Signatures of Life on an Exoplanet
Although scientists continue to discover planets outside the solar system, they really want an answer to a different question: Do any of these planets host life? Answering this question requires knowledge about a definitive (or at least likely) signature of life on a planet as well as the ability to detect that signature. Given the expected advances in telescope technology over the next decade, free oxygen in an exoplanet’s atmosphere is the best signature to seek.
Without question, the last two decades witnessed unparalleled advances in our knowledge of planets. While the number of planets within the solar system decreased from nine to eight (which some didn’t like), the number of known planets beyond the solar system grew by leaps and bounds. Astronomers now know of nearly 1,000 exoplanets (with another couple thousand likely candidates). Yet this progress still leaves the real question in most astronomers’ minds unanswered—does life exist beyond Earth’s confines? Fortunately, research into the signatures of exoplanet life and continued improvements in telescope technology offer the hope of beginning to answer this question over the next decade.
Science news announcements often hail the discovery of a “habitable” planet. These planets meet some minimum size and orbit requirements that give them the potential to host liquid water on the surface. Yet given the constraints in telescope technology, astronomers cannot actually determine whether any liquid water exists on these planets. The main difficulty arises in trying to find the light from the planet amidst the far brighter light coming from its host star.
James Kasting, a prominent scientist in the search for extraterrestrial life, notes that astronomers continue to make progress in solving this problem. In an Astrobiology article, he points out that the Thirty Meter Telescope (TMT) should commence operations within the next decade and could detect the existence of liquid water on some nearby exoplanets.1 (The article also mentions the space-based Terrestrial Planet Finder, but congressional budget issues resulted in the indefinite suspension of its development.)
As a ground-based telescope, the TMT will use adaptive optics to correct for atmospheric distortions. The collection area associated with such a large telescope makes it sensitive to the small signals resulting from different components of exoplanet surfaces. It may not find any water, but the TMT will open new horizons in exoplanet research.
However, it is worth noting that the presence of liquid water on a planet’s surface does not guarantee the existence of life. Consequently, astronomers seek other signatures indicative of life. The detection of free oxygen in the atmosphere represents one promising biosignature. Oxygen’s highly reactive nature means that it must be replenished continually. However, validating such a signature requires demonstrating that the planet’s atmosphere represents the best (if not only) plausible option for generating it.
Scientists have identified nonbiological processes that produce free oxygen continually, but a recent study demonstrated that those processes operate with much less efficiency than what is required for life on any planet in the liquid water habitable zone. Specifically, industrial processes are able to produce free oxygen using specific kinds of catalysts, but research showed that these catalysts would not occur naturally under the conditions required for a habitable planet.2
Bottom line: astronomers have made incredible strides in finding planets outside our solar system. While some of those exoplanets meet a minimum set of requirements—which could make it possible for liquid water to exist on their surfaces—we have no way to detect the presence of liquid water. However, telescope technology advances in the next decade may bring us to the brink of addressing the elusive question of whether life exists beyond Earth. That is one exciting prospect!
1. James F. Kasting, “Habitable Planets: What Are We Learning from Kepler and Ground-Based Searches?” Astrobiology, 11 (May 20, 2011): 363–66.
2. A. Léger et al., “Is the Presence of Oxygen on an Exoplanet a Reliable Biosignature?” Astrobiology, 11 (May 20, 2011): 335–41.