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Astronomer: We Can’t Just Assume Countless Earths Out There

He points out that the Principle of Mediocrity is based on faulty logical reasoning

Dartmouth physicist and astronomer Marcelo Gleiser wrote recently that the Copernican Principle has been misused to imply that Earth is somehow insignificant. That, he says, is a philosophical attitude, unrelated to the science. We don’t know where Earth stands in relation to other planets because we do not yet have telescopes capable of getting much detail about planets outside our solar system.

Marcelo Gleiser

Gleiser, author of The Island of Knowledge (2014), has also tackled the Mediocrity Principle (because Earth is nothing special, there must be countless intelligent civilizations out there).

According to Britannica, “Widely believed by astronomers since the work of Nicolaus Copernicus, this principle states that the properties and evolution of the solar system are not unusual in any important way. Consequently, the processes on Earth that led to life, and eventually to thinking beings, could have occurred throughout the cosmos.”

Gleiser finds the Mediocrity Principle’s logic similarly wanting:

If we constrain the Copernican Principle to a statement about Earth not being a special planet in terms of its location in the universe, all is well. Trouble starts when we extrapolate to statements about the ubiquity of life in the universe, following the faulty notion that if Earth is not special then neither is life. This is a massive non sequitur. It becomes exponentially nonsensical when elevated to the so-called principle of mediocrity: Given that there is life on Earth and Earth is not a special place, life should be abundant in Earth-like planets around the universe, including intelligent life. In other words, the principle states that life is so abundant out there that it’s a mediocre property of the universe. This sort of thinking is not only bad science but also bad philosophy, and it has serious repercussions on our current project of civilization. If our planet and the abundant life in it are so trivial to the point of being mediocre, why respect either?

Marcelo Gleiser, “The mediocrity of the mediocrity principle (for life in the universe)” at Big Think (October 6, 2021)

He concedes that the Principle can be quite sound under controlled circumstances: If most balls in a box are red, you are more likely to randomly draw a red one. Where stars are otherwise alike, the Principle can be useful for making probability decisions in astronomy.

But the probability of life on other planets presents us with a very different situation: The Principle assumes, in the absence of any evidence, that Earth is typical in terms of its properties and life forms, not simply its position. And that is what he sees as an unwarranted assumption:

A quick look at our solar system neighbors should dispel this notion. Mars is a frozen desert; if it had life in its early years, it didn’t offer enough stability to support it for very long. The same applies to Venus, now a hellish furnace. Farther away, there are many “Earth-like” exoplanets, but only in the sense that they have a similar mass and orbit a star at a distance that is within the habitable zone, where water, if present on the surface, is liquid. These preconditions for life are a far cry from life itself.

Marcelo Gleiser, “The mediocrity of the mediocrity principle (for life in the universe)” at Big Think (October 6, 2021)

Realistically, he notes, life must exist on a planet for a long time before the ways it changes a planet’s atmosphere could be detected from many light years away. Intelligent life may take longer and be far more tenuous. Put another way, on our own planet, water bears can survive many catastrophes that humans cannot. But they don’t think or seek to communicate with anyone and likely never will.

It’s an open question whether the notion of Earth’s mediocrity is a distraction from concern about protecting our environment. On that score, we are better off, probably, with the Rare Earth Principle or the Privileged Planet Hypothesis.


You may also wish to read: Physicist: Copernican Principle doesn’t make Earth insignificant. That, Marcelo Gleiser says, is a philosophical attitude, unrelated to the science. Theoretical physicist Gleiser notes that we’ve only begun to point huge telescopes at exoplanets. There are too many unknowns to be sure of our status.


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Astronomer: We Can’t Just Assume Countless Earths Out There