An astrobiologist on the wonderful, weird planets that probably don’t harbor life.

An Astrobiologist’s Love Note to the Weird Planets Out There

An Astrobiologist’s Love Note to the Weird Planets Out There

The citizen’s guide to the future.
June 8 2015 11:28 AM

Seeing Stars

I’m an astrobiologist. But I love the weird planets that are unlikely to harbor life anyway.

The unconfirmed planet Tau Ceti e might be habitable, but even if it’s not, it could be just as fascinating.

Photo illustration courtesy PHL/UPR Arecibo via Wikimedia Commons

I recently broke the hearts of Star Trek fans.

In a paper for the Astrophysical Journal, my colleagues and I looked at several star systems to see which might be most likely to be compatible with life. One of those was the star Tau Ceti, which is the closest sunlike star to Earth, making it popular in science fiction, including the Star Trek universe. But we found that Tau Ceti has an unusual ratio with other important rock-forming elements, magnesium and silicon. It lies on the high end of the magnesium-to-silicon scale, with a ratio of 1.8 in favor of magnesium. It is 70 percent more magnesium-rich than our sun.

Because of the unusual composition of Tau Ceti, we put it low on our list of potentially habitable star systems. When they saw the results, Star Trek fans were disappointed: One of their favorite solar systems—home of the Traveler and the spacecraft Kobayashi Maru—had been demoted. Social media was teeming with comments like “This delivers an unfortunate swat to the dreams of many.”


But this isn’t bad news! Tau Ceti may not host alien life, but it would be amazing to observe a planet so vastly different than Earth, even if the differences are mostly scientific jargon at the moment. Science fiction gets one thing right: We live in a strange universe of constantly surprising discoveries.

Life on other planets is scientifically sexy—which is why the search for life has fascinated us over the history of humanity. From ancient Greece to modern scientists like Frank Drake and Enrico Fermi, people have debated the idea of our galaxy being inhabited. Extraterrestrial life litters our bookshelves and our movie screens. The poetry of life in the cosmos is partially what drove me to call myself a stellar astrobiologist (stellar as in star, of course) and not an astrophysicist. The same ideals that drive our quest to find life also drive us to explore the unknown. But when it comes to finding planets, maybe we’re a little too focused on life—our search for aliens means that we overlook incredibly amazing things about non-habitable worlds.

Planets aren’t very special: The Kepler spacecraft found its 1,000th planet earlier this year. Now, we are more interested in whether they are habitable. Traditionally, the word habitable isn’t about whether a planet currently supports life, but rather if it lies in an area adeptly called the “habitable zone.” The implication is that these Goldilocks planets could have a temperature conducive to liquid water. The temperature of the star and the distance from the planet to the star classically determine this zone. However, a few years ago, my Arizona State University colleague Patrick Young, a theoretical astrophysicist, looked at how chemical abundances of stars affect habitable zones, concluding that oxygen plays a big role in this calculation, and habitable zones are not as simple as we would hope. (Disclosure: Future Tense is a partnership of ASU, Slate, and New America.)

People often wonder whether dismissing “non-habitable” planets is shortsighted—what about life that is different than that we know of on Earth? However, no one will pay us to look for something we don't understand or even know could exist. We have just one data point when it comes to life: Earth. We don’t even really know how it started here. When we look for planets to explore as possibly habitable, we look for planets about the size of Earth, having water, and of a similar temperature. This is a reasonable bet, but I’m deviating from it a little. In my work, my first instinct isn’t to look for stars that are most like our sun but toward the extreme chemical anomalies.


Carbon and oxygen are two of the most important rock-forming elements on Earth and are natural starting points for exoplanetary mineralogy. A few years ago, a group at Yale University, led by researcher Nikku Madhusudhan, looked at the super-Earth 55 Cancri e and determined the carbon-to-oxygen ratio was large enough that carbon would dominate the planetary system. It was coined a Diamond Planet, because a diamond layer could exist at high pressure inside the planet. Now scientists have to consider how a layer of diamond would affect mantle dynamics. New classes of planets entirely outside our observable solar system began appearing.

There was a time when the motto of NASA’s Astrobiology Institute was “follow the water.” We began looking for water in all the celestial bodies around us. Of course, just because a planet has liquid water does not mean that it will necessarily have life. We discovered that water was actually common and could be found on moons around gas planets, such as Jupiter’s Europa, all the way to the atmospheres of giant exoplanets. We realized that just finding water would not help us narrow our search for life. So we began looking for the elements we know life requires. This is one of the objectives of NASA’s new coalition, the Nexus for Exoplanet System Science, or NExSS. Another of my colleagues, Natalie Hinkel, has created something called the Hypatia Catalog, which records the elemental abundances of stars as found by many scientists. She is building an interactive website to let people explore the makeup of their favorite stars and to help scientists better understand the stars and planets they study. Now we can get an idea about the range of possibilities for stars and planets in our galaxy. Some stars, like Tau Ceti, will fall off our list of possibly habitable worlds, but this does not mean we should ignore them. The opposite is true. They are so strange, we must study them.

Judging by their dismayed reactions, I feel like most fans forgot the Star Trek intro narration, or at least two-thirds of it. Yes, we would love to seek out new life and civilizations, but we also need to explore these strange new worlds and boldly go where we have never been before. This doesn’t only mean flying a spaceship to parts unknown. We need to start thinking, studying, and being fascinated by worlds that a few decades ago we couldn’t fathom existed.

This article is part of Future Tense, a collaboration among Arizona State University, New America, and Slate. Future Tense explores the ways emerging technologies affect society, policy, and culture. To read more, visit the Future Tense blog and the Future Tense home page. You can also follow us on Twitter.

Michael Pagano is a postdoctoral research fellow at Arizona State University’s School of Earth and Space Exploration.