Sections

Services

Information

UK United Kingdom

Search for alien life could remain fruitless, study finds

Given that we are unlikely to be visiting an exoplanet any time soon, astronomers have been contemplating whether it might…

Aliens will only ever be toys for us. bflv, CC BY-NC

Given that we are unlikely to be visiting an exoplanet any time soon, astronomers have been contemplating whether it might be possible to detect indications of simple life – a biosignature – from a distance. Many think that the strongest case for extraterrestrial life would be the discovery of oxygen and methane on the same body. They also think that the likelihood of finding such a biosignature is greatest on an Earth-like planet that is orbiting a sun-like star.

Astronomers who hope to search for these biosignatures in expolanets, however, may be in for a disappointment. New research finds that there is no way we can confirm that such a signature is actually the result of extraterrestrial life. The problem, it turns out, is that an exomoon’s atmosphere will be indistinguishable from the one of the planet it orbits.

Finding E.T.

Searching for extraterrestrial life is no easy feat. Astronomers have to first search for a star that has planets. Then they have to ensure that there is at least one planet that orbits this star in the habitable zone, which is a region around the star in which we might expect liquid water. Finally, they have to record the faint light that originated from the bright star and was reflected off the exoplanet after having passed through its atmosphere.

This faint light, even if only a handful of photons, when compared with light from the parent star is enough to give some indication of the chemicals in the atmosphere of this planet. Life as we know it creates two gases that wouldn’t naturally be present in an atmosphere at the same time – oxygen from photosynthesis and methane from microbes.

Both oxygen and methane can be created independently by non-living processes, so their individual presence is of little interest. What scientists are looking for is both of them in the atmosphere of a single body. If these reactive gases are not constantly replenished by living things, they will react with each other, creating carbon dioxide and water. As a result, we should not observe them in the same atmosphere without a large, living source.

False hopes

In the new study, published in the Proceedings of the National Academy of Sciences, Hanno Rein at the University of Toronto and his colleagues wanted to know whether anything else could mimic this biosignature. While working through potential false positives, which are signals that would show signs of life but in reality there isn’t life, he found a big one: exomoons. Rein found that observers on Earth will not be able to tell whether the signs of methane and oxygen originate from a single celestial body, or come from two nearby worlds.

This could happen because, just as Earth has a moon, there is a chance that exoplanets will have exomoons. While we have yet to find an exomoon, looking at the various moons of our solar system’s planets suggests that exomoons ought to be plentiful. However, even if they are plentiful, chances are that exomoons will be difficult to spot.

If both these celestial bodies have an atmosphere and in their atmospheres the exoplanet has oxygen and the exomoon has methane (or vice-versa), then an observer on Earth will record an oxygen-methane biosignature. This might seems like evidence for life, whereas in reality both these gases are being produced by non-living processes on two separate celestial bodies. Since they can’t react with each other, they will be able to build up to high levels.

Futile technology

“Even if we somehow developed ways of finding exomoons, we won’t be able to tease out the difference between their atmospheres given the limited amount of light that reaches us,” Rein said. This fundamental limit on the light that reaches us is called photo noise.

Rein limited his analysis to biosignatures coming from Earth-like planets orbiting a sun-like star, which is the combination that astronomers are betting has the greatest chance of hosting life. The American space agency NASA recently announced that they had found such an Earth-sized planet less than 500 light years away, although the star it orbits isn’t sun-like.

While their analysis might seem quite restrictive and involves a number of assumptions, it does not really matter: interpretation of biosignatures needs to be flawless. According to David Cullen at the University of Cranfield, “This study seems to highlight a real issue that will needed to be considered, along with other issues, when interpreting biosignatures.”

Rein himself was surprised to find such a limitation. However, he sees the results of his work in positive light. “Finding such a limitation tells us what we should focus on in the future. Rather than a restricted search for Earth-like planets orbiting sun-like stars, we should broaden our search,” he said.

What this research shows is a need to move away from a highly focused search for extraterrestrial life that is currently in place. Rein points out that the chances of eliminating such false positive biosignatures increases as the star becomes dimmer or larger planets are considered. Perhaps alien life is not just unlike that on Earth, but it is also resides in a place that is unlike Earth.

Sign in to Favourite

Join the conversation

14 Comments sorted by

  1. Thomas Goodey

    Researcher

    This is not correct. If the exomoon were periodically eclipsed by its planet (from our point of view) which would be quite likely, then we could tell their optical signatures apart.

    report
    1. Akshat Rathi

      Science and Data Editor at The Conversation

      In reply to Thomas Goodey

      You are right. However, the chances of a random observer looking at Earth when our moon is being eclipsed is a mere 2%, as the authors point out in the paper. This means that we will have to wait many years and for many observations before we can be sure that we are catching the signal when the exomoon is in such a position. Of course this is assuming that we can spot the exomoon in the first place. We are yet to find a method of confirming whether an exoplanet has an exomoon (or two), which makes it all the more less likely that we will not be able to tease apart the optical signature with certainty.

      report
    2. Thomas Goodey

      Researcher

      In reply to Akshat Rathi

      Aha! But the Earth is an exception in being a small planet having such a large moon. Most large exomoons detected will presumably be circling around gas giants, and will be eclipsed frequently from a wide angle, as Io and Europa are in our system.

