The Forensic Astrophysics of Superman

Kal-El, native of the planet Krypton, came to Earth and was adopted by Jonathan and Martha Kent. Being a Kryptonian, he found that he had superhuman powers and used them for good as Superman[1].

We all know the story. But what do we know about Krypton? Any information would be a dramatic coup for astronomers of all varieties. Superman couldn't tell us, he was just an infant when he left[2]. We might have to do some deduction. I propose we start by trying to determine the orbital period of Krypton - in other words, how long a Kryptonian year was.

To do this, we need the equation for orbital period. Without pausing for derivation, it's:


What we don't have is r, the orbital distance from Krypton to its sun (which happens to be named Rao), and M the mass of Rao. But I think we can estimate them. Rao is a red giant star[3], and those tend to put out a total amount of light that is much higher than the sun. Arcturus is a red giant of presumably similar classification, and has a total power output around 180 times that of the sun.

So clearly Krypton will have to be much farther away from Rao than the Earth is from the Sun, assuming the Kryptonian biosphere requires a similar level of heat and light as Earth's life. This is a fairer assumption then it sounds: a depowered Superman is able to survive on the Earth on several occasions with no trouble at all[4].

Fig 1: Not Krypton's star, but one of the same kind

Radiant intensity falls off as the distance squared. To reduce a power output of 180 times the sun to the same as the sun, you'll have to increase your distance by a factor of the square root of 180. This happens to be about 13.4. That times the orbital radius of the earth is about 2 billion kilometers. This is a little farther than the orbit of Saturn.

So we're half done. What's the mass of Rao? Here we're in sketchier territory. I assume it's low mass by red giant standards to give it as long of a lifetime as possible given what we know in footnote 3. I'm going to give it half a solar mass, though red giants can exist as massive as around 6 solar masses. Bigger and they tend to be supergiants, another creature entirely. My 0.5 solar mass estimate has its problems but I think it's about the closest we can get.

Plugging all that into the equation and I get a period of 69.1 years. It's a very rough estimate based on shoddy information, and even if it's accurate it doesn't tell us all that much. But it might tell us some things. Life on earth is well adapted for the seasons which occur about once every 365 earth days. Assuming Krypton has an orbital tilt, how might life react to seasons which last for decades of earth time? It would be an interesting kink in the biology of Kryptonian life.

Fig 2: An exobiological type specimen

Maybe something a Daily Planet reporter might want to explore in the science pages. Especially if he happened to have some personal experience[5] in the field...

1. A lot of people think Superman disguises himself as Clark Kent. It's the other way around. Clark Kent psychologically thinks of himself as Clark, not Superman. Clark is who he is, Superman is what he does. (Batman is the opposite. Batman is who he is, Bruce Wayne is a convenient disguise.)

2. Depending on who's writing the story, his spaceship may have contained a wealth of information. But we'll assume we're working on a version of the Superman mythos without this knowledge, or that it's inaccessible to Earth scientists.

3. The Sandman: Endless Nights explicitly gives that classification for Rao, though the timeline seems sketchy. That story seems to happen around 3 billion years ago, and red giants burn fuel too quickly to continue existing until Krypton exploded 3 billion years later. Rao as a red dwarf seems more plausible, but I'll defer to the story and assume I'm misinterpreting the timeline.

4. Some old comics also have Earth people visiting Krypton via time travel. The temperature and atmosphere seem to suit them fine, but Krypton's gravity is stronger than earth and unprotected humans can't tolerate it at all. Which, incidentally, probably puts the acceleration due to gravity at Krypton's surface at about 5-20 times that of Earth, very roughly.

5. How is it that no other Kryptonian happened to be off of the planet when it exploded? It's a long story.

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Just curious, why do more massive red giants burn out more quickly, and is that also the case for other types of stars?

Yes, generally. More massive stars have a greater gravitational pull holding them together. This causes increased internal pressure and thus more nuclear fusion to maintain the balance between the the hot gas trying to expand and gravity trying to compress. Keeping up that high rate of fusion means burning through the nuclear fuel more quickly.

Thing is... a red giant is not a TYPE of star, it's a PHASE of stellar evolution. And one that lasts a for comparatively short time late in the star's life. A planet that was habitable in a 69 year orbit around a red giant star would have been something like -300ºF for the billions of years that the star was on the main sequence.

A follow on from dr's point would be how would the Kyptonian's have been able to evolve comparatively quickly, as Rao would have been nearing it's life.

I think Hunter/Prey and it's implications gives us a pretty good idea.

