Now what does it mean for planet to harbor life or to be habitable? We simply don’t know. We can make educated guesses, but without an example of extra-terrestrial life to go on we don’t know which conditions are essential to the development of life. The consensus is that liquid water is a requirement, so the concept of a “habitable zone” or “goldilocks zone” was developed.[i] The zone is the distance from star where a planet can maintain liquid water. This distance changes depending on the size and brightness of the star. If the planet is too small or large it will not retain its water. We give priority to the exoplanets that we find that are both a similar size to our own planet and lie in the “just right” goldilocks zone. This reduction of life-giving features (based on Earth’s conditions) has divided the scientific community on whether life will develop inevitably given ideal conditions (whatever those may be), or if Earth is special because life is rare. Obviously, knowing the exoplanet may contain liquid water is not enough for us to know if it contains the unknown, but necessary components of life. The upshot being that if Earth has liquid water and intelligent life, then planets that also have liquid water could also have intelligent life. The argument is not about how rare the Earth is, but how rare intelligent life is. Since we do not know if life is an inevitable occurrence given the correct conditions, the rare-Earth argument has become an argument for the rarity of intelligent life. I believe that the distinction between the ideas of rarity of Earth-like planets and the rarity of intelligent life is necessary.
It seems that the goldilocks zone’s only use is to narrow the search for extra-terrestrial life. The number of planets we have found is in the thousands; using the habitable zone criteria, the number of potentially habitable planets falls to 34.[ii] The habitable zone provides a way to decrease the amount of work we must do to analyze these planets once the technology improves. An improvement in technology would allow scientist to analyze exoplanet atmospheres for bio-signature gases by looking at spectroscopy. These are gases that could be markers of life. On Earth, the abundance of highly reactive oxygen would be a sign that something is producing it. Without a continuously replenishing source the oxygen would react with the other elements. For any aliens looking at our atmosphere’s makeup it would tip them off that something is going on Earth.[iii]
Earlier this year, NASA teased its Instagram followers with a secret announcement. Before making the reveal, they let us know, perhaps the assuaging the disappointment of its nearly 20 million followers, that it was definitely not aliens. But it was something much more grounded in reality: a planetary system with seven Earth-like planets only 40 light-years away. Even cooler is that three of them lie in their star’s habitable zone.[iv] While NASA tried to mitigate our imaginations, we immediately concluded that there were aliens, some memes even calling to the faraway aliens to save us from our Earth-bound existence. Although the public is already dreaming of little green men, the excitement comes from the astronomers too. This is a momentous discovery, marking the most number of habitable planets found in a single planetary system. Scientists do not have separate minds for research and for their everyday life; rather, their opinions about what they are studying can affect how they view the results of their research.
We presume that it is less outlandish for us to make claims of a planet’s habitability, than to announce the discovery of aliens. The information we have about the Trappist 1 system is limited. We only know of the masses of the planets, the type of star, as well as the orbiting distance. We know nothing of their atmosphere of these planets, or their surface conditions. The star of this seven-Earth solar system, is one- twelfth the size of our Sun. The star is a red, “ultra-cool dwarf” and found more commonly than our Sun.[v]
The Sun shapes many of the conditions on Earth. Sunlight refracted by atmospheric particles and water, give us our trademark blue sky and our planet’s distinctive blue coloring. How Earth-like can these planets be orbiting a red star? Is a planet with a red sky really Earth-like? Having reduced Earth’s tangled web of interactions between life on the planet and the planet itself, to just two conditions–mass and density—it seems a stretch to say that these planets are Earth-like, rather than Earth-sized. Confidently declaring the seven Trappist 1 planets as Earth-like, is just another form of our Earth Bias finding its way into our scientific language.
We know that Earth is more than conditions on Earth’s surface and we know nothing of the surface conditions on these 7 planets that reside 235 trillion miles away. It’s like using our nearsighted vision to look out at two blurry people of the same height and shape out in the distance, and declaring with confidence that they are twins. How is this any more outlandish then declaring that these planets are inhabited? Robert Miller, author of The Christians as the Romans Saw Them, questions, “Yet is it not equally true that how something appears how it is perceived by others is an aspect of what it is?”[vi]While this question deals with attitudes towards early Christians in the Roman Empire, I think it is apt to our current situation. When scientists declare that a planet is Earth-like, the people think the planet is inhabited. If scientists hesitate to make the claim of aliens, they should also be wary of declaring something Earth-like, because people arrive at the same conclusion: alien life.
[i] Close, Laird. “Lecture 26.” Astronomy 204: Great Debates in Astronomy, 2001. http://exoplanet.as.arizona.edu/~lclose/teaching/a202/lect26.html.
[ii] “Exoplanets Data Explorer | Exoplanets – Form Search | DataIR.” Accessed September 18, 2016. http://datair.soe.ucsc.edu/experiment/exoplanet/search/exact.
[iii] Dvorsky, George. “A New Equation Reveals Our Exact Odds of Finding Alien Life.” io9. Accessed September 14, 2016. http://io9.com/what-a-brand-new-equation-reveals-about-our-odds-of-fin-531575395.
[iv] Gillon, Michaël, Amaury H. M. J. Triaud, Brice-Olivier Demory, Emmanuël Jehin, Eric Agol, Katherine M. Deck, Susan M. Lederer, et al. “Seven Temperate Terrestrial Planets around the Nearby Ultracool Dwarf Star TRAPPIST-1.” Nature 542, no. 7642 (February 23, 2017): 456–60. doi:10.1038/nature21360.
[v] Gillon, Michaël. “Dwarf Planetary Systems Will Transform the Hunt for Alien Life,” n.d. https://aeon.co/ideas/dwarf-planetary-systems-will-transform-the-hunt-for-alien-life?utm_source=Aeon+Newsletter&utm_campaign=842570fcee-EMAIL_CAMPAIGN_2017_05_05&utm
[vi] Miller, Robert. The Christians as the Roman Saw Them. Yale University Press. 1986
