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.
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.
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.
What do you need in order to have a planet that supports complex life? First, you need liquid water at the surface of the planet. But there is only a narrow range of temperatures that can support liquid water. It turns out that the size of the star that your planet orbits around has a lot to do with whether you get liquid water or not.
A heavy, metal-rich star allows you to have a habitable planet far enough from the star so the planet can support liquid water on the planet’s surface while still being able to spin on its axis. The zone where a planet can have liquid water at the surface is called the circumstellar habitable zone (CHZ). A metal-rich star like our Sun is very massive, which moves the habitable zone out further away from the star.
If our star were smaller, we would have to orbit much closer to the star in order to have liquid water at the surface. Unfortunately, if you go too close to the star, then your planet becomes tidally locked, like the moon is tidally locked to Earth. Tidally locked planets are inhospitable to life. So we need a star massive enough to give us a nice wide habitable zone far away from the Sun itself.
But even with the right size star, which we have in our solar sytem, we still have CHZ problems. Just because a planet starts off in the circumstellar habitable zone, it doesn’t mean that it will stay there.
Jay Richards tweeted about this new article from the New Scientist, which talks about that very problem.
Excerpt: (links removed)
Earth’s comfortable temperatures may be thanks to Saturn’s good behaviour. If the ringed giant’s orbit had been slightly different, Earth’s orbit could have been wildly elongated, like that of a long-period comet.
Our solar system is a tidy sort of place: planetary orbits here tend to be circular and lie in the same plane, unlike the highly eccentric orbits of many exoplanets. Elke Pilat-Lohinger of the University of Vienna, Austria, was interested in the idea that the combined influence of Jupiter and Saturn – the solar system’s heavyweights – could have shaped other planets’ orbits. She used computer models to study how changing the orbits of these two giant planets might affect the Earth.
Earth’s orbit is so nearly circular that its distance from the sun only varies between 147 and 152 million kilometres, or around 2 per cent about the average. Moving Saturn’s orbit just 10 percent closer in would disrupt that by creating a resonance – essentially a periodic tug – that would stretch out the Earth’s orbit by tens of millions of kilometres. That would result in the Earth spending part of each year outside the habitable zone, the ring around the sun where temperatures are right for liquid water.
Tilting Saturn’s orbit would also stretch out Earth’s orbit. According to a simple model that did not include other inner planets, the greater the tilt, the more the elongation increased. Adding Venus and Mars to the model stabilised the orbits of all three planets, but the elongation nonetheless rose as Saturn’s orbit got more tilted. Pilat-Lohinger says a 20-degree tilt would bring the innermost part of Earth’s orbit closer to the sun than Venus.
So the evidence for a our solar system being fine-tuned for life keeps piling up. It’s just another factor that has to be just right so that complex, embodied life could exist here. All of these factors need to be just right, not just the orbits of any other massive planets. And you need at least one massive planet to attract comets and other such unwelcome intruders away from the life-permitting planets.
Here’s a good clip explaining the circumstellar habitable zone:
The factor I blogged about today is just one of the things you need in order to get a planet that supports life.
Here are a few of the more well-known ones:
a solar system with a single massive Sun than can serve as a long-lived, stable source of energy
a terrestrial planet (non-gaseous)
the planet must be the right distance from the sun in order to preserve liquid water at the surface – if it’s too close, the water is burnt off in a runaway greenhouse effect, if it’s too far, the water is permanently frozen in a runaway glaciation
the solar system must be placed at the right place in the galaxy – not too near dangerous radiation, but close enough to other stars to be able to absorb heavy elements after neighboring stars die
a moon of sufficient mass to stabilize the tilt of the planet’s rotation
an oxygen-rich atmosphere
a sweeper planet to deflect comets, etc.
planetary neighbors must have non-eccentric orbits