However, SN 2005gl shows that we may not be as clued up on stellar evolution as we once thought

Nice job! Came here from Unqualified Offerings. This is exactly what I try to do - use the comics as a hook, and then employ sneaky ninja physics to slip is some real learnin'!

It's always fun to see how many people correct aspects of my discussion concerning the gravity on Krypton, pointing out how to improve the calculation. But then they always get to the same phrase: Look, it's just not realistic that.... - forgetting that we are talking about the Last Son of Krypton, coming to Earth with powers and abilities far beyond mortal men! But the point is not to correctly determine the gravity of a fictional planet, but to use the fictional planet as a venue to discuss gravity and Newton's laws.

Keep up the good work.

Your Friendly Neighborhood Physics Professor,


Krypton was actually assigned to the Antares star system in at least one work (the novel Superman: Last Son of Krypton by Elliot Maggin).

Also, human life isn't native to Krypton; it was settled a relatively short time before the planet exploded. 5,000 years total inhabitancy, perhaps? Not long compared to stellar phases. That doesn't help with the endemic lifeforms, though.

There are other serious astronomical problems with Krypton, of course. For example, it's claimed that gold is more common than iron (and that gold is thereby as cheap as any base metal). How metal-rich must its sun be, and how ridiculously shortened its lifespan? How do you even build an Earth-like planet (with 3 moons) from an accretion disk with that kind of elemental composition? You'd end up with a Jupiter-sized solid planet, with surface gravity of, what, 250 gee or so?

One point I think you may be missing is that under a red sun, Kryptonians have no extra-baseline abilities. They're just normal humans. That's been a constant, established sometime in the Golden Age of comics, I believe, and kept through all subsequent revisions.

Therefore, it doesn't really matter that a depowered Superman can easily flourish on Earth. His superhuman powers aren't really artifacts of his extraplanetary origin, and they have nothing much to do with Krypton itself. This has to be true; if Krypton were as radically different an environment as it would have to be for, say, a difference in native gravity field to adequately explain Superman's enormous physical strength and durability, it would be impossible for Earth type carbon based humanoids to live there, and even more absurd to posit that they had evolved there (although, as another commenter points out, various stories have also established that both Krypton and Daxam were 'seeded' with humanoid life, which presumably either evolved or was originally created elsewhere).

My own presumption is that, given how many times we've seen non powered Earth humans visit a pre-apocalypse Krypton without needing any kind of protective gear, Krypton is simply another Earthlike planet, albeit one that orbits a red sun. To explain Superman's powers, as well as those of other Kryptonians, and those of races like the Daxamites, who have similar powers to Kryptonians but who were (at least, at one time) defined as coming from a solar system with a different colored sun (orange, I think), I use an entirely different hypothesis.

With Kryptonians and Daxamites, we have entire species of human beings who have no extra baseline powers under their native suns, but who exhibit frankly astounding levels of superhuman performance when they are removed from those systems. There have also been various stories published depicting humans from Earth who have no superpowers under a yellow sun like Sol, but who gain Kryptonian/Daxamitesque powers under the influence of other kinds of sunlight. This is pretty bizarre, but seems to me to pose a vital clue to what is actually going on... namely, at some point in the very distant past of the DC Universe, some godlike being or beings created several different races of superpowerful humanoid beings, and to keep those humanoid beings from getting out of hand, they built into each of those beings different triggers that would effectively 'turn off' their powers.

This would allow these beings to safely stash their superpowerful action figures in various 'toy boxes' around the galaxy, only taking a select few of them out to play with every once in a while. This also provides a rationale for the destruction of Krypton; either the creators/owners of the Kryptonian 'soldiers' got tired of the prototype and decided to get rid of it, or a rival godlike being did the honors.

But, again... the various planets that the various humanoids of the DC Universe live on have little or nothing to do with the superhuman powers those humanoids exhibit when they are removed from those planets. Apparently the powers are always there; they are simply in some cases deactivated only by sunlight of a specific spectral frequency, or in other cases, activated only by other specific solar frequencies. The planets themselves should be largely similar... although, yeah, if the suns themselves are different, the orbital distances that would support Earthlike human life would doubtless vary.

Very interesting.

In the Silver Age, the Kryptonian Year was established as 1.39 Terran Year (Superman #157, Nov. '62, see M. Fleisher, The Superman Encyclopedia, p. 130).

hey i was just wondering if youve seen the 'unified theory of superman's powers' any input? its quite interesting and fun, gives a somewhat convincing theory about superman's powers...