what makes a habitable planet?

Red dwarf and planet. Image pilfered from the Hubble Space telescope site, a great source of space pictures, all of which are free to use.

Last week, scientists analysing data obtained by the Keck ‘planethunting’ observatory (located at the summit of Mauna Kea, Hawaii) revealed data about the GJ 581 solar system that suggests there is a terrestrial planet in the habitable zone. Until recently, all exoplanets discovered have been either gas giants or terrestrial planets orbiting too close to the star to be suitable for life. The observatory has been looking at the GJ 581 system for 11 years, and has so far uncovered six planets, all of which in a closer orbit than Mars. The latest discovery, the fifth from the star, is a terrestrial planet three times the diameter of Earth, but with less density, resulting in roughly the same gravity. GJ 581 orbits that star in 36.6 days at a distance of approximately 0.15AU (15% of the Earth-Sun distance; yes, that’s really close). GJ 581 is a red dwarf with a stellar output of less than 2% of the Sun’s. The planet is tidally locked; one side is always dark and the other is in permanent daylight. The planet is, very unromantically, named GJ 581g, but I like the unofficial name, after the wife of the scientist leading the team (Stephen Vogt), Zarmina’s World.

What is unusual about this discovery is that it comes earlier than expected, and that the star involved is closer than expected, at ‘only’ 20 light years away. The paper announcing the discovery states: _if the local stellar neighborhood is a representative sample of the galaxy as a whole, our Milky Way could be teeming with potentially habitable planets_ (Vogt et al., 2010 – link to the full scientific paper here ).

Wow. I mean – wow.

This is the stuff an SF writer’s dreams are made out of. Let’s just forget about the tiny detail of how to get there.

After reading this news, I wanted to take one step back and analyse: what makes a planet habitable?
A simple search on Wikipedia delivers a set of conditions:

– suitable star
– far enough from the star
– must have elements that sustain life (hydrogen, nitrogen, carbon and oxygen)

This page also mentions as a fourth condition that the planet musty not be tidally locked, but that condition has already been broken. We are finding potential for life in places where this has not been considered before. So I went hunting for additional information. This paper goes into a lot more scientific detail than you will ever require. Below, I have distilled some of the important points.

(Note: this is a work in progress. I also wish to state that I’m not officially qualified in any of the scientific disciplines I’m touching on below. If you are, I am happy to post corrections or additions either in this post of follow-ups.)

A suitable star system
This comes down to two factors: the life cycle of the star and its output. Very bright stars burn their energy quickly. If a star is more than 1.4 solar masses, its life will end in spectacular fashion much sooner than the life of our sun, without having a long stable period of stellar output which allows habitable conditions to develop. Which is OK, because bright stars (O, B and A class) only make up a very small percentage of stars anyway. The sun is a G class star and is about 4.5 billion years old. It will last for another 5 billion or so before we will see noticeable changes. Less hot stars will last much longer. An M-class red dwarf is likely to be stable in the long term. There are potential problems with planetary atmospheres being buffeted with solar winds for long periods of time (especially close to the star), but this could be mitigated if the planet has a magnetic field. At three times Earth mass, that is not unlikely.

Distance from the star
‘If you sit too close to the fire, you’re going to get burnt’

The habitable zone is a band inside which a terrestrial planet with water on its surface will neither become too hot to develop a runaway greenhouse effect (Venus), nor too cold for CO2 to condense on its surface (more about CO2 later). People call this the Goldilocks zone. Can’t think why 😉
The width of the habitable zone is determined by a number of factors and may vary quite a lot depending on those factors. First of all, there is the star’s output. It comes as a no-brainer that the habitable zone around the Sun is going to be further away than that around GJ 581, an M-class dwarf with less than two percent of the sun’s output.

However, the width of the habitable zone is also dependent on the planet, or rather, the planet’s atmosphere. This is where CO2 comes in. As greenhouse gas, it has a major function in regulating surface temperature. The main path by which it initially enters the atmosphere is through volcanism. This is why a number of panel members at Worldcon mentioned that geological activity is essential to a healthy planet that can sustain the evolution of life (I had wondered about that, since no one explained). The presence, or absence, of CO2 can shift the outer boundary of the habitable zone by a considerable distance. Without CO2, Earth would be permanently frozen. With a significant atmosphere containing CO2, Mars is well within the habitable zone. Volcanism is the result of compaction of planetary mass through gravity and/or the effect of the pull of other celestial bodies on a planet (or moon). Earth’s core is molten because Earth is big enough to have enough gravity to condense its core to a level that the rock melts. Io is volcanic because the other large Galilean moons and Jupiter are constantly pulling at it. Mars is too small to remain volcanic for long after it was formed, and then was too small to hang onto its atmosphere. The planetary mass should be at least 0.5 times that of Earth.

On the other side of the spectrum, massive planets retain too large an envelope of hydrogen and helium (as in the cloud layers of the gas giants). The pressure of this layer (large planet = high gravity) prevents water being liquid on the surface. There is a risk that this could happen for planets more than 6 times the mass of Earth.

In summary: what makes a habitable planet:
A planet’s composition, temperature, atmosphere and surface pressure are such that liquid water exists on the surface.

What distinguishes an inhabited planet from a habitable planet (big difference):
The right chemistry and the conditions above have persisted for long enough (a few billion years) for evolution to occur.