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Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres

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Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres universe Review Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres Sara Seager 1,2,3,*, Janusz J. Petkowski 1 , Maximilian N. Günther 2,† , William Bains 1,4 , Thomas Mikal-Evans 2 and Drake Deming 5 1 Department of Earth, Atmospheric, and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; [email protected] (J.J.P.); [email protected] (W.B.) 2 Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; [email protected] (M.N.G.); [email protected] (T.M.-E.) 3 Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 4 School of Physics & Astronomy, Cardiff University, 4 The Parade, Cardiff CF24 3AA, UK 5 Department of Astronomy, University of Maryland at College Park, College Park, MD 20742, USA; [email protected] * Correspondence: [email protected] † Juan Carlos Torres Fellow. Abstract: The search for signs of life through the detection of exoplanet atmosphere biosignature gases is gaining momentum. Yet, only a handful of rocky exoplanet atmospheres are suitable for observation with planned next-generation telescopes. To broaden prospects, we describe the possibilities for an aerial, liquid water cloud-based biosphere in the atmospheres of sub Neptune- sized temperate exoplanets, those receiving Earth-like irradiation from their host stars. One such Citation: Seager, S.; Petkowski, J.J.; planet is known (K2-18b) and other candidates are being followed up. Sub Neptunes are common and Günther, M.N.; Bains, W.; easier to study observationally than rocky exoplanets because of their larger sizes, lower densities, Mikal-Evans, T.; Deming, D. and extended atmospheres or envelopes. Yet, sub Neptunes lack any solid surface as we know it, so Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized it is worthwhile considering whether their atmospheres can support an aerial biosphere. We review, Exoplanet Atmospheres. Universe synthesize, and build upon existing research. Passive microbial-like life particles must persist aloft 2021, 7, 172. https://doi.org/ in a region with liquid water clouds for long enough to metabolize, reproduce, and spread before 10.3390/universe7060172 downward transport to lower altitudes that may be too hot for life of any kind to survive. Dynamical studies are needed to flesh out quantitative details of life particle residence times. A sub Neptune Academic Editor: Apurva V. Oza would need to be a part of a planetary system with an unstable asteroid belt in order for meteoritic material to provide nutrients, though life would also need to efficiently reuse and recycle metals. The Received: 19 February 2021 origin of life may be the most severe limiting challenge. Regardless of the uncertainties, we can keep Accepted: 24 May 2021 an open mind to the search for biosignature gases as a part of general observational studies of sub Published: 31 May 2021 Neptune exoplanets. Publisher’s Note: MDPI stays neutral Keywords: exoplanets; exoplanet atmospheres; biosignature gases with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction We aim to expand our understanding of what kind of exoplanets have habitable environments. We begin with the motivation to consider the atmospheres of a type of Copyright: © 2021 by the authors. exoplanet called “sub Neptunes” (Section 1.1). We then summarize Earth’s atmospheric Licensee MDPI, Basel, Switzerland. This article is an open access article aerial biosphere (Section 1.2), followed by a review of theoretical studies on giant planet distributed under the terms and and brown dwarf atmospheres as potential abodes for life (Section 1.3). conditions of the Creative Commons 1.1. Motivation to Expand the Pool of Planets Considered to Be Potentially Habitable Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The growing excitement of the chance to discover signs of life on exoplanets by 4.0/). way of exoplanet atmosphere biosignature gases is motivating many new simulations to Universe 2021, 7, 172. https://doi.org/10.3390/universe7060172 https://www.mdpi.com/journal/universe Universe 2021, 11, x FOR PEER REVIEW 2 of 31 Universe 2021, 7, 172 2 of 30 1.1. Motivation to Expand the Pool of Planets Considered to Be Potentially Habitable The growing excitement of the chance to discover signs of life on exoplanets by way assessof exoplanet detectability. atmosphere The excitementbiosignature is gases tempered is motivating by the reality many that new only simulations a small numberto assess ofdetectability. potentially The habitable excitement rocky is planets tempered will by be the accessible reality that for suchonly observationsa small number with of