      Moreover, you should consider a selection effect: many of these exoplanets are detected because they pass over the disk of their suns, from our point of view. This means that we are more or less in the ecliptic plane of their systems, assuming there is a well-defined ecliptic plane there. It's reasonable to think that most systems have such a preferential plane, which is the general plane of the angular momentum of the primordial planetary disk, I suppose. In that case we will probably be much more likely to be near the rotational plane of exomoons of large planets in the system.

      report
    3. Akshat Rathi

      Science and Data Editor at The Conversation

      In reply to Thomas Goodey

      This paper's analysis is restricted to Earth-like exoplanets. We only have one example to go by. Rein too told me that we just don't know what size normal exomoons for Earth-like planets would be.

      So considering gas giants doesn't fit this story. That point speaks to what Rein said in "broadening our search".

      Your latter point is taken into consideration in the 2% probability.

      report
    4. Thomas Goodey

      Researcher

      In reply to Akshat Rathi

      Then I think the analysis is too restrictive and almost meaningless. I understand that it is generally believed that our large Moon is a bit of a freak, so if we are considering only Earth-sized exoplanets, we've more or less ruled out interesting exomoons already Just going by our solar system, we have the example of three Earth-sized planets with one large moon, and two big gas giants with five large moons (Io, Europa, Ganymede, Callisto, Titan). The size of these large moons might be just big enough to hold an atmosphere to support life, in the right other circumstances. So I think that, in terms of evaluating the possibility of life on exomoons, the population that should be considered is exomoons of large gas giant exoplanets, not exomoons of Earth-sized exoplanets. And, as I pointed out, the former population might well be periodically eclipsed, which would sort out the chemical signatures very effectively.

      report
    5. Akshat Rathi

      Science and Data Editor at The Conversation

      In reply to Thomas Goodey

      This answers your point: "While their analysis might seem quite restrictive and involves a number of assumptions, it does not really matter: interpretation of biosignatures needs to be flawless."

      Given the significance such a biosignature, we have to absolutely certain that it isn't a false positive. They have pointed out one among many. If you don't see it, I think we will have to agree to disagree.

      report
    6. Max Wallis

      research associate

      In reply to Akshat Rathi

      Astronomers don't have a monopoly for detecting "indications of simple life – a biosignature – from a distance". We search for bio-signatures in meteorites, especially those from Mars (> 100 so far identified), from comets and occasional ones from further afield. We can progress beyond microfossils and bioetching to molecular fossils like hopanes. Once this becomes well established science, based on the 'ground truth' from meteorites, detecting the molecular signatures of such complex biochemicals in IR spectra opens up the possibility of extra-solar system detection. Arguments and inferences based on methane and oxygen address only a small subset of the possibilities.

      report
    7. Thomas Goodey

      Researcher

      In reply to Akshat Rathi

      Ah, I see the point, which hasn't been expressed sufficiently clearly. We are both right. Let's consider the matter deeper. Suppose we look at a star and detect an exoplanet going around it, with maybe an associated exomoon; we don't know. Suppose that we detect both oxygen and methane signatures from the exo-object possible-combination. This could either imply oxygen and methane existing together on one world (planet or moon) which would suggest life ("Case A"); or it could imply oxygen on one world…

      Read more
    8. Tim Benham

      Student of Statistics

      In reply to Thomas Goodey

      Paper is over-drawn. Exomoons of Earth-like planets are unlikely to support atmospheres. Earth's moon is considered abnormally large yet it it is far too small to retain an atmosphere.

      report
    9. Thomas Goodey

      Researcher

      In reply to Tim Benham

      Quite so. As I commented earlier, the interesting case is an exomoon of a large planet, perhaps a Jupiter-like planet. The example of our Titan shows that such an exomoon could easily be big enough to hold a substantial atmosphere. Then indeed confusion of the type described in the article could arise. I commented that, if the exomoon is in such an orbit as to be eclipsed from our point of view, we might be able to resolve that confusion.

      report
    10. Tim Benham

      Student of Statistics

      In reply to Thomas Goodey

      I think demanding eclipses is being over-strict, but they are simply unlikely, not impossible. Exoplanet transits are also unlikely, yet many hundreds if not thousands exoplanets have been detected this way. Moreover, moon orbits are often going to be almost coplanar with planetary orbits, so a transiting planet is quite likely to eclipse its own moon.

      report
    11. Thomas Goodey

      Researcher

      In reply to Tim Benham

      I said that IF eclipses took place, they would be useful for discrimination. The rest of your post is exactly what I said in a previous post.

      report
  2. Oliver König

    logged in via email @live.de

    I strongly agree with the closing remark that alien life need not be like Earth-life, since the history of life on earth would not re-occur elsewhere in the exact same fashion even if all other factors were largely identical. There are numerous lifeforms on earth that do not breathe oxygen/carbondioxide, for example sulfur-reducing bacteria, and while they are not widespread, only slightly different conditions over the planetary history of an otherwise earthlike planet could perhaps induce them to…

    Read more
  3. Vern Wall

    Retired engineer

    Scientists can't even identify life right here on Earth. There is a rock alleged to be from Mars, and scientists can't agree on whether it has little corpses, fossil turds, or just curly rocks.

    report