The Quest for Biosignatures of Extraterrestrial Life – A Holy Grail of Exoplanet Research

by Dr. Tahir Yaqoob on March 3, 2013

Exoplanet research papers currently cover a wide variety of topics. A large proportion of papers present detailed analysis of the data, or a feature of the data, for a single system, either from ground-based or space-based observations. Others present studies of subsamples consisting of groups of systems, along with statistical or collective results and deductions. Some papers explore data analysis techniques, discussing either proposed new methods, or issues with old methods. Then there are the theory papers, which try to construct models to account for the data in the context of current theories, mostly of planet formation and migration. However a significant fraction of theory papers deal with exoplanet atmospheres since the problem is extremely complex, especially for the close-in hot jupiters. The detailed microphysics of exoplanet atmospheres have bulk, macroscopic effects on the properties of an exoplanet system, yet many critical details are unknown, leading to ambiguity and uncertainty in predictions. A much smaller number of papers deal with studying possible biosignatures of extraterrestrial life.

Typically, between one and six or so papers are posted to the preprint server, arxiv.org, every day. However, the kind of paper that pauses, takes a step back, and looks at the big picture to ponder where things are going, is relatively rare. One such paper was posted in February 2013, entitled, Finding extraterrestrial life using ground-based high-dispersion spectroscopy, by I. A. G. Snellen et al. The paper is interesting on several fronts, one of them being that it is a harsh reminder of just how long it takes to open up new arenas of discovery in this business, compared to a human lifetime. For example, the X-ray observatory called Chandra was conceived in the 1970s. It did not launch until 1999, and then only after many metamorphoses compared to the origin plan. (It is still operating today and is still the highest resolution imaging X-ray telescope made by humans.) From concept to seeing results was a quarter or so of a human lifetime. And what about the atom? The atom was debated and argued over for a hundred years from the time of Dalton’s deduction from the pressure of gases before it was accepted by the scientific community. To live through that must have been at once exciting and frustrating. Would you rather live through some of it with the prospect of not knowing the answers, or would you rather read about it a hundred years later?

And so it is with exoplanets. We are living through it. We want to know the answers but there is no fast-forward button. Would you rather be living hundreds of years from now, when we might possibly have made contact with other life forms in the Universe? Or are you happy now, when we hardly know anything, but there is the excitement of starting the journey and unraveling a few “bits and bobs” here and there?

The Snellen et al. paper opens the abstract with the “no messing around” sentence, “Exoplanet observations promise one day to unveil the presence of extraterrestrial life.” Indeed, yes, it would be quite boring to study balls of rock and gas without that prospect. But then things rapidly descend into stating some depressing facts. “The cancellation of both the Terrestrial Planet Finder (TPF) and Darwin missions means that it is unlikely that a dedicated space telescope to search for biomarker gases in exoplanet atmospheres will be launched within the next 25 years.” (Darwin was a proposed European mission that never happened.) Twenty-five years. Ponder that for a moment. For those people who have just had a newborn baby this year, that baby will already have left college in that time, and indeed, that baby might be able to do a Ph.D. on the data. Or not, if there are further delays.

Despite the spelling-out of the oncoming “desert” in space-based exoplanet exploration, the point of the Snellen et al. paper is to promote some optimism, with the positive spin that it will still be possible to make some progress with ground-based telescopes, albeit nowhere near as capable as space-based endeavors. The authors give credence to their thesis with some quantitative calculations, along with the caveats. The main caveat is that for an Earth-like planet (in their calculations, an exact twin), detection of oxygen will not be possible for a system in which the host star is a “twin” of our sun, using ground-based observations only. However, the paper demonstrates that if the host star is what is known as an M-dwarf (a star smaller and much less massive than our sun, and cooler), then detection of oxygen with planned future ground-based telescopes will be possible with “a few dozen transits.” The reason for this is that a transit signal is inversely proportional to the square of the radius of the host star. This is easy to understand if you just think about the fractional area of the star (and therefore the fractional change in signal from the star) that is blocked out by a planet of a given size passing in front of the star. So targeting a smaller star boosts the observer’s sensitivity to the information conveyed in the transit data, such as absorption due to oxygen in the planet’s atmosphere.

However, the problem is that the expected absolute brightness of the nearest M-dwarf hosting an Earth-like planet is beyond the reach of current telescopes. On the other hand, the authors performed detailed simulations for specific future large-area telescopes and demonstrate that it will be feasible. They also compared the simulation results with what would be expected from the successor to the Hubble Space Telescope (the James Webb Space Telescope, or JWST), to be launched in 2018. This is not a dedicated exoplanet mission but it does have the advantage of being space-based. Even so, the authors conclude that the planned European Extremely Large Telescope (E-ELT) will be about a factor of 10 “more efficient than the JWST in detecting biomarker gases in Earth-twin atmospheres.”

The paper also advocates building precursors to the very large ground-based telescopes, called flux collectors, which are cheap because they have poor imaging quality. However, they have a large area, or light-collecting ability, and that’s what’s required for measuring atomic and molecular features, not the image quality. The authors state that a couple of hectares of area (a couple of hundred-thousand square feet) could achieve the goal for the hypothetical earth-like planet around an M star in a single orbit. The orbital time is an important consideration because, obviously for an exact earth twin around a sun-like star the orbital time is one year, and “several dozen” orbits correspond to half a human lifetime or more. In contrast, the orbital time of an earth-like planet around an M star is of the order of weeks. That’s a huge difference.

In the final (discussion) section of the paper we come back to the present with a bump, with the concisely phrased opening sentence, “It is a very long, difficult, and costly process to prove new technological concepts for highly innovative space missions such as Darwin and TPF.” The authors then remind us in some rather formal language something along the lines of how we could possibly know how to plan a mission 25 years into the future when exoplanet science is in its infancy and developing and changing so rapidly. The authors are sort of implying that you can’t go wrong with sinking a lot of money into ground-based telescopes because they can be used for other science.

The paper ends with some caution and a forthright comment. The caution is that the detection of oxygen will not prove the existence of extraterrestrial life, since it is known that certain abiotic photochemical processes can produce oxygen as well. The comment concerns doubts raised in the literature about whether biological activity could occur on planets orbiting M-dwarfs. To this, the authors firmly dismiss such criticism and say, “we just do not know.”

In some ways there are parallels to the long process in which humans discovered and explored their own planet (and this is still ongoing in the deep oceans). In terms of making direct contact with extraterrestrial life, be it intelligent or not, the story may not be told for hundreds or perhaps even thousands of years. And if we look too far into the future we realize that if our future descendants were able to come back to tell us the story, we may be ill-equipped to have any clue what they are talking about. Just think about going back in time and trying to explain to King Henry what a quark is. Not easy is it? And what would he make of a smartphone? Would he try to open it to see if there were tiny people inside? We are only talking about 500 years.

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