As astronomers continue to detect planets orbiting far-away stars, they are beginning to wonder — if there are so many planets out there, how likely is it that our planet is unique? Could any of these planets sustain life? Is there, perhaps, already life there?
The best way of searching for life on these planets — since they are much too far away to send spacecraft — is by scanning the planets’ atmospheres for gases that might indicate the presence of life.
A new study from NASA scientists helps pinpoint which combinations of gases astronomers should look for. The scientists developed simulations of a variety of different types of atmospheres around different types of suns, to find the possible molecular signatures one might expect to see from non-biological sources. They published their results in The Astrophysical Journal last week.
The presence of certain gases is unexpected on lifeless planets, because they will react with other molecules present in the atmosphere. For them to persist, there needs to be a source — life. Therefore, to search for life, astronomers could look for those suggestive gases in the atmosphere.
Oxygen might seem like a good place to start. Oxygen on earth is produced predominantly by photosynthesis from living creatures. Oxygen is mostly found in the molecular form of O2, or two oxygen molecules bonded together. Ozone (O3) is produced in the atmosphere from O2 interacting with sunlight. Both of these are possible indicators of life.
But there are also non-biological processes that produce oxygen, which could serve as a red-herring. Carbon dixoide (CO2) for instance, is produced by volcanic activity on earth, and when it interacts with ultraviolet light in the atmosphere, it produces oxygen, leaving behind carbon monoxide (CO).
To eliminate false positives, the scientists concluded, it would help to know about the presence of other gases, too. Oxygen in combination with carbon monoxide (CO), for instance, indicates that the source of the oxygen might be CO2, and not alien life.
But oxygen in combination with methane (CH4) is a likely signal of life, because oxygen interacts with related molecules when both are present in the atmosphere, depleting the supply of both — unless there’s another source.
To look for such gases in distant atmospheres, scientists study the spectrum of a star with a planet in orbit around it. (The spectrum of light is basically a measurement of what colors of light the star emits. But stars emit light beyond the range of what humans can see, which must be measured as well.) They measure the spectrum of light as the planet eclipses the star, which tells them what light is absorbed by the planet’s atmosphere.
It’s a tricky measurement. Analyzing the results, scientists have now shown, will be tricky as well.
Image Source: ”Triple-star sunset” by NASA/JPL-Caltech. Original uploader was SnoopY at en.wikipedia - Transferred from en.wikipedia. Licensed under Public domain via Wikimedia Commons