I've read in a few articles now that the fifth planet is considered the most habitable, why is this?
The TRAPPIST-1 system contains a total of seven planets, all around the size of Earth. Three of them -- TRAPPIST-1e, f and g -- dwell in their star's so-called "habitable zone."ť The habitable zone, or Goldilocks zone, is a band around every star (shown here in green) where astronomers have calculated that temperatures are just right -- not too hot, not too cold -- for liquid water to pool on the surface of an Earth-like world.While TRAPPIST-1b, c and d are too close to be in the system's likely habitable zone, and TRAPPIST-1h is too far away, the planets' discoverers say more optimistic scenarios could allow any or all of the planets to harbor liquid water. In particular, the strikingly small orbits of these worlds make it likely that most, if not all of them, perpetually show the same face to their star, the way our moon always shows the same face to the Earth. This would result in an extreme range of temperatures from the day to night sides, allowing for situations not factored into the traditional habitable zone definition. The illustrations shown for the various planets depict a range of possible scenarios of what they could look like.
Those brown, green, and blue zones in that TRAPPIST-1 system image seem to be IMO oddly placed. If we're looking just at irradiation and comparing to the Solar System, d should be easily inside the green zone, g well outside in the blue and f just about on the edge of green and blue. Or is there some more detail going into defining those zones (e.g. different stellar spectrum)?.
Yes, I think the runaway greenhouse limit for the smallest red dwarfs is around 0.9 S_Earth. So if scaled properly, the HZ is slightly further out for a smaller star. The paper preprint linked above says that they did climate models on all of them, and b, c, d ran into the runaway greenhouse state, while e, f, g remained temperate (h is likely too cold). EDIT: if we are looking for a name for the system, I think we should call it ...
Andrew LePage February 22, 2017 at 15:02> In particular, let’s see what Andrew LePage comes up with in his own Habitable Zone Reality Check.Well, I’ve got a lot of data to digest before a write a “Habitable Planet Reality Check” (hopefully to come out in the next few days), but at first blush there does indeed seem to be reason to believe that at least one of these worlds is “potentially habitable”… maybe more. And the fact that TRAPPIST-1e has a fairly well determined radius and mass with a resulting density suggestive of a volatile-rich planet means that Earth-size planets orbiting small red dwarfs *CAN* hold onto their water and atmospheres despite flare activity, excessive X-ray/XUV flux, etc.. That’s a hopeful sign about the potential habitability of exoplanets like Proxima Centauri b or even Kepler 186f, among many others.
Lords of Kobol! Could it be the Great Colonies?
Arxiv preprint on UV / XUV and habitability in the Trappist-1 system. Long story short, if there is an Earth-like moderately dense atmosphere with an ozone layer - all good. That failing, not even UV-resistant bacteria can make it.https://arxiv.org/abs/1702.06936