'Habitable' Exoplanets Might Not Be Very Earth-Like After All
More caution is needed in describing newly-found exoplanets as habitable given the limitations of observations, researchers warn.
One of the most exciting moments in exoplanet science came in late February, when NASA's Spitzer Space Telescope announced the discovery of seven rocky planets orbiting in or near the habitable zone of their parent star, TRAPPIST-1, which lies 40 light years away from Earth.
"The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system," NASA said in a statement. "All of these seven planets could have liquid water - key to life as we know it - under the right atmospheric conditions, but the chances are highest with the three in the habitable zone."
Comparing the habitability of our own planet to the conditions on newly-discovered exoplanets, however, could be misleading, according to the authors of a commentary in the journal Nature Astronomy. They argue that even though scientists have found hundreds of Earth-sized planets, there is no available technology to show us if their surfaces are remotely Earth-like.
Lead author Elizabeth Tasker said the language used by NASA and others can be "unnecessary and dangerous," in the sense that the public could get overly hopeful that life exists on these other planets.
Tasker, who is an associate professor in the department of solar system science at the Japan Aerospace Exploration Agency, was not involved in the TRAPPIST-1 discovery.
In studying the data, however, she noted that the team found that many of the planets orbit in resonant configurations, meaning that one planet's orbit is a direct ratio of another. In other words: The inner planet in the TRAPPIST-1 system, for example, orbits eight times in the period it takes the outer planet to orbit two.
Tasker said this probably suggests these worlds formed further out from the star and over the years, their mutual gravitational attraction pulled them in closer. "They may not be terrestrial planets, but maybe the cores of gas giants because they formed in a similar region to our own solar system," she said.
Tasker's interest in the habitability of new-found planets sprung from the announcement of Gliese 832c in 2014. In the scientific paper announcing the discovery of the planet, the authors cautioned that this "super-Earth" is more likely a "super-Venus" with a massive atmosphere. On Venus, the surface roasts at oven temperatures and can crush unprotected spacecraft in moments. Yet, Tasker said that many of the media stories fawned over Gliese 832c as a potentially habitable world.
Tasker said the oversimplification of comparing exoplanets to our own planet arises from something called the Earth similarity index. The index uses four parameters: the radius of the planet, the radius of Earth, the stellar flux (or radiation) of the exoplanet's star, and the solar flux of our own sun. This metric is used by entities such as the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, which ranks planets on a habitability index.
"In practice, these [parameters] aren't independent," Tasker said. For example, the flux coming from the star gives equilibrium temperature, which is different from planetary surface temperature - and doesn't take into account how much radiation the planet receives, she said.
Tasker acknowledged it is difficult for people to stop comparing planets to one another, but she did advocate for - at the least - a better description of what constitutes a habitable zone. She said that the media should specify that the zone is where liquid water could exist on a rocky planet orbiting the star, rather than just saying where liquid water could exist. She also suggested that if possible, reports specify that the habitable zone is dependent on many factors, such as whether the star has variable radiation, the tilt of the planet's axis, and the planet's atmosphere.
"Our knowledge is far from sufficient to comparatively rank the ability of planets to support life," the Nature commentary concludes. "Unless we want to risk destroying the chance to find out if the Earth is unique, we need to stop pretending that we already know."
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