SET-E: The Search for Extraterrestrial Environmentalism
DDraft version April 1, 2016
Preprint typeset using L A TEX style emulateapj v. 5/2/11
SET-E: THE SEARCH FOR EXTRATERRESTRIAL ENVIRONMENTALISM
Ben Montet and Ryan Loomis (Dated: April 1, 2016)
Draft version April 1, 2016
ABSTRACTThere is currently no evidence for life on any known exoplanet. Here, we propose a form of “galacticanthropology” to detect not only the existence of life on transiting exoplanets, but also the existenceof environmental movements. By observing the planet’s atmosphere over long time baselines, thedestruction and recovery of a hole in an exoplanet’s ozone layer may be observable. While not readilydetectable for any one system with JWST, by binning together observations of hundreds of systemswe can finally determine the occurrence rate of environmental movements on Earthlike planets in thegalaxy, a number we term η Green Earth. INTRODUCTION
Now that the detection of planets orbiting other starsis pass´e, we can begin searching these known planets forsignatures of extrasolar life. The search has spanned theobservational (Wright et al. 2014; Griffith et al. 2015) tothe theoretical (Rauer et al. 2011; Grenfell et al. 2014),making some predictions testable with current or soon-to-be-available instruments (Loeb & Zaldarriaga 2007;Davenport 2013; Seager et al. 2013; Snellen et al. 2013)and others testable in the more distant future (Loeb &Turner 2012; Loeb 2014; Stevens et al. 2015).Should extraterrestrial life be discovered, there is noclear consensus as to whether humans should attemptto interact with it or not (Serling 1962; Brin 1983).While biosignatures will be detectable with upcomingtelescopes such as the James Webb Space Telescope(JWST), it will be difficult to ascertain whether thesesignatures are the result of relatively simple organismsor of intelligent civilizations.Given that we may wish to avoid interaction withmalevolent or carelessly destructive lifeforms, it wouldbe advantageous to find observable signatures of thesetraits. Atmospheric changes as a result of human inter-actions with the environment serve as a good startingtemplate. Certainly, other species have significantly af-fected the environment on Earth: for example, plant lifeand phytoplankton have, over hundreds of millions ofyears, provided the Earth’s atmosphere with a consider-able quantity of oxygen. However, humans are the onlyknown species to affect the atmosphere on a timescaleof decades. Evidence of an exo-atmosphere changing ona similar timescale may then be evidence for intelligentlife.Searching for signs of an advanced civilization destroy-ing themselves and their environment is a rather grimendeavour (e.g. Stevens et al. 2015), so we propose in- [email protected] Cahill Center for Astronomy and Astrophysics, CaliforniaInstitute of Technology, 1200 E. California Blvd., MC 249-17,Pasadena, CA 91106, USA Harvard-Smithsonian Center for Astrophysics, 60 GardenStreet, Cambridge, MA 02138, USA But we did this on our own time, please don’t blame ourinstitutions. https://twitter.com/JasonFRowe/status/696792227629113344 stead to search for evidence of extraterrestrial environ-mentalism. In this brief paper, we describe how givenhigh enough photometric precision and a long time base-line, it would be possible to observe the creation anddiminution of a hole in the ozone layer of an exo-Earth.Such a hole could affect both the transmission signatureof a planet and the transit light curve shape, and couldbe observed as seasonal variations in ozone abundancethrough reflected light spectroscopy. ATMOSPHERIC CHANGES CAUSED BY(UN)INTELLIGENT LIFE
The Earth’s Ozone Hole
Ozone in the Earth’s atmosphere is concentrated in thestratosphere, shielding life on Earth’s surface from ultra-violet radiation (Solomon 1999). The mass production ofchloro-fluoro-carbons (CFCs) in the mid-1900s, however,has caused substantial changes to the ozone layer on arelatively short timescale (e.g. Solomon 1988; Andersonet al. 1991). Thermal gradients in the atmosphere leadto the concentration of these ozone destroying moleculesinto seasonal clouds over the South Pole, severely de-pleting ozone and forming an ‘ozone hole’ every spring(e.g. Crutzen & Arnold 1986; Toon et al. 1986; Shin-dell et al. 1998). Although global ozone concentrationshave exhibited a long-term decline over the past decades,swift international action through the Montreal Protocolhas significantly reduced CFC output and ozone levelsare projected to recover in the coming century (Elkinset al. 1993; Weatherhead & Andersen 2006; Newmanet al. 2006). This has been widely accepted as one ofthe greatest successes of modern environmentalism.
