# What types of stars do not have a CHZ/Goldilocks zone?

Most stars have a continuously habitable zone (CHZ), but I'm specifically trying to think up ones that do not. Aside from pulsars and black dwarfs (theoretical, as the universe isn't old enough to have one yet, but included both for argument's sake and because this is for a sci-fi setting), can you think of any types of stars that would havd a 0 CMZ?

Edit: corrected CHZ to reflect what I am more appropriately looking for. Continuous means the star lives long enough to support complex life and that the zone remains habitable throughout the star's main-sequence life span.

• This question should be moved to astronomy.stackexchange.com Sep 22, 2019 at 1:38

Not in normal cases, but you might find some on the extremes.

Consider a pulsar for example, it might have a habitable zone, but it wouldn't be overly friendly to humans.

Although pulsars regularly blast out deadly gamma rays and X-rays, alien planets in orbit around them could theoretically be habitable, a new study finds.

The researchers noted the habitability of planets around pulsars depended on at least two key findings. For one, atmospheres a million times thicker than Earth's are likely needed to protect any life on their surfaces from pulsar radiation. This would make conditions on these worlds similar to ones found in Earth's deep seas.

Or something cool enough to have the CHZ very close to the star, that then periodically flares up to such an extent that it roasts everything.

On August 13, 2017, the Next Generation Transit Survey (NGTS) telescope spotted an intense solar flare from a tiny star barely bigger than Jupiter. But despite this sun’s diminutive size, the flare gave off as much energy as 80 billion megatons of TNT. That’s 10 times as powerful as the strongest flare ever observed on our own sun. It’s also the coolest star ever observed to give off such a hot flare, and the spectacular outburst is teaching astronomers the power of small stars.

If you include the criterion that planets must be able to stay in the CHZ long enough for life to evolve, then any star more than about four times the mass of the Sun would qualify — class O and B stars, roughly speaking. Those stars will only live for a few hundred million years at best.

Brown dwarfs (BDs) are a good candidate for planets not lasting long in the habitable zone. BDs burn deuterium but not hydrogen. That means that, once the deuterium is gone, BDs just cool off in time. They have habitable zones that simply sweep inward. Here is an example:

A planet on a fixed orbit will start off too hot, then enter the habitable zone, and then leave it. Different-mass brown dwarfs evolve at different rates, such that planets have a range of maximum habitable zone lifetimes, which are typically 100 million years to a billion years.

On a side note, the concept of the "continuously habitable zone" needs some numbers on it to be useful. Stars brighten slowly during the main sequence because the mean molecular weight of the core increases as H becomes He, and this moves the habitable zone slowly outward (more on that here: https://planetplanet.net/2019/02/13/how-planets-die-roasted-toasted-and-swallowed-by-their-evolving-stars/). But all stars die eventually (even red dwarfs, although it probably takes hundreds of billions to trillions of years).

The Goldilocks zone is that region of space around a star where the energy flow is sufficient to allow conditions compatible with life.

Since energy flow diminishes with the square of the distance r, $$I(r)=I_0/r^2$$, the only way for not having a goldilocks zone at a certain distance is to have the star emitting already less than the bare minimum to sustain energy flow needed to sustain life.

Any other star will have a distance at which the energy flow has the right value.

• But it is not just the energy emissions that determine the chz. The cabonite-syllicate cycle also affects the chz. Also, I edited as you were responding I guess, but I meant a "continuous" habitable zone, or one that exists a) long enough to support conplex life and b) exists throughout the star's main sequence lifespan Sep 20, 2019 at 7:13
• The cabonite-syllicate cycle affects if life can exist on a planet, but other than that is not related to the CHZ/Goldilocks Zone, which is determined only by the amount of energy a planet gets from a star. Sep 20, 2019 at 7:21
• @Mark Ripley I was lead to understand that the c-s cycle does affect the goldilocks zone, causing most to be moved farther out from stars than previously determined solely based on their energy output. Sep 20, 2019 at 7:37
• @HA Harvey You may be correct that the composition of the planet affects the goldilocks zone; for example, some planetary heat occurs due to radioactive decay, which would allow life to occur in larger orbits because some of the energy required would not be needed from the orbited star. Sep 20, 2019 at 7:45

Not possible. The only way such a thing could exist would be if there were a type of star which emits so little radiation (in any part of the E-M spectrum) that even a planet immediately adjacent to its surface would not be warmed to a temperature of 0C.