wp-plugin-hostgator domain was triggered too early. This is usually an indicator for some code in the plugin or theme running too early. Translations should be loaded at the init action or later. Please see Debugging in WordPress for more information. (This message was added in version 6.7.0.) in /home4/scienrds/scienceandnerds/wp-includes/functions.php on line 6114ol-scrapes domain was triggered too early. This is usually an indicator for some code in the plugin or theme running too early. Translations should be loaded at the init action or later. Please see Debugging in WordPress for more information. (This message was added in version 6.7.0.) in /home4/scienrds/scienceandnerds/wp-includes/functions.php on line 6114Source:https:\/\/www.quantamagazine.org\/doubts-grow-about-the-biosignature-approach-to-alien-hunting-20240319\/#comments<\/a><\/br> In 2020, scientists detected a gas called phosphine in the atmosphere of an Earth-size rocky planet. Knowing of no way that phosphine could be produced except through biological processes, \u201cthe scientists assert that something now alive is the only explanation for the chemical\u2019s source,\u201d the New York Times<\/em> reported. As \u201cbiosignature gases\u201d go, the phosphine seemed like a home run.<\/p>\n Until it wasn\u2019t.<\/p>\n The planet was Venus, and the claim about a potential biosignature in the Venusian sky is still mired in controversy, even years later. Scientists can\u2019t agree on whether phosphine is even present there, let alone whether it would be strong evidence of an alien biosphere on our twin planet.<\/p>\n What turned out to be hard for Venus will only be harder for exoplanets many light-years away.<\/p>\n NASA\u2019s James Webb Space Telescope (JWST), which launched in 2021, has already beamed back data on the atmospheric composition of a midsize exoplanet dubbed K2-18 b that some have interpreted \u2014 controversially \u2014 as possible evidence of life. But even as hopes for a biosignature detection soar, some scientists are starting to openly ask whether gases in the atmosphere of an exoplanet will ever be convincing evidence for aliens.<\/p>\n A slew of recent papers explore the daunting uncertainties in exoplanet biosignature detection. One key challenge they identify is what the philosopher of science Peter Vickers<\/a> at Durham University calls the problem of unconceived alternatives<\/a>. Put simply, how can scientists be sure they\u2019ve ruled out every possible nonbiological explanation for the presence of a gas \u2014 especially so long as exoplanet geology and chemistry remain nearly as mysterious as alien life?<\/p>\n \u201cNew ideas are being explored all the time, and there could be some abiotic mechanism for that phenomenon that just hasn\u2019t been conceived of yet,\u201d Vickers said. \u201cThat\u2019s the problem of unconceived alternatives in astrobiology.\u201d<\/p>\n \u201cIt\u2019s a bit of this elephant in the room,\u201d said the astronomer Daniel Angerhausen<\/a> of the Swiss Federal Institute of Technology Zurich, who is a project scientist on the LIFE mission, a proposed space telescope that would search for biosignature gases on Earth-like exoplanets.<\/p>\n If or when scientists detect a putative biosignature gas on a distant planet, they can use a formula called Bayes\u2019 theorem to calculate the chance of life existing there based on three probabilities. Two have to do with biology. The first is the probability of life emerging on that planet given everything else that\u2019s known about it. The second is the probability that, if there is life, it would create the biosignature we observe. Both factors carry significant uncertainties, according to the astrobiologists Cole Mathis<\/a> of Arizona State University and Harrison Smith<\/a> of the Earth-Life Science Institute of the Tokyo Institute of Technology, who explored this kind of reasoning in a paper<\/a> last fall.<\/p>\n The third factor is the probability of a lifeless planet producing the observed signal \u2014 an equally serious challenge, researchers now realize, that\u2019s tangled up in the problem of unconceived abiotic alternatives.<\/p>\n \u201cThat\u2019s the probability that we argue you can\u2019t fill in responsibly,\u201d Vickers said. \u201cIt could almost range from anything from zero to 1.\u201d<\/p>\n Consider the case of K2-18 b, a \u201cmini-Neptune\u201d that\u2019s intermediate in size between Earth and Neptune. In 2023, JWST data revealed a statistically weak sign of dimethyl sulfide (DMS) in its atmosphere. On Earth, DMS is produced by marine organisms. The researchers who tentatively detected it on K2-18 b<\/a> interpreted the other gases discovered in its sky to mean that the planet is a \u201cwater world\u201d with a habitable surface ocean, supporting their theory that the DMS there comes from marine life. But other scientists interpret the same observations as evidence of an inhospitable, gaseous planetary composition more like Neptune\u2019s.