Weird Worlds: Gleise 581

A quick note before I jump into the meat of this week's post. From next week, I'm going to be on holiday for about two weeks. I'm going to try to queue up a couple of blog posts to self-update while I'm gone, but if something goes wrong I may not be able to do anything about it until I'm back in normal-internet land.

So this week, I thought I'd do another entry in my Weird Worlds series. This time about a planetary system which has gained quite a bit of media attention: Gliese 581. (Pronounced something like "glee-zeh", if you're curious.)

You may have heard of Gliese 581 discussed in the media with phrases like "second Earth" or "super Earth in the habitable zone" being thrown around. So how like Earth are we talking? Which part of the habitable zone? What sort of star is Gliese 581 anyway? Read on!

The Star

First the basics: Gliese 581 (which I will now refer to as Gliefeo because I am lazy and typing numbers is annoying it rolls off the tongue better) has four confirmed planets and two unconfirmed planets. For the sake of not getting too carried away I will focus on the four confirmed planets, although it was one of the unconfirmed ones which drew a large slice of media attention.

Gliefeo itself is a red dwarf star only about a third of the mass of our sun and a bit less than a third of the size. This means that to be within the habitable zone, its planets need to be significantly close to it than Earth is to Sol. Furthermore, the stellar environment these planets will be living with is very different to ours. Here is a nice Space.com article about it.

All of Gliefeo's planets were detected using the radial velocity method.


The Planets

As I mentioned above, Gliefeo's four confirmed planets are all quite close to their sun. In fact, as the image below (taken from my favourite exoplanet app) shows, all four are well within the orbit of Mercury (the grey circle). And the second image below (also taken from my favourite exoplanet app) shows two blue circles indicating radial distances of 0.1 AU and 0.3 AU (an AU is the distance between Earth and sun).

Gliese 581 and its four confirmed planets designated, from innermost to outer: e, b, c, d. The grey outline shows where the orbit of Mercury would be if Mercury's orbit were picked up and plonked around Gliese 581. The green annulus shows the habitable zone for Gliese 581 and both Gliese c and d are within the habitable zone for at least part of their orbits. Image from Exoplanet iOS app by Hanno Rein.

A representation of the Gliese 581 system. The inner blue outline denotes an orbit of radius 0.1 AU and the fainter outer circle (click to enlarge) shows an orbit of radius 0.3 AU. The four planets from inner to outer are designated e, b, c and d. Image from Exoplanet iOS app by Hanno Rein.

Some technical details on each of the planets (all taken from the Extrasolar Planets Encyclopaedia) :

• Gliese 581 e:
• Furthest distance from Gliefeo: 0.03 AU
• Length of year: 3.15 Earth days
• Mass: 1.94 Earth masses
• Gliese 581 b:
• Furthest distance from Gliefeo: 0.041 AU
• Length of year: 5.37 Earth days
• Mass: 15.64 Earth masses = 0.91 Neptune masses = 1.08 Uranus masses
•  Gliese 581 c:
• Furthest distance from Gliefeo: 0.07 AU
• Length of year: 12.9 Earth days
• Mass: 5.36 Earth masses
• Gliese 581 d:
• Furthest distance from Gliefeo: 0.22 AU
• Length of year: 66.8 Earth days
• Mass: 7.09 Earth masses = 0.41 Neptune masses = 0.49 Uranus masses

The last two planets, c and d, are the most interesting because of their location in the habitable zone.

Habitability

So are the two outermost planets of Gliefeo habitable? Well, probably not. In fact, chances are, none of these planets are able to support Earthlike life. Ignoring the issues with the star itself, as mentioned in the Space.com article I linked above, there are problems with all four planets.

Gliese 581 e is the closest to Earth in mass which, depending on its size (and composition) could mean that we could safely walk upon its surface from a gravitational point of view (well, safely is relative, but we probably wouldn't die instantly). However, it's so incredibly close to the star that it would definitely be to hot to support life.

Gliese 581 b is the largest of the four at around the size of Neptune. This almost certainly makes it a gas giant with a very dense atmosphere that would crush us if we tried to find a surface. It would also be very hot, not just because of it's proximity to the star, but because of the insulating effect of that thick atmosphere.

Gliese 581 c skirts the inner edge of the habitable zone, probably making it too warm for comfortable life. It's heavier mass also suggests a thicker atmosphere than Earth's (although this is purely speculation) and it could be more similar to Venus in terms of climate. That is to say: very inhospitable. It's surface gravity would probably also be a bit too strong for us, though this would depend a bit on composition.

Gliese 581 d falls into the class of planets somewhere between Earthlike and (mini) gas giant. We're not completely sure at what mass point planets stop being rocky and turn into mini gas giants.

(Side note: the mini and the giant should really cancel out, shouldn't they? Maybe we should call them gas balls to distinguish from larger gassy planets like the gas giants Jupiter and Saturn. But then, where would you draw the line? I suspect it would end up depending on the composition of the gas at lesat partially.)

So while Gliese 581 d spends roughly half its time in the habitable zone and half beyond it, it's probably not inhabitable itself. However, if it had any rocky moons like the solar system gas giants we know and love, those moons have a reasonable chance of being habitable. With the added advantage that they're going to be (probably) tidally locked to the planet, not to the sun, allowing the sun to more evenly warm its surface.

It's interesting, really, that of all the exoplanets discovered so far—not just around Gliefeo—the area most likely to be habitable is a moon of a gas giant. Really that says more about our detection techniques than anything else, but it does suggest some interesting possible world building.

Requests?

As I mentioned in the intro, I'm going away and hope to have some blog posts prepared in advance. However, my brain is all used up organising travel-related things without much room left for thinking about stories or reading new articles (my primary sources of blog post inspiration). So if any of you lovely readers have any requests for future posts, drop me a line in the comments!