# If Thorne–Żytkow Objects Exist, How Far/Wide Would Their Habitable Zones Be?

First proposed in 1977 by physicist Kim Thorne and astronomer Anna Żytkow, a...you know what? Let's just call it a TZO, sounds easier that way. Anyway, a TZO is supposedly a giant or even a supergiant that has swallowed up a neutron star and merged it as the larger star's core. This is a neat idea, as neutron stars are all that remains of a supernova that have become super compressed--on average, a 12-mile-wide object that is half more massive than our own sun. And despite being monstrous, TZOs can live as long as our sun can, and this paragraph from Astronomy explains why:

Ordinary red supergiants, like other stars, are powered by nuclear fusion in their cores. So when that energy runs out, their uncontested gravity causes them to implode before erupting as a supernova. But TZOs can live such long lives because they do not rely on sustained nuclear fusion in their cores to avoid collapse. Instead, a TZO’s neutron star core, which is already extremely compressed, largely prevents the rapid and uncontested gravitational collapse of the surrounding supergiant layers.

Astrobiologists have been looking for stars that are bright enough and long-lasting enough for life to thrive on any planet orbiting that star within a zone suitable enough for surface liquid water. The reason that TZOs were never the focus on that topic is that their very existence remains questionable.

But what if they DO exist?

In this scenario, there are plenty of Earthlike planets orbiting one TZO (starting with one for simplicity's sake) that fit all the criteria for making life possible--plate tectonics, liquid surface water, atmosphere, magnetic field--except that all of them are lacking that one crucial ingredient: Life itself. The star in question is 1200 times brighter than our sun. So in that respect, how far and how wide would the habitable zone of this hybrid star be?

• I'm not a cosmologist but in order to be habitable you'll probably want a fair portion of the planet to have a surface temperature of between 40 and -40 C. However, the planet's temperature will depend on how much light is given off by the TZO (which will be impacted by what elements make up the enclaving gas giant, how large the gas giant is, and how large the star is), as well as the atmosphere of the planet (which might be malleable). If you can figure out how much of the star's light is absorbed by the gas giant, you can figure out how much energy via light the TZO gives off. Apr 14 at 15:50
• At that point, light spreading follows an inverse-square law so you can figure out how much energy in light hits the planet based on the planet's size and distance from the TZO, and then how much of that is absorbed and trapped in the atmosphere based on the atmospheric makeup. Note that this can provide a broad range of results because of all the variables. You'll probably want your planet to have a similar surface gravity to Earth, so that may limit its size somewhat. Apr 14 at 15:55
• Apr 14 at 23:18

It's quite difficult to distinguish a Thorne–Żytkow object from an ordinary red giant or supergiant. The defining characteristics involve spectral lines from lithium and heavier metals (strontium, rubidium, etc. - see Levesque et al. 2014). While that's unfortunate for observational astronomers, it's fortunate for us, as the habitable zone should be essentially identical to that of a red giant or supergiant of corresponding luminosity.

A luminosity of $$L=1200L_{\odot}$$ is actually quite low for a TŻO. Since Thorne and Żytkow's original paper, it's believed that these objects should have luminosities in the range of $$L\sim10^4\text{-}10^5L_{\odot}$$. As the effective temperature of a planet scales as $$T_{eff}^4\propto L/D^2$$, with $$D$$ the orbital radius, we can conclude that the habitable zone around a TŻO should be centered somewhere between 100-300 AU, with a width of perhaps tens of AU or so. Were we to use your luminosity value, the zone would instead be centered around 35 AU.

That said, the lifetimes of Thorne–Żytkow objects are believed to be on the order of 1 million years or so, which is not long enough to allow life to develop from nothing (the statement in the question that they live as long as the Sun isn't correct!). On the other hand, the progenitor star of a TŻO should already have spent millions to billions of years on the main sequence and time on the horizontal and red giant branches, so there has been plenty of time for the ingredients for life to come together well in advance.

Of course, this naïve calculations fails to take several things into account. For instance, the neutron star must have formed as the result of a supernova, which could have ablated away much of the planet's atmosphere. Additionally, TŻOs have cool surfaces, with peak temperatures of perhaps ~3000 Kelvin, so photosynthesis would be harder, with less light near the peak absorption points of chlorophyll.

What do you mean by the habitable zone? Do you mean a zone where an otherwise suitable planet could be habitable for lifeforms similar to Earth life, the more general case, or a zone where an otherwise suitable planet could be habitable for humans and beings with similar enviromental requirements, the more specific case?

The way to find the inner and outer edges of a star's circumstellar habitable zone is simple, you find the star's luminosity relative to that of the Sun, and then multiply the inner and other edges of the Sun's circumstellar habitable zone by the square root of the luminosity difference between the star and the Sun.

So what are the distances from the Sun of the inner and outer edges of the Sun's circumstellar habitable zone?

This list includes a number of estimates of the inner or outer edges, or both, of Sun's circumstellar habitable zone:

https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates[1]

Note how much some of them disagree with other estimates. The narrowest habitable zone from combining two estimates would be only 0.014 AU wide, from 0.99 to 1.004 AU, while the widest habitable zone from combinng two estimates would 9.62 AU wide, from 0.38 to 10 AU.

I note that the only estimate in that list that is specifically for the special case for planets habitable for humans is that of Dole, 1964, from Habitable planets for Man, Stephen H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[2]

I think that all of the later estimates are for the more general case of liquid water using lifeforms.

Some of the more extreme inner andouter edges are calculated to be valid only for planets with specific types of atmospheres, for example. Some of the more distant outer edges are calculated for planets with high hydgrogen content in their atmospheres, which is inconsistent with the high oxygen content necessary to be habitable for humans.

So an optimistic writer could estimate the Sun'scircumstellar habitable zone as large as 9.62 AU wide and multply by the square root of a star's luminosity difference, while a more pessimistic writer, or one more worrid about their story later being proved to be impossible, could estimate the Sun'scircumstellar habitable zone as narrow as 0.014 AU and multply by the square root of a star's luminosity difference.

And you should check out the Ultimate Solar System section of the PlanetPlanet blog, the section where Sean Raymond designs imaginary solar systems with as many planets in the habitable zone as possible.

https://planetplanet.net/the-ultimate-solar-system/[3]