thepo- next-generationtentially habitable telescopes rocky planets expected will in be the accessible next two for decades, such observations including the wit Jamesh the Webb next- Spacegeneration Telescope telescopes (JWST) expected [1], the in “extremely the next two large” decades ground-based, including telescopesthe James W nowebb under Space constructionTelescope (JWST) [2–4], [1] and, the possible “extremely space large observatories” ground-base suchd telescopes as the Starshade now under Rendezvous construc- Probetion [2 [–54]],, Theand HabExpossible Observatory space observatories [6], the LUVOIRsuch as the telescope Starshade [7], Rendezvous or the Origins Probe Space [5], TelescopeThe HabEx [8 ].Observatory [6], the LUVOIR telescope [7], or the Origins Space Telescope [8]. TheThe “traditional” habitable habitable planet planet is is rocky rocky,, wi withth a thin a thin atmosphere, atmosphere, most most usually usually con- consideredsidered to have to have a high a high mean mean molecular molecular weight weight atmosphere, atmosphere, such such as asone one dominated dominated by byN2 Nor2 COor CO2. Such2. Such habitable habitable planets, planets, however, however, present present formidable formidable challenges challenges in detecting thethe componentscomponents ofof theirtheir thinthin atmospheres.atmospheres. IfIf planetsplanets moremore favorablefavorable forfor observationalobservational studystudy thanthan small,small, rocky rocky exoplanets exoplanets are are valid valid targets targets for for the the detection detection of signs of signs of life, of then life, wethen need we toneed expand to expand our understanding our understanding of the of habitable the habitable planet planet parameter parameter space. space. InIn thisthis paper,paper, wewe focusfocus onon thethe temperatetemperate subsub Neptune-sizedNeptune-sized exoplanetsexoplanets (hereafter(hereafter calledcalled temperatetemperate subsub Neptunes)Neptunes) asas candidatecandidate abodesabodes forfor life.life. SubSub NeptunesNeptunes areare looselyloosely defineddefined asas planetsplanets largerlarger thanthan EarthEarth butbut smallersmaller thanthan Neptune,Neptune, withwith radiiradii betweenbetween 1.61.6and and 33 oror 44 EarthEarth radiiradii (R (REarthEarth)).. Sub Neptunes are distinguisheddistinguished fromfrom ‘super-Earths’,‘super-Earths’, whichwhich areare alsoalso planetsplanets largerlarger thanthan EarthEarth butbut smallersmaller thanthan subsub Neptunes,Neptunes, byby havinghaving thickthick envelopesenvelopes ofof HH22 oror HH22-He-He thatthat comprisecomprise 11 toto 10%10% ofof thethe planetaryplanetary massmass [[99,10],10].. WeWe furtherfurther definedefine “temperate”“temperate” to to highlight highlight the the sub sub NeptunesNeptunes thatthat receivereceiveEarth-like Earth-likeamounts amountsof ofenergy energyfrom from theirtheir hosthost starstar (e.g.,(e.g., K2-18bK2-18b [[1111]]),), becausebecause ofof suchsuch planets’planets’ specialspecial potentialpotential toto havehave liquidliquid waterwater eithereither inin cloudcloud formform [[1212]] or or eveneven asas anan oceanocean beneathbeneath thethe atmosphereatmosphere [ 13[13,14],14]. SubSub NeptunesNeptunes are are favorable favorable for for both both detection detection and and atmospheric atmospheric study study because because of their of largetheir sizeslarge andsizes lower and lower densities densities as compared as compared to rocky to rocky exoplanets exoplanets [15]. [15] Those. Those transiting transit- reding dwarfred dwarf stars stars have have larger larger transit transit signals signals as compared as compared to those to those transiting transiting sun-sized sun- stars.sized Hundredsstars. Hundreds of sub of Neptunes sub Neptunes are known are known (Figure (Figure1), such 1) that, such they that appear they appear to be the to mostbe the commonmost common type of type planet of planet in our in Galaxy our Galaxy (out to (out orbital to orbital periods periods of 100 of days) 100 [days)16–18 [].16–18]. FigureFigure 1.1. The k knownnown population population of of exoplanets exoplanets with with the the sub sub Neptune Neptune exoplanets exoplanets marked marked by bythe the blueblue bands.bands. TheThe leftleft (right)(right) panelpanel showsshows the the planet planet radius radius (mass) (mass) in in Earth Earth radii radii (mass) (mass) versus versus orbital or- periodbital period in days. in days. The blue-shaded The blue-shaded region region marks marks the typically the typically adopted adopted sub Neptune sub Neptune defining defining range of approximatelyrange of approximately 1.6–4 Earth 1.6 radii–4 Earth (2 to radii 10 Earth (2 to masses). 10 Earth Only mass confirmedes).
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