Looking for The Right Atmosphere

The next step in the search for habitable planets.

Artist's illustration of the planets, TRAPPIST-1b and TRAPPIST-1c, passing in front of their parent star, which is much smaller and cooler than our sun. (NASA/ESA/STScI/J. de Wit (MIT))
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Now that scientists have identified thousands of planets around other stars, they’re moving on to a more difficult problem: identifying which ones might be suitable for life.

In a new study by Julien de Wit of MIT and colleagues, reported in the journal Nature, the researchers took a close look at the atmospheric composition of two previously discovered,  potentially habitable planets in the TRAPPIST-1 system, which is only about 40 light years from Earth. When the planets passed in front of their central star, de Wit’s team analyzed the light filtering through their atmospheres and found that neither planet has an atmosphere dominated by hydrogen or helium. Thus, we can rule out that these two planets are gas giants like Jupiter, which was still a possibility, since their masses remain unmeasured.

The authors go on to say that the spectra recorded during the planetary transits are consistent with the atmospheres of rocky, terrestrial planets in our own solar system, such as Venus, Earth, and Mars. However, the results would also be consistent with a planet such as Mercury, which has no atmosphere at all.

Also, the two planets in question orbit very close to an “ultracool” dwarf star with an effective temperature of less than 2,700 degrees Kelvin. This category also includes brown dwarfs, and such a star would be unable to sustain hydrogen fusion in its interior, meaning that the solar system studied by de Wit and his colleagues is likely to be very different from our own. Many different types of atmospheres are imaginable, beside just the ones we observe in our planetary neighborhood.

Any planet hosting life would have to meet many conditions, apart from being rocky and being located within a star’s habitable zone (where water is stable in liquid form). To start with, life needs an energy source, polymeric chemistry and a suitable solvent. There has to be a magnetosphere to protect any living things from solar flares and cosmic radiation. An efficient recycling mechanism such as plate tectonics has to be in place for a significant biosphere, and certainly for Earth-like complex life. And topographic complexity—mountains and valleys—on the planetary surface would be highly beneficial, as this would enhance evolution and biodiversity. These are only some of the necessary conditions for life. We’ll be able to determine some of these parameters for some exoplanets after the James Webb Space Telescope is launched, but until then we’re pretty much in the dark when searching for a second Earth.

Nevertheless, the new work by de Wit and colleagues is significant, and shows that we can at least gather some preliminary information on planetary atmospheres. Perhaps even more impressive, the researchers put together their proposal to use the Hubble Space Telescope in less than 24 hours, and got an immediate review, which came out positive, allowing them to catch the double transit in time.

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About Dirk Schulze-Makuch
Dirk Schulze-Makuch

Dirk Schulze-Makuch is a professor of astrobiology at Washington State University and has published seven books related to the field of astrobiology and planetary habitability. In addition, he is an adjunct professor at the Beyond Center at Arizona State University and currently also holds a guest professorship at the Technical University Berlin in Germany.

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