Earth could indeed be a rare planet

Just because a planet resembles Earth in size (and mass), it need not be conducive to life. Take the example of Venus. It is almost an identical twin in terms of mass, and surface gravity, but otherwise it is very different. Some of its features include high temperatures, with its surface temperature reaching a temperature of 700 celsius and an toxic atmosphere of sulphuric acid clouds among others. The atmosphere of Venus is also a hundred times denser.

So what exactly makes a planet habitable? On Earth, two thirds of the surface is covered by liquid water. Subduction of water is possibly responsible for plate tectonics. Presence of a magnetic field (to deflect high energy cosmic ray particles, including solar wind which would otherwise cause mutations and cell radiation damage) and a large moon to stabilise its rotation axis are absent in the case of Venus.

There may be other subtle effects. Apart from the requirement that the planet should not be less than a third of the Earth mass (otherwise it would lose most of its atmosphere including water vapour in a comparatively short time as has happened in the case of Mars and the Moon) the thickness of the crust is also perhaps crucial. Much of the crust is composed of various oxides like silica, iron oxides and others.

If the crust were barely another 20 metres thicker (it is left to the readers to estimate the total mass of rock), most of the remaining atmospheric oxygen (which has a mass of around 1,015 tonnes in the atmosphere) could have reacted and been absorbed to form oxides when Earth’s surface was molten.

There could have perhaps been an atmosphere made of pure nitrogen and carbon dioxide. These are the dominant gases in the martian atmosphere. Also a warmer and deeper ocean would have absorbed much of the carbon dioxide, resulting in plant life not being able to thrive.

Hitherto discussions have centred on small changes in coupling strengths of the basic interactions as crucial for existence of life. For instance, if the nuclear coupling was four per cent weaker, deuterium would not form, or if it is two per cent stronger, the pp reaction in the sun would be explosive. It turns out that more mundane parameters like thickness of planetary crusts, or ocean depths could also play subtle roles.

While a suitable planet, as we have seen above, should not be lighter than a third of the terrestrial mass, it turns out that a planet heavier than about three Earth masses would also be not too comfortable. The escape velocity from such a planet  would be more than 20 km/s and the surface gravity more than twice Earth’s. This would make the atmosphere far denser (even if the planet is at the habitable zone) and together with the higher surface gravity, would imply crushing atmospheric pressure.

As the majority of stars are M-dwarfs with a typical luminosity of about 0.1 per cent of sun, the planet must be 30 times nearer to the star than the Earth-sun distance. This would make it tidal locked and tidal forces would be 100 times that experienced by Earth. The moon’s distance from Earth also matters. If the moon was only a third of its present distance, the tidal forces would be 30 times stronger. All these suggest that perhaps only a small fraction of Earth-like (or even a twin) exoplanets could host biological life. Earth could indeed be a rare Earth!