Criticisms

Because Astrobiology relies mostly on scientific extrapolations, over solid, factual evidence, the authenticity of astrobiology as a science can be questioned. Astrobiology is more theoretical than scientific. While other branches of science remain heavily theoretical, there is a greater degree of mathematical, pragmatic and/or observational evidence supporting the theories. For example, while science cannot prove string theory, there is a great deal of mathematical computation which implies the existence of strings of energy. Such evidence does not exist with Astrobiology, save for an asteroid segment which is believed to have fossilized Martian microbes. [55] the University of Glamorgan, UK, started just such a degree in 2006.[56]

Characterization of non-Earth life is extraordinarily unsettled; hypotheses and predictions as to its existence and origin vary wildly; true astrobiological experiments (with modest exceptions such as the study of the ALH84001 meteorite and searches for indications of life in Earthshine) simply cannot occur at present. Finally, astrobiology has been criticized for being unimaginative in the tacit assumption that Earth-like life presents the most likely template for life elsewhere. For example, Michael Crow, the president of Arizona State University, said the following:[57]

For the last 3,000 years of our science, we really haven't gotten around to the notion that there might be something going on somewhere other than in this small, rural village [called Earth], in this isolated corner of our own galaxy or the Universe itself.

Biologist Jack Cohen and mathematician Ian Stewart, amongst others, consider xenobiology separate from astrobiology for this reason. Cohen and Stewart stipulate that astrobiology is the search for earth-like life outside of our solar system and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining characteristics of life. See carbon chauvinism.

As with all space exploration, there is the classic argument that there is still a lot more scientists have to learn about Earth. Critics of astrobiology may prefer that federal funding remain dedicated towards searching for unknown species in our own terrestrial biosphere. They feel that earth is the most plausible and practical region to search for and study life.

Rare Earth hypothesis

In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, is probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).

Peter Ward, one of the authors, said the following:[1]

How do we define life as we do know it? Life on Earth has DNA, a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we really should expect radically different chemistries.

Rare Earth hypothesis

In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, is probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).

Peter Ward, one of the authors, said the following:[1]

How do we define life as we do know it? Life on Earth has DNA, a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we really should expect radically different chemistries.