One of the interesting things about science is how things can change. When I wrote my ebook Planetary Formation and Biogenesis, finishing in 2011, it was generally accepted that the star Tau ceti had no planets, and all the star had orbiting it was a collection of rocks or lumps of ice, in short, debris that had never accreted. Now, it appears, five planets have been claimed to orbit it, and most have very low eccentricities. (The eccentricity measures the difference between closest and farthest distance from the star in an elliptical orbit. If the eccentricity is zero, the orbit is circular.) Orbits close to zero indicate that there have been no major disruptions to the planetary system, which can occur if the planets get too big. Once they get to a size where their gravitational pull acts on each other, the planets may play a sort of game of planetary billiards, often ejecting one from the system, and leaving the rest with highly elliptical orbits, and sometime planets very close to the star.
The question then is, how does my theory perform? The theory suggests that planets form due to chemical interactions, at least to begin, although once they reach a certain size, gravity is the driving force. This has a rather odd consequence in that while the planets are small, their differences of composition are marked, but once they get big enough to accrete everything, they become much more similar, until they become giants, in which case they appear more or less the same. The chemical interactions depend on temperature, and for the rocky planets, on a sequence of temperatures. The first important temperature is during stellar accretion, when temperatures become rather high in the rocky planet zone. For example, the material that led to the start of Earth had to get to at least 1538 degrees Centigrade, so that iron would melt. All the iron bearing meteorites almost certainly reached this temperature, as there is no other obvious way to melt the iron that forms them. At the same time, a number or silicates melt and phase separate. (That is forming two layers, like oil and water.) There is then a second important temperature. When the star has finished forming, which occurs when most of the available gas has reached it, there remains a much lower density gas disk, which cools.
The initial high temperatures are caused by large amounts of gas falling towards the star, and it gets hot due to friction as it loses potential energy. Accordingly, the potential energy depends on the gravitational field of the star, which is proportional to the mass of the star. The heat also depends on the rate of gas falling in, i.e. how much is falling, and very approximately that depends on the square of the mass of the star. Unfortunately, it also depends on how efficient the disk was at radiating heat, and that is unknowable. Accordingly, if all systems have the same pattern of disk cooling, then very very roughly, the same sort of planet will be at a distance proportional to the cube of the stellar mass, at least on this theory.
There are only two solitary stars within 12 light years from Earth that are sufficiently similar to our star that they might be considered to be of interest as supporting life, and only one, Tau ceti is old enough to be of interest as potentially having life. Tau ceti has a mass of approximately 0.78 times our sun’s mass, so on my theory the prediction of the location of the earth equivalent based on our system being a standard (which it may well not be, but with a sample of one, a statistical analysis is not possible) would be at approximately 0.48 AU, an AU (astronomical unit) being the distance from Earth to the sun. The planets present are at 0.105 AU, 0.195 AU, 0.374 AU, 0.552 A.U. and 1.35 AU. If the 0.552 planet is an Earth equivalent, all the others are somewhat further from the planet than expected, or alternatively, if the 0.374 AU planet is the earth equivalent, they are much closer than expected. Which it is within the theory depends on how fast the star formed or how transparent was the disk, both of which are unknowable. Alternatively, the Earth equivalent would be defined by its composition, which again is currently unknowable. The Jupiter equivalent should be at about 2.5 AU on my theory. If it were to be the 1.35 AU planet that was the Jupiter equivalent (mainly water ice) then the 0.552 planet would be the Mars equivalent, and while it would be just in the habitable zone (0.55 – 1.16 AU estimate) the core would have the chemistry of Mars, plus whatever it accreted gravitationally.
Tau ceti is thus the closest star where we have seen a planet in the habitable zone. The planet in the habitable zone is about 4.3 times as massive as earth, so it would be expected to have a stronger gravitational acceleration at its surface, but possibly not that much more because Earth’s gravity is enhanced by its reasonably massive iron core. Planets that accrete much of their mass through simple gravity probably also accrete a lot more water towards the end because water is more common than rock in the disk, apart from the initial stones and iron concentrated through melting. So, with a planet possibly in the habitable zone, but of unknown water content, and unknown nature, if we had the technology would you vote to send a probe to find out what it is like? The expense would make the basic NASA probes look like chickenfeed, and of course, we would never get an answer in our lifetimes, unless someone develops a motor capable of reaching relativistic speeds.
The Fermi paradox, raised by Enrico Fermi, posed the question, if alien life is possible on other planets, given that there are so many stars older than our sun, why haven’t we been visited (assuming we have not)? For all those who say there are better things to spend their money on, they answer that question. The sheer expense of getting started may mean that all civilizations prefer to stay in their own system. Not, of course, that that stops me and others from writing science fictional stories about them.