergoing nuclear fusion. Once formed they are steadily cooling down, so their habitable zone will get progressively closer and smaller. How long would it take for a white dwarf to become too cold to have a habitable zone? The first reference I found was Smithsonian Magazine, which wrote in an article about white dwarfs:

NASA estimates that the sun will stay a white dwarf for around 10 billion years. However, other estimates suggest stars can stay in this phase for 1015, or a quadrillion, years.

But this is incredibly misleading. These estimates are not, in fact, at odds with each other. Further, how long it would take for a hot object to cool through radiation is perhaps one of the most well worked-out aspects of physics, so how could there be such wildly divergent estimates? Astronomy Magazine has a more thorough explanation. Over around 10 billion years white dwarfs will cool until they are near the temperature of the cosmic background radiation (CMB). If they start at a temperature of 100,000 K, after 2 billion years they will be only 8,000 K. The habitable zone time window is therefore very short, from an evolutionary perspective. After 10 billion years they will be near the CMB temperature. The question is – when will they cool all the way to the CMB temperature? At the tail end we need to consider exotic sources of heat from quantum effects, and the fact that the CMB continues to cool as well. But this does not change the very short window for a white dwarf habitable zone.

I suppose it’s possible that if surface ice on a distant world thawed into a surface ocean once the planet settled into a close orbit, life could arise in that ocean, but it would not have time to evolve beyond the single-celled life stage before the ocean froze once more into a block of ice.

On the timescale of a civilization, however, the habitable-zone window would be significant, let’s say a few million years. This would be enough time to do some advanced terraforming and build a settlement. The fact that the world will freeze in a few million years is likely not much of a concern. But I’m not sure why a civilization would choose to do that, rather than terraform a planet around a star that will provide stability for billions of years. Further, the habitable zone of even a newly-minted white dwarf is small and close. The current planet candidate (it has not yet been confirmed) is 1/60th the distance from its white dwarf as the Earth is from our Sun. That means the planet is almost certainly tidally-locked with one side always facing the white dwarf. This isn’t necessarily a deal-killer, as ocean and atmospheric currents can distribute a lot of the heat and there could be a livable band around the terminator between the light and dark sides. But still – not ideal.

What kind of light does a white dwarf put out? Initially they glow with white-blue visible light. That’s good. But they also put out a lot of X-rays and ultraviolet light. That’s not-so-good. But perhaps that can be mitigated with planetary engineering – create a nice thick ozone layer. White dwarfs do not give off a solar wind, so that’s a plus – nothing to strip away the atmosphere. All things considered, not great but but somewhat feasible, for an advanced civilization.

There may be reasons why a white dwarf planet would be chosen as the site of a settlement. As I have discussed before, interstellar travel will likely always be extremely difficult, and even advanced civilizations may stick close to their home world. Perhaps there are slim pickings for habitable zones in their tiny slice of the galaxy. Perhaps the location is of some strategic importance.

I think the bottom line, however, is that, all things considered, white dwarfs are not good targets to search for planets that may have life on them. They will most likely not have live that evolved on the planet, except the transient beginnings of single-celled life. They may have been settled by an advance civilization, but then we should be looking for technological signatures. What is scientifically interesting about this find is simply more data points on how stellar-systems evolve. But we are obsessed with finding life, and so we focus on this tiny side-point.


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