If we assume their homeworld is similar to Titan, but instead orbiting a Neptune sized gas giant and a bit bigger and receiving the same amount of Solar energy as Titan, The star is a main sequence G type star, and the moon has a surface gravity of 0.28 G's. The atmosphere isn't smoggy like Titan so more light reaches the surface. What wavelengths are they likely to see in? If they came to Earth and observed the planet would they be able to see it like Humans do? Also would it be possible for them to have eyes that resemble some of Earth's species?

  • $\begingroup$ I'm not really sure where to start in regards to trying to come up with an answer for this. I'd say it depends a lot on to many conditions. Assuming they would have any similarities to us, look at creatures that live in dark places. If they have eyes, they are generally large and sensitive, so earth would be too bright. Unless there is warming from the planets radiation it is going to be very cold, so earth would be too hot. Honestly, I'd say use your imagination, think things through, and go with that. You're as likely to be right as anyone. $\endgroup$
    – AndyD273
    Jul 26, 2016 at 5:22
  • $\begingroup$ If this exomoon planet receives the same insolation as Titan, it must the same distance as Saturn from the Sun. Roughly a light hour away from its primary star. $\endgroup$
    – a4android
    Jul 26, 2016 at 9:19
  • $\begingroup$ Stephanie, I am grateful for your question. It helped solve a problem I had with aliens who inhabited a gas giant moon not at all too dissimilar from Titan and at a similar distance from their primary star. I couldn't work out what their vision would be like, however, having to think about your question gave me my answer too. Many thanks and an upvote for luck. It's the least I can do. $\endgroup$
    – a4android
    Jul 29, 2016 at 6:52

1 Answer 1


The amount of light will be 0.0178 of the light received by the Earth. Less than 2%. If vision is in the optical range, their eyes will have to maximize their light catching capacity. Basically they will be very big. Visiting Earth would be blindingly bright for big eyed aliens.

However, there is an alternative vision scheme. This was proposed earlier for another answer, but it certainly fits here.

This article by Robert Freitas called "Extraterrestrial Zoology", originally published in Analog, July 1981, has an interesting discussion of alien senses.

Vision, of course, is simply the detection of one narrow set of wavelengths of light within the entire electromagnetic spectrum. One alternative to “visual” sight is infrared (IR) vision, or seeing with heat waves. The rattlesnake is quite good at this – the creature has two imaging eyeballs operating in the visible, and two conical pits on either side of the head which permit binocular IR sensing of temperature differences as little as 0.002 °C. The theory of optics predicts that alien infrared eyeballs with resolution close to that of the human eye could have apertures as small as 4 centimeters at 93,000 Angstroms (the peak wavelength of black body radiation emitted by a warm human body). This compares well with the size of the eye of the Indian elephant (4.1 cm), the horse (5 cm), the blue whale (14.5 cm), and the largest cephalopods (up to 37 cm).

The fact that rattlesnake have binocular IR sensing and that optics can allow for IR eyes having the same resolution as the human eye, then alien organisms living on a low light planet would be able to see quite well if they evolved infrared eyesight. The ability to see such remarkably small heat differences means excellent vision even on a cold planet. Visiting Earth would be blindingly bright for infrared eyed aliens too.

Aliens with infrared eyesight would look grotesque. There would be large holes in their heads where we have eyes.

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    $\begingroup$ The animals on such a world would be adapted to seeing things around the ambient temperature, not Earthly temperature. A warm human body would be like a iron smelting pot is to us. $\endgroup$
    – JDługosz
    Jul 26, 2016 at 8:11
  • $\begingroup$ Large holes meaning there would be no eyes? or no visible pupils just black? $\endgroup$
    – Stephanie
    Jul 26, 2016 at 9:34
  • $\begingroup$ Neither. The holes are their eyes. The thermoreceptors line what are effectively hollow spaces. our eyes are filled with the vitreous humour which is jelly, but it would absorb infrared. Infrared eyes look like hollows because they are overgrown versions of the rattlesnake's heat sensing pits. Look up details of rattlesnake heat pits and beef them up to suit your aliens. $\endgroup$
    – a4android
    Jul 27, 2016 at 8:31
  • $\begingroup$ Pit viper infrared sensors are open pits because a) IR is absorbed by water, and b) they have to be kept cool to avoid interference from the snake's own body heat. Neither of those apply to a methane-based creature on a cryogenic world. There's no reason they couldn't evolve eyes structurally just like ours. $\endgroup$ Jan 6, 2023 at 17:16

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