<\/p>\n Unconceived alternatives have already forced astrobiologists multiple times to revise their ideas about what makes a good biosignature. When phosphine was detected on Venus<\/a>, scientists didn\u2019t know of any ways it could be produced on a lifeless rocky world. Since then, they\u2019ve identified several feasible abiotic sources of the gas<\/a>. One scenario is that volcanoes release chemical compounds called phosphides, which could react with sulfur dioxide in Venus\u2019 atmosphere to form phosphine \u2014 a plausible explanation given that scientists have found evidence of active volcanism on our twin planet. Likewise, oxygen was considered a biosignature gas until the 2010s, when researchers including Victoria Meadows at the NASA Astrobiology Institute\u2019s Virtual Planetary Laboratory began<\/a> to find<\/a> ways<\/a> that rocky planets could accumulate oxygen<\/a> without a biosphere. For example, oxygen can form<\/a> from sulfur dioxide, which abounds on worlds as diverse as Venus and Europa.<\/p>\n Today, astrobiologists have largely abandoned the idea that a single gas could be a biosignature. Instead, they focus on identifying \u201censembles,\u201d or sets of gases that couldn\u2019t coexist without life. If anything can be called today\u2019s gold-standard biosignature, it\u2019s the combination of oxygen and methane. Methane rapidly degrades in oxygen-rich atmospheres. On Earth, the two gases only coexist because the biosphere continuously replenishes them.<\/p>\n So far, scientists haven\u2019t managed to come up with an abiotic explanation for oxygen-methane biosignatures. But Vickers, Smith and Mathis doubt that this particular pair \u2014 or perhaps any mix of gases \u2014 will ever be convincing. \u201cThere\u2019s no way to be certain that what we\u2019re looking at is actually a consequence of life, as opposed to a consequence of some unknown geochemical process,\u201d Smith said.<\/p>\n \u201cJWST is not a life detector. It\u2019s a telescope that can tell us what gases are in the atmosphere of a planet,\u201d Mathis said.<\/p>\n Sarah Rugheimer<\/a>, an astrobiologist at York University who studies exoplanet atmospheres, is more sanguine. She\u2019s actively looking into alternate abiotic explanations for ensemble biosignatures like oxygen and methane. Still, she says, \u201cI would be popping open a bottle of champagne \u2014 very expensive champagne\u00a0\u2014 if we saw oxygen, methane, and water and CO2<\/sub>\u201d on an exoplanet.<\/p>\n Pouring drinks over an exciting result in private is, of course, different from telling the world they\u2019ve found aliens.<\/p>\n Rugheimer and the other researchers who spoke to Quanta<\/em> for this story wonder how best to talk in public about the uncertainty around biosignatures \u2014 and they wonder how swings in astrobiological opinion about a given detection might undermine public trust in science. They\u2019re not alone in their worry. As the Venus phosphine saga moved toward a climax in 2021, NASA administrators and scientists implored the astrobiology community to establish firm standards for certainty in biosignature detection. In 2022, hundreds of astrobiologists came together<\/a> for a virtual workshop to discuss the issue\u00a0\u2014 though there\u2019s still no official standard for, or even definition of, a biosignature. \u201cRight now, I\u2019m pretty happy that we all agreed, first of all, that this is a bit of a problem,\u201d Angerhausen said.<\/p>\n Research moves ahead despite uncertainty \u2014\u00a0as it should, Vickers says. Running into dead ends and having to backtrack is natural for a fledgling field like astrobiology. \u201cThis is something that people should try to better understand about how science works overall,\u201d Smith said. \u201cIt\u2019s OK to update what we know.\u201d And bold claims about biosignatures have a way of lighting a fire under scientists to falsify them, Smith and Vickers say \u2014 to go hunting for unconceived alternatives.<\/p>\n \u201cWe still don\u2019t know what the hell\u2019s happening on Venus, and so of course it feels hopeless,\u201d said the astrochemist Clara Sousa-Silva of Bard College, an expert on phosphine who helped make the Venus detection. To her, the next step is clear: \u201cLet\u2019s think about Venus again.\u201d Astronomers practically ignored Venus for decades. The biosignature controversy sparked new efforts not only to discover previously unconsidered abiotic sources of phosphine, but also to better understand our sister planet in its own right.\u00a0(At least five missions to Venus<\/a> are planned for the coming decades.) \u201cI think that is also the source of hope for exoplanets.\u201d<\/p>\n<\/div>\n <\/br><\/br><\/br><\/p>\n
\nDoubts Grow About the Biosignature Approach to Alien-Hunting<\/br>
\n2024-03-20 21:58:57<\/br><\/p>\n