Hmm. Interesting. A white dwarf is about the size of Earth -- roughly 1/100 the radius of the Sun. However, a planet in a white dwarf's habitable zone must be about 1/100 the Earth-Sun distance. (∼0.006 AU to ∼0.06 AU.) This dual scaling cancels out neatly. A white dwarf in that planet's sky would appear similar to the sun in our sky.

White dwarfs are also indefinitely stable; they keep cooling over billions of years to become black dwarfs. Then, over >10^100 years, all elements heavier than iron will decay to 56Fe by various processes such as fission and alpha emission. All atoms lighter than iron combine by nuclear fusion reactions, building gradually up to 56Fe. All of this can happen via quantum processes at zero temperature. So they end as lumps of indefinitely stable cold iron.

Intelligent life can hang around white dwarfs for a long time. Good candidate star type for Dyson spheres.

A planet formed out of an accretion disk is generally a planet under continued bombardment from further accretion.

The only exception that we know of is our own Solar System. Where Jupiter acts like a vacuum cleaner to reduce material that might hit Earth. We knkw of no other system with both Earth-like planets and gas giants. Given how catastrophic the remaining asteroids have been for us (bye bye dinosaurs), this is unlikely to be a coincidence.

Unless we find a similar arrangement around a white dwarf, the time spent in the habitable zone isn't the only important factor for stability over evolutionary time frames.

"Dragon's Egg" by Robert L. Forward, published in 1980, is a hard sci-fi novel about intelligent life forms living on a neutron star. It's superb.

I'd never seek to deprive anyone of entertaining alien-life theories, but in general people wildly underestimate the factors that play into making Earth habitable. The only evidence for a habitable planet-type is the very precise conditions that comprise this one. Worse, it's unclear as to how much those conditions rely on the complete configuration of the greater solar system.

Meanwhile on the HN-equivalent on some planet around a white dwarf: “can life emerge around a yellow star?”

The orbital speed of such a planet would be extreme, something like 300 km/s.

This also means other planets (like Venus or Mars are to Earth) would be visible in the sky as perceptible disks.

suedo - science

and less fun than speculating about Gods preferenc in tooth care, paste or powder?

Everyone agrees, right!, that we need a several orders of magnitude greater power space based telescope, solely devoted to examining extra solar planets,and then just see what is there. No matter what, it will be very interesting,and generate data usefull in many lines of study. That, or there is an astronomicaly slim chance we will get lucky and someone out there is broadcasting on a frequency that we are look8ng at. Starship changes the equasion, and its time to get the job, done.Now. Think about it, the whole world will drop tools and watch, everytime a new planet comes into focus. Way better than what we are watching now, and will cost, a whole lot less. tick twitch mega goog tube, blarg!

Here's a (not completely flippant) answer: yes. At least in the case of Sirius B, and let's be hones, this is the case we care about, a planet that orbits it would be very close to Sirius A too. The distance between Sirius A and Sirius B is always between about 8 AU and 32 AU (1 AU = Sun-Earth distance). Since Sirius A is about 25 times more luminous than the Sun and Sirius B about 50 times dimmer, it is very likely that a planet orbiting Sirius B will receive most of its light/heat from Sirius A, and it would be somewhat comparable to what the Earth and Mars receive from the Sun.

compare Forward, Dragon's Egg (1980)

One unclear point: how would these planets form?

Stars go through the red giant phase before becoming white dwarves, so they will swallow all the planets close to the star. And blow away any remnants of the accretion disk.

White dwarfs have a surface, with 50km of crust. And they cool. They have a hydrogen atmosphere.

Life doesn’t take very long to appear, perhaps just a few million years. We keep revising the number down on earth.

It’s possible, and perhaps even likely, that most white dwarfs host life at some point in their lifecycle, when they’re cool enough.

Nice read! FWIW, I built a system the other day, its name, "whitedwf" :)

The end stage of stellar evolution cannot have life around it. There’s not enough water beyond the frost line to renew any planet around a white dwarf.

Depends how big his cory is

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