Something is either unique or not unique. You can’t be more or less unique. It would perhaps be better worded “Why is Earth unique in so many ways?”
Right.
We have only recently actually seen planets around other suns. But really only as a dot seen via instruments – not enough to identify any features of extrasolar planets.
Prior to this, I believe we had semi-confirmed that extrasolar planets actually existed, by noting their effect on their sun. But we’d never actually seen them. And only semi-confirmed; some scientists thought there might be other explanations for those effects on their sun.
And before that, we had only theories that if our sun could have planets, and there are billions of suns, there are probably other suns that have planets. But not much evidence either way.
One more unique thing is the way the material was in so many supernovae before…
the clouds of gas floating off into space… I think this caused the general area to be filled with blocks of ice… which came to earth to wet it.
Just right for us (because we’re the proverbial water in the puddle).
If you like boiling hot rains of sulphuric acid however, Earth is wholly unsatisfying and Venus is where the party’s at. Boiling hot rains of sulphuric acid are pretty unique, too ! 
Oh I get that, Douglas Adams reference and all.
Indeed.
We are in the Goldilocks Zone for Goldilocks. A young flaxen haired human girl. We have a reasonably deep understanding of chemistry and physics in the Goldilocks temperature range, and a vastly more limited understanding outside that zone. Not that the laws are different, but that experimentally it is a lot of work to perform experiments, and since it is a lot of work to get there, the pay-offs are not as great. Our understanding of cryogenic processes - especially superconductivity and the like - are tiny compared to our understanding of the nuances of chemistry and physics at room temperature. What this IMHO leads to is an inward looking lack of imagination as to what the Goldilocks zones are. Look at the Kuiper Belt objects we know about - like Pluto and Triton. It quite possible highly complex chemistry and physics occurs within those environments. Whether it would lead to enough complexity to provide the springboard for evolutionary processes to gain a foothold and lead to something that would pass muster as a life form, well that is another matter, but discounting it on the basis of our current limited understanding of the possibilities is naive.
With a sample size of one, all we have is an existence proof that the Universe is capable of allowing life as we define it to occur. After that things get awfully thin.
This is quite simply not true. If you take “unique” as an absolute, then there is no such thing as “not unique”. Everything is unique. Uranus and Neptune are unique, too: Each of them has its own mass, and its own radius, and its own composition, and its own average temperature, and so on. They are, in fact, unique in all of the same properties that the Earth is. But for most of those properties, Uranus and Neptune are, while not exactly identical, still very similar to each other. How else can we express this than by saying that Uranus and Neptune are less unique than Earth?
In no way is that unique. All the planets in this system condensed from the very same cloud of gas and dust (nebula in the argot). Which means they all had the same pre-formation earlier history, whatever it may have been.
As well, there is zero evidence the nebula our system formed from was in any way unusual. Certainly the nature of the average nebula has changed over the billions of years the universe has existed in its present incarnation. Early first generation stars and first generation nebulae were different from ones formed just a few billion years ago as ours was. And in turn they differ a bit from ones forming today or those to be formed a few dozen billion years from now.
But overall there is no reason to conclude there’s anything unusual about our system’s history or formation compared to its millions of contemporaries.
Interesting thread. I’d like to summarize the traits that make Earth habitable for us. Please add, subtract, correct, or clarify! These aren’t listed as necessary for any kind of life, just what we tend to see here on Earth.
- Atmosphere, enabled by:
– far enough from sun
– larger core, produced by collision with planetoid
– magnetosphere caused by molten iron core, blocking solar wind that would blow it away
– liquid water oceans with dissolved calcium, allowing CO2 to precipitate as limestone - Geological activity (plate tectonics), providing turnover of materials and participating in a chemical balancing act
- Close enough to sun to enable solar-powered biota
- The Moon, stabilizing spin and making weather less extreme (more on this please?)
- Water, provided by asteroid collisions, stabilizing atmosphere and as basic building block and solvent
Is the chemical composition of Earth unusual, compared to other rocky planets (as far as we know)?
Meh. I’ve seen bigger.
I agree that it’s a dumb descriptor for planets. I was simply arguing the grammar, not the logic.
It’s equally dumb to consider the word “unique” to be absolute, in any context. It’s not just that all planets are unique; all everything is unique. The only way the word “unique” has any usefulness at all, in any context, is if some things are more unique than others.
You’re (sort of) both right:
u·nique
adjective
**Usage note **
Many authors of usage guides, editors, teachers, and others feel strongly that such “absolute” words as complete, equal, perfect, and especially unique cannot be compared because of their “meaning”: a word that denotes an absolute condition cannot be described as denoting more or less than that absolute condition. However, all such words have undergone semantic development and are used in a number of senses, some of which can be compared by words like more, very, most, absolutely, somewhat, and totally and some of which cannot.
The earliest meanings of unique when it entered English around the beginning of the 17th century were “single, sole” and “having no equal.” By the mid-19th century unique had developed a wider meaning, “not typical, unusual,” and it is in this wider sense that it is compared: The foliage on the late-blooming plants is more unique than that on the earlier varieties.The comparison of so-called absolutes in senses that are not absolute is standard in all varieties of speech and writing.
It’s certainly not a point about which I’m passionate enough to argue