Recently, I picked up the audio book version of “The Privileged Planet”, which is a book by philosopher / economist Jay Richards and astronomer Guillermo Gonzalez. This book is different from books about the origin of the universe and cosmic fine-tuning. It talks about what it takes to make a habitable planet. And it talks about how the habitable places in the universe are also the best places to make scientific discoveries.
Christian scholar Jay Richards was interviewed by Brian Auten of Apologetics 315.
Science Daily reports on a new factor that affects planetary habitability: tides. Specifically, tides can affect the surface temperature of a planet, which has to be within a certain range in order to support liquid water – a requirement for life of any conceivable kind.
Excerpt:
Tides can render the so-called “habitable zone” around low-mass stars uninhabitable. This is the main result of a recently published study by a team of astronomers led by René Heller of the Astrophysical Institute Potsdam.
[…]Until now, the two main drivers thought to determine a planet’s temperature were the distance to the central star and the composition of the planet’s atmosphere. By studying the tides caused by low-mass stars on their potential earth-like companions, Heller and his colleagues have concluded that tidal effects modify the traditional concept of the habitable zone.
Heller deduced this from three different effects. Firstly, tides can cause the axis of a planet`s rotation to become perpendicular to its orbit in just a few million years. In comparison, Earth’s axis of rotation is inclined by 23.5 degrees — an effect which causes our seasons. Owing to this effect, there would be no seasonal variation on such Earth-like planets in the habitable zone of low-mass stars. These planets would have huge temperature differences between their poles, which would be in perpetual deep freeze, and their hot equators which in the long run would evaporate any atmosphere. This temperature difference would cause extreme winds and storms.
The second effect of these tides would be to heat up the exoplanet, similar to the tidal heating of Io, a moon of Jupiter that shows global vulcanism.
Finally, tides can cause the rotational period of the planet (the planet’s “day”) to synchronize with the orbital period (the planet’s “year”). This situation is identical to the Earth-moon setup: the moon only shows Earth one face, the other side being known as “the dark side of the moon.” As a result one half of the exoplanet receives extreme radiation from the star while the other half freezes in eternal darkness.
The habitable zone around low-mass stars is therefore not very comfortable — it may even be uninhabitable.
Here is my previous post on the factors needed for a habitable planet. Now we just have one more. I actually find this article sort of odd, because my understanding of stars was that only high-mass stars could support life at all. This is because if the mass of the planet was too low, the habitable zone wouldbe very close to the star. Being too close to the star causes tidal locking, which means that the planet doesn’t spin on its axis at all, and the same side faces the star. This is a life killer.
This astrophysicist who teaches at the University of Wisconsin explains it better than me.
Excerpt:
Higher-mass stars tend to be larger and luminous than their lower-mass counterparts. Therefore, their habitable zones are situated further out. In addition, however, their HZs are much broader. As an illustration,
a 0.2 solar-mass star’s HZ extends from 0.1 to 0.2 AU
a 1.0 solar-mass star’s HZ extends from 1 to 2 AU
a 40 solar-mass star’s HZ extends from 350 to 600 AU
On these grounds, it would seem that high-mass starts are the best candidates for finding planets within a habitable zone. However, these stars emit most of their radiation in the far ultraviolet (FUV), which can be highly damaging to life, and also contributes to photodissociation and the loss of water. Furthermore, the lifetimes of these stars is so short (around 10 million years) that there is not enough time for life to begin.
Very low mass stars have the longest lifetimes of all, but their HZs are very close in and very narrow. Therefore, the chances of a planet being formed within the HZ are small. Additionally, even if a planet did form within the HZ, it would become tidally locked, so that the same hemisphere always faced the star. Even though liquid water might exist on such a planet, the climactic conditions would probably be too severe to permit life.
In between the high- and low-mass stars lie those like our own Sun, which make up about 15% percent of the stars in the galaxy. These have reasonably-broad HZs, do not suffer from FUV irradiation, and have lifetimes of the order of 10 billion years. Therefore, they are the best candidates for harbouring planets where life might be able to begin.
