The calculation of probability by Pierluigi Peruzzi

The Milky Way (our galaxy) contains around 150,000,000,000 stars.

We know of approximately 100,000,000,000 galaxies, or perhaps even more.

Estimating the approximate number of stars is very simple
150,000,000,000 × 100,000,000,000 = 15,000,000,000,000,000,000,000.

Assuming that one in every 100 stars has planets suitable for life, and that life has emerged on one in every 100 of these planets, then we simply need to divide the figure above by 10,000. That gives us
15,000,000,000,000,000,000,000,000 / 10,000 = 1,500,000,000,000,000,000 planets
on which life exists in our universe.

I would also like to say to religious people that denying the existence of life on other planets is pure blasphemy. In other words, you do not believe that God is capable of creating life on other planets.

The exploration of our solar system

Research into the Saturn moon Titan has provided scientific proof that life (on Titan) can also arise on other planets. Titan is home to vast seas of methane and contains traces of at least a dozen organic compounds, including methane, ethane, propane, ethyne and hydrogen cyanide. There is no oxygen on Titan. Nor is this possible, as at such extremely low temperatures no plants capable of producing oxygen can survive. Oxygen is, after all, a by-product of plants, produced through photosynthesis. Oxygen binds quickly when no new oxygen is being produced. This planet (or moon) has cooled over millions of years. New life can hardly arise at such low temperatures. However, ancient life could now be frozen there.

Let’s get to the point: For biological life (a Cambrian as we understand it) to emerge on any planet, certain conditions are required. The Cambrian period lasted approximately 550–600 million years. In my view, the conditions necessary for the emergence of the Cambrian from the primordial soup are:

• the right temperature,
• the right duration of that temperature,
• a specific gravitational force,
• the planet’s rotation at the right speed,
• a specific amount of light,
• perhaps an atmosphere (or perhaps not),
• perhaps there needs to be a great deal of water (or perhaps not).

The right temperature for the Primordial Soup

I presume that an average temperature of 15–35 °C is required for a primordial soup based on hydrocarbons to form. Perhaps even a little less, or a little more. At even higher temperatures, a different kind of biological compound might possibly form. At lower temperatures, this is less likely, but we don’t know for sure.

That said, we shouldn’t overlook the fact that most planets have an equator and two poles. This means that the right temperature is bound to be found somewhere in between. That makes the whole thing even easier.

The correct duration of this temperature

The correct duration of this temperature must be no more than 550 million years, the length of the Cambrian period. However, life itself – that is, simple single-celled organisms and microorganisms – can emerge after just a few million years.

A certain gravity

I do not regard gravity as a decisive factor in the origin of life. Titan (a moon of Saturn) has, after all, vast seas of methane, which must clearly have been formed by past life, as methane is a waste product of biological organisms. It also has a gravity of just 0.15 g (1.4 m/s2). The Earth, of course, has 1 g (9.8 m/s²) of gravity.

In my view, there simply needs to be enough gravity for the primordial soup to remain on the ground. So 0.1 g (1 m/s2) to 1.2 g (12 m/s2) would be perfectly sufficient. And such gravity is found on almost all medium-sized planets with a diameter between 3,000 and 15,000 km.

The rotation of a planet at the right speed

In my view, a planet’s rotation is very important if life is to develop on its surface or in its oceans. The frenzied rotation of Saturn or Jupiter automatically gives rise to storms that would sweep everything away. The resulting electrical discharges (lightning) are also unlikely to be ideal for a certain standard of living.

On the other hand, a planet must be able to rotate so that it is heated evenly from all sides by its star. Mercury and our Moon, for example, do not rotate. As a result, the side of Mercury facing the Sun reaches temperatures of up to 600°C, whilst the side facing away from the Sun experiences sub-zero temperatures. This is certainly not ideal for the emergence of life.

Perhaps a certain amount of light (or perhaps not)

Our plants need a certain amount of light for photosynthesis. But in the dark depths of the ocean, at 4,000 metres, there are a great many forms of life. So light is not necessarily the decisive factor.

A planet’s atmosphere

Perhaps the Cambrian period on another planet would also require an atmosphere. Nevertheless, in warm waters or underground, some form of life could develop according to Darwinian principles.

Water

Water must inevitably already exist on all planets, as H and O are simple, fundamental elements.

Every star system must have planets, because all star systems were formed in the same way following the Big Bang.
To claim that not every star has planets lacks any factual or astrophysical basis. There is simply no way to justify this. Perhaps there is an exception somewhere, but that is all.

Of course, the number of planets will vary, but if we consider our own system of Venus, Earth and Mars, then surely every second star has a habitable planet. Furthermore, given the cooling of all stars that has taken place over the last few billion years, there must at some point have been a habitable planet orbiting every star. If we now factor in Darwin and the Cambrian period, there is a vast multitude of different life forms in the universe.

The idea that there is no life on other planets is more a reflection of humanity’s deep psychological desire to feel more significant and to be at the centre of events.

Moreover, our small solar system has produced various habitable celestial bodies.Titan and Mars (in the past) and Earth (today) and, very likely, Venus (in the future). Why on earth, over the course of billions of years, should no habitable planets have developed in all the other star systems?! I find it hard to believe that this can be objectively justified. It has more to do with deep-seated fears that we mere mortals might not be anything special in this universe.