I think it is probable, that analogue planets will exist, not exact copies of course; planets will be like fingerprints or Zebra stripes...all different but some could be similar to an extent. Of course, Earth is a very complicated and diverse planet. It would probably be quite a special planet in the universe, from our viewpoint. If life could develop here, and the rules of the universe is the same everywhere, life could exist on other planets given appropriate conditions. Even here life exists on fairly extreme parts of the globe, within oceans, on the ice and in desert regions. Humans are quite adaptable when one adds technology to their basic tools. So, I think the answer is YES, probably....there are so many solar systems out in the universe, that it is basically unthinkable that there are no analogue planets.

As a geologist I sometimes wonder whether the influence of earth's plate tectonics on the evolution of a complex biosphere is underestimated: as a creator of geographic barriers, pathways, compartments; as a cause of global disturbances that make ecosystems on earth more resilient; as a recycling mechanism for crustal rocks and equilibrator of extreme surface conditions (e.g. snowball earth states that are ended through a phase of increased volcanism), as a provider of an energy source independent from the sun (see e.g. hydrothermal-vent-related ecosystems), to name some points.

Hi Michael, very true, plate tectonics, mantle convection, coupling of mantle to the crust etc. all play long term roles in the Earth's dynamics. Think of the fluid core causing the magnetic field (where would we be without it)... so I think you are quite right, plate tectonics is probably underestimated as we typically think in terms of relatively shorter periods. The complexity of the biosphere is further increased by geomorphological processes, e.g. physical and chemical weathering, soil formation (what life would we have without soil?) and of course the atmospheric processes.

It is very difficult to find an exact analogue to any known planet, not only Earth. Variability among solar system planets is very high, and it would be difficult to find an exoplanet exact analogue, for example, of Venus or Mars since the history of every large body is almost unique, more or less like people: similar but diverse. On the Earth there is the further point of life, which we consider interesting by itself, but I suggest to separate the two issues: many many geophysical and astrophysical processes have modeled the Earth (global composition, differentiation, plate tectonics, meteoroid bombardment, volcanism, formation of the Moon, orbital evolution, seasonal variability, atmospheric composition and evolution, climatology, etc etc etc...) but we do not know for sure how each of them has been essential to the raise of terrestrial life. So in principle is not inconceivable to have life in environments different from Earth.

Hi to all and thanks for your answers and for share your knowledge.

Michael, we Biologist know well the key role that plate tectonics has played in evolution, in fact, Evolution Theory comes from the diversity of life observed by Darwin, that has been product of geographical barriers; different niches, that have been product of the diversity in the relief; different distribution and concentration of nutrients, product of weathering and volcanism, etc. In part, precisely for the role of plate tectonics, I asked about Earth exact analogous.

If all planets are like fingerprints or Zebra stripes as Ludwig said and Emiliano extend (I agree with that too), and as we can see in our solar system or in the Kepler catalog, then there are a very little probability that we could find an Earth exact analogous, in terms of size, orbit and star type. These 3 features have been used on many papers to define an Earth-Like planet. I think we need more features in order to declare a planet Earth-Like. Features like plate tectonics, active volcanoes, continents and oceans, even an O2-N2 atmosphere and a Moon (in the strict case). I understand that with our current technology and methodologies, only those 3 features is all that we could know for an exoplanet, but then, rather than Earth-Like planets they could be named Rocky planets or Sun Solar System Inner Planets-Like, or otherwise. In fact, in many papers are two more useful concepts named incorrectly (in my opinion), these are Habitable Planet and Circumstellar Habitable Zone.

As you (Michael) and Emiliano said, many geophysical and astrophysical (and biological) processes have modeled the Earth and even these 3 processes have modeled Life in Earth. I consider that, just when we are able, in the short or middle term, of analyze the surface of exoplanets using some new technology or methodologies of remote sensing, or why not, robotic probes, or even better manned missions, we will able to find a true Earth-Like planet, very likely an “Early Earth-Like” planet more than a “Current Earth-Like” planet.

But if we find a “Current Earth-Like” planet, we will be face to face with one of the greatest milestone in the history. Nowadays, the only way known that a planet have the levels of oxygen and nitrogen that we currently have on Earth, is the presence of carbon-based photosynthetic life. If we found a current Earth analogous, automatically that planet will be uninhabitable for us, because our presence on that planet could jeopardize that life on it and viceversa, that life could be a danger to us. The same if we find an Early Earth-Like Planet inhabited by similar Methanogenic and Thermophilic early Earth Life. Of course, we “could” kill those life forms (as Hollywood teach us) and inhabit their planets, but we shouldn’t do that, at least we scientific ones, isn’t? ^_^

If it is actually quite complicated, to find a planet similar to Earth in at least 5 or 6 features, finding a habitable planet is almost impossible (in my opinion). Petigura et al. (2013) when analyze the Kepler brightness measurements of about 42,500 Sun-like stars in order to find Earth-size planets with Earth-like orbits around stars G-K type, reported 10 Earth-Size planets (1-2 E radii) in the "Habitable Zone". They estimate that in our Galaxy, 5.5% of Sun-Like Stars could harbor Earth-size planets in the "Habitable Zone". According to Francois Fressin, from the Harvard-Smithsonian Center for Astrophysics, a planet that has a radius twice the size of Earth's might not even be rocky, So, How many of them could be Earth-Like in at least 5 or 6 characteristics? How many could be Venus-Like? How many are Giants Gas cores with an eroded Solar-wind atmosphere, or planets with oceans hundreds of miles deep above a rocky core?