This guy is just someone I found through a web search. He has a support-the-unions-sticker on his web page, so he’s a liberal crackpot. But he makes my point, anyway, so that’s good enough for me.
Maybe the new discovery is talking about this now, but I already knew about the tides and habitability, because I watched The Privileged Planet DVD. Actually that whole video is online, and the clip that talks about the habitable zone and water is linked in this blog post I wrote before.
Here’s an article from Evolution News that talks about liquid water and tidal locking, but it has even more factors that need to be fine-tuned for habitability.
Excerpt:
Stars with masses of 0.1-0.5 solar mass make up 75 percent of the stars in our Milky Way galaxy.6 These represent the red dwarfs, the M class. But these stars have low effective temperatures, and thus emit their peak radiation at longer wavelengths (red and near-infrared).7 They can have stable continuously habitable zones over long time scales, up to 10 billion years, barring other disruptions. It is also easier to detect terrestrial sized planets around them.8 But a serious problem with red dwarf stars in the K and M classes is their energetic flares and coronal mass ejection events. Potentially habitable planets need to orbit these stars closer, to be in these stars’ habitable zones. Yet the exposure to their stellar winds and more frequent and energetic flares becomes a serious issue for habitability. Because of these stars’ smaller mass, ejections get released with more violence.9 Any planet’s atmosphere would be subject to this ionizing radiation, and likely expose any surface life to much more damaging radiation.10 The loss of atmospheres in these conditions is likely, but the timescales are dependent on several factors including the planet’s mass, the extent of its atmosphere, the distance from the parent star, and the strength of the planet’s magnetic field.11 To protect its atmosphere for a long period, like billions of years, a planet with more mass and thus higher gravity could hold on to the gases better. But this larger planet would then hold on to lighter gases, like hydrogen and helium, and prevent an atmosphere similar to Earth’s from forming.12 Another consequence is that the increased surface pressure would prevent water from being in the liquid phase.13
So again, you need to have a huge, massive star in order to hold the planet in orbit over LONG distances. If it’s a short distance, you not only have the tidal-locking problem, but you also have this solar activity problem (flares, coronal ejections).
But wait! There’s more:
Another stellar parameter for advanced life has to do with UV (ultra-violet) radiation. The life-support star must provide just enough UV radiation, but not too much. UV radiation’s negative effects on DNA are well known, and any life support body must be able to sustain an atmosphere to shield them. Yet the energy from UV radiation is also needed for biochemical reactions. So life needs enough UV radiant to allow chemical reactions, but not so much as to destroy complex carbon molecules like DNA. Just this flux requirement alone requires the host star have a minimum stellar mass of 0.6 solar masses, and a maximum mass of 1.9 solar masses.14
So the ultra-violet radiation that is emitted has to be finely-tuned. (I’m guessing this assumes some sort of chemical origin-of-life scenario)
Still more:
Another requirement for habitable planets is a strong magnetic field to prevent their atmosphere from being lost to the solar winds. Planets orbiting a red dwarf star are also more affected by the star’s tidal effects, slowing the planet’s rotation rate. It is thought that strong magnetic fields are generated in part by the planet’s rotation.15 If the planet is tidally braked, then any potential for a significant magnetic field is likely to be seriously degraded. This will lead to loss of water and other gases from the planet’s atmosphere to the stellar winds.16 We see this in our solar system, where both Mercury and Venus, which orbit closer to the Sun than Earth, have very slow rotation rates, and very modest magnetic fields. Mercury has very little water, and surprisingly, neither does Venus. Even though Venus has a very dense atmosphere, it is very dry. This is due to UV radiation splitting the water molecules when they get high in the atmosphere, and then the hydrogen is lost to space, primarily, again, by solar wind.17
You have to hold on to your umbrella (atmosphere), or you get hit with dangerous rain (solar winds).
So a few more factors there, and remember, this is just the tip of the iceberg when it comes to circumstellar habitability constraints.
WINTERY KNIGHT 