Yes, my obsession with the Gliese 581 star system continues ever onwards. This time, however, I’m not talking about Gliese 581g! Yes, surprisingly, it is Gliese 581d that I turn my attention to for the duration of this post. Gliese 581d was discovered in 2007, but was quickly written off as not a potential candidate for harbouring life, due to its status as a super Earth (its mass is 5.6 times the size of the Earth), and its distance from its home star. Recent simulations, however, have shown that Gliese 581d lies just in the outer fringes of the habitability zone of Gliese 581, and with a large enough greenhouse effect, could easily support liquid water. 

Let us, for a second then, assume life exists on the surface of Gliese 581d. Further still, let’s assume that intelligent, sapient life exists on the planet. What would it look like to gaze up at the skies of Gliese 581d? What would its star look like in the sky? For one, it would be white, rather than the yellowish-white of our star. Its size, however, is a bit tricky to calculate. To do so, one must employ the small angle formula, which uses the principles of trigonometry to approximate the size of arcs in angles. This is the perfect formula for the size of objects in the sky, at least in astronomical terms, is measured in degrees of angular diameters. For instance, our star, Sol, better known as simply the sun, is about .52º. The formula, after I made some adjustments, was as follows:

θ=360º/2π*d/D

Where:

  • θ (the Greek letter theta) is the angular diameter
  • d is the diameter of Gliese 581 (the star) in kilometres
  • D is the distance between Gliese 581d and its home star in kilometres

With this formula, I simply plugged in the values to the equation, and got an angular diameter of .71º. In terms that we can comprehend, that means that the star in their sky is around 36.4% bigger than ours. Once again, that is a bit hard to visualise, so I did you all the favour of creating a photo comparison between our star in our skies, and their star in their skies. Here is the image as it appears on Earth: 

And here is the image again, this time as if it was taken on the surface of Gliese 581d, looking at Gliese 581:

And there you have it! The star of Gliese 581d, as it appears on its surface.