Hi Sandro, I agree with you in that "habitable zone" and "Earth-like planet" are not well-chosed terms, since they are easily misunderstood. I think these terms arose more for sensationalism than scientific pertinence. In fact, we cannot bring them away from the context of the exoplanetary diversity. If we look at the extraordinary strangeness of the discovered exoplanets (hot Jupiters, hot Neptunes, planets around binary stars, around pulsar, etc) that was never imagined before, we see that a "simple" rocky planet of the size of the Earth quietly orbiting around a main-sequence star seems very "Earth-like"...But in this case we can also say "Venus-like" in the same manner.

"Habitable zone" I guess is a similar case. It was introduced several decades ago in order to define where we can expect liquid water on a planetary surface. This could be an important point for life, but its true relationship with habitability is questionable (we do not even know if liquid water is essential...).

I add a further comment about the "in situ" exploration of exoplanets: for sure we can exclude it for long time, since the distances are so large that any meaningful mission would require to travel at a significant fraction of the speed of light. We will not have technological and economical ability for that for long time. So, the only information we can count on is from remote sensing (of unresolved objects moreover...).

I suspect that we will eventually will find life sustaining planets, but they

will have followed their own evolutionary history, and their life would be

vastly different from that found on Earth.

Since we would not have any resistance to the microbes on that planet,

we would not live long when exposed to them. The microbes would find us to

be an easy meal, if the larger animals with the usual accommodations of

sharp teeth and claws did not get to us first.

We assume that a Habitable Zone Planet would contain water in a combination

of solid, liquid, and gas, but not completely solid only, nor not completely as

a gas only, which confines the gravity-temperature-pressure-atmospheric

thickness to a narrow window, but, we do not know for sure.

I suspect that Earth has only had this window of opportunity for about the

last 630 million years of roughly 4567 million year existence. This implies

a planetary evolution where a habitable planet passes up and through

a period of habitable conditions. Or, more correctly, the Habitable window

passes through the planet from a closer Solar orbit, progressively to an

ever higher Solar orbit.

I suspect that the Earth has "used up" about 80 % of its Habitable

Time window with the peak of Habitation having occurred back when

dinosaurs reached their maximum allowable sizes around 150 million

years ago. Over the next 200 million years, the tendency should be

toward increasing temperatures and pressures of the atmosphere,

and toward decreasing maximum allowable sizes of life forms,

here on Earth.

The maximum allowable sizes of life forms are a structural limitation

of the materials ( bone and muscles ) in combination with the

applied forces on the structure ( increasing gravity ) of a planet

that is slowly gaining more mass, and is thus increasing in Radius,

Volume, Surface Area, average density, volume of expelled gas and

water, and geologic activities.

The increase in each of these is the only thing that I can conceive of

that would allow a planet to remain in the " Habitable Conditions"

for such a protracted period of time while still releasing more and more

gas into the atmosphere.

Plate Tectonics can account for the recycling of solids and some

liquids, but it can not account for recycling of gases.

Growing and evolving planet tectonics can account for the increase in: volume

of the Atmosphere, volume of the Water, volume of the Crust, the long

period of fluctuation in temperatures, pressures, sea level changes,

glaciations, melt downs, geologic surface feature changes, and the

bending of the rock at the edge of the continents that creates the illusion

of subduction and material recycling. It can also account for the

evolution of plants and animals from small to very large sizes,

and then back toward smaller sizes, at least on land, and in the air,

and for some of the ancient sea critters of enormous sizes.

To understand if a Habitable Life sustaining process is possible on

another planet, we first need to get a more correct and more system

encompassing overview of how and why our own planetary processes

have evolved, and why they have continued to remain in the

narrow window between about 13 C and 26 C for such a protracted

period of geologic time. The dominate temperature has been around

22 C for much of that time period.

How, and Why? What is oscillating while still growing to make this

possible? Well as usual, I have digressed.

After a time of good discussions and learning I dare to conclude several things about habitability and the searching of LIFE. I wrote those things here:

https://www.researchgate.net/post/The_Habitable_Zone_Gallery?cp=re65_x_p2&ch=reg&loginT=i-C8BQfZSYGitMum-AzFHVshIo_PuIUrcAspwI5k49pfYqpLTeDTTA%2A%2A&pli=1#view=5318cea7d2fd64767d8b461f

Please I would like to know all the different opinions about this and about I wrote. At your orders, I remain.