Extraterrestrial Ozone Holes
Due to its radiation shielding abilities, the presenceof ozone in an exoplanet atmosphere is likely valuableto carbon based lifeforms. If extraterrestrial intelli-gent life has the same propensity for ingenuity and self-destruction as humans do, however, they are likely toalso find ways to damage their ozone layer. Very littleresearch has been done on what such an ‘exo-ozone hole’might look like, but we speculate that there will likelybe a long-term decline of total atmospheric ozone and Wildly a r X i v : . [ a s t r o - ph . E P ] M a r Fig. 1.—
Simulated transit (not to scale) of an Earth analog passing in front of a Sunlike star. The Earth’s ozone signature is highlighted,with bluer colors corresponding to positions on the Earth with less ozone and redder colors to positions with more ozone. Observing atransit at wavelengths sensitive to ozone (around 9 . µ m) might cause observable variations in the transit shape or time of ingress becauseof the non-uniform distribution of atmospheric ozone. Ozone data from the Ozone Monitoring Instrument on the NASA AURA spacecraft. seasonal variations similar to those seen on Earth. Theexpected latitudinal thermal gradient and transport ofatmospheric particulates between atmospheric cells alsosuggests that ozone depleting clouds will likely form pre-dominantly in polar regions. OBSERVING ALIEN OZONE
Transmission Spectroscopy
Assuming a favorable orbital geometry for the system,a planet will transit the face of its host star once per yearof the planet (Figure 1).Ozone is detectable in the spectrum of the Earth in a ∼ µ m wide feature centered at 9.7 µ m (Hovis et al.1968). This bandpass will be observable with MIRI on JWST . Previously, this instrument has been consideredby Barstow et al. (2015) as a possible tool to detect ozonefeatures, however they only considered detecting the ex-istence of ozone, not searching for long-term variations.By stacking together many spectral observations, theseauthors find that ozone will be detectable in the atmo-sphere of an Earth-like planet orbiting an M dwarf usingonly observations from 30 transits with
JWST .We propose observing transits with both
JWST andits successor. Given a long time baseline and enoughtransits, one could then fit a variable ozone model (suchas a long-term trend) to the observations and infer therate of change of ozone in the atmosphere. Here, the most important consideration is the long time baselinerequired. Since
JWST will have a short lifespan rela-tive to the timescale for the evolution of our own ozonelayer, future mid-IR space observatories will be essentialto complete this study.
Transit Morphology
The signature of an ozone hole, rather than a uniformdecrement in the atmospheric ozone, could also be ob-servable in changes in the transit shape as a function ofwavelength. The Earth is largely spherical (Eratosthenes240 BC), and typical transit models reflect this (Man-del & Agol 2002). However, by observing in a narrowwavelength range, centered on the region of the spec-trum affected by ozone, the Earth would be asymmetric:it would have a smaller effective radius at the latitudescorresponding to the ozone hole and a larger effective ra-dius where there is no ozone hole, making the silhouetteof the Earth slightly egg-shaped.Since atmpospheric ozone is typically located in thestratosphere approximately 20 km above the surface ofthe Earth (Shindell et al. 1998), a hole covering 20% ofthe surface would affect the observed transit depth by upto 0.1 parts per million, a roughly 0.1% effect (Figure 2).The hole could also affect the time of ingress/egress byup to 0.6 seconds, depending on the orientation of thetransiting planet with respect to its host star. Throughobservations with very high resolution photometry andvery fine time sampling, taken simultaneously in a band-pass focused on ozone and one at a different wavelength,this could be trivial to detect. Fig. 2.— (Top) Simulated transit of an Earthlike planet transit-ing a Sunlike star, with a full ozone layer (solid line). This transitis significantly affected by removing the ozone layer on the bottom20% of the Earth’s atmosphere, effectively decreasing the observedradius of the planet in that region by 20 km, although the differ-ence cannot be seen on this scale. (Bottom) The residuals betweenthese two models, considering an alignment where the ozone hole isaligned such that it is pointed in the direction of motion of Earth.The difference at first and third contact are due to this hole’s ge-ometry, where the overall change in transit depth is observed atall times. This effect may be small, but for a larger planet or asmaller star, the signal can grow arbitrarily large.
Ozone in Reflected Light
While variations in ozone in transit may be hard todetect with current facilities for any single system, wemight have more hope in detecting seasonal variationsin ozone over the course of the planet’s orbit. This isalready a well-established problem. As the Earth’s or-bital speed around the sun is 30 km s − , by observingthe planet at various points during its orbit, one can sep-arate the planetary spectrum from the stellar spectrumby searching for changes in the wavelengths of spectralfeatures (e.g. Snellen et al. 2014; Brogi et al. 2016). Sincethe seasonal variations in ozone can be 25% or more (Car-iolle et al. 1990), observing throughout an orbit could beuseful in detecting environmental movements. DEMOGRAPHICS OF OZONE HOLES IN THE GALAXY
As previously noted, Barstow et al. (2015) show thatozone features could be detected by binning togethermultiple transits observed with
JWST . However, evenignoring the relatively short time baseline offered by
JWST , observing changes in the ozone layer for any par-ticular system may be difficult: a 4% change in the totalozone would only represent a 0.1 σ effect in the observedozone signature. We leave this exercise to the reader. Or future
Perhaps we should, instead of focusing on any one sys-tem, try to understand the population of ozone holesin the galaxy. Very small effects can be understood ona population scale by binning together observations ofmany similar objects. For example, Sheets & Deming(2014) combined secondary eclipses of sub-Saturn plan-ets in
Kepler data to estimate the average albedo of theseplanets, while Zuluaga et al. (2015) combine transit pho-tometry of giant planets in a statistical search for exo-rings.We can do the same to better understand the under-lying statistics of ozone holes. Instead of analyzing anysingle system, we can instead observe many transits of ∼ σ effect in the binnedtransmission spectrum. Depending on what fraction ofthese