With the James Webb Space Telescope still more than four years away from launch astronomers are coming up with ideas on how to maximise its usefulness. A proposal published in Astrobiology outlines a way it could be used to detect signatures of life on planets around nearby stars.
While it is always possible that life around extrasolar worlds will be “not as we know it”, the obvious place to start looking is on planets as similar as possible to our own. This includes the presence of liquid water, which in turn requires a substantial atmosphere. Low atmospheric pressure means water boils as soon as it melts.
However, according to the new paper, “Current proposed methods for measuring pressure by using remote-sensing techniques that could be applicable to exoplanetary atmospheres are challenging.” The distinctive scattering of blue light that gives our sky its color is one favored suggestion for the signature of an atmosphere like our own. However, the authors point to work showing that ozone or sulphur dioxide in the atmosphere, amongst other gasses, will mask this. The peak wavelengths for light scattered in this way are also outside the James Webb's range.
A better way, the authors suggest, is to look for dimers. Dimers are formed when two molecules become bound together without forming a new molecule. In particular two O2 molecules can become temporarily bound together, and in the process absorb light differently from individual O2 molecules (monomers). One of the differences is that dimer behavior is more sensitive to atmospheric pressure than that of monomers, with a feature known as optical depth proportional to the square of the pressure, rather than the pressure to the power of one as for monomers.
Lead author and PhD student Amit Misra of the University of Washington recommends using two bands in the near infrared, with wavelengths 0.76μm and 1.06μm to measure whether the atmosphere of a planet is dense enough to maintain liquid water. “So the idea is that if we were able to do this for another planet, we could look for this characteristic pattern of absorption from dimer molecules to identify them,” Misra says.
Misra and his co-authors generated data for fictitious planets around other stars and demonstrated that with the James Webb they would be able to measure atmospheric pressures equal or greater to that of Earth's, and some techniques might push the detection to one quarter of our own pressure, depending on confounding factors.
While knowing the pressure in the atmosphere is important, just picking up O2-O2 dimers could matter for another reason. Oxygen wouldn't last in our atmosphere without photosynthetic lifeforms constantly replacing it. If Oxygen dimers were detected it would be a strong hint that life is flourishing on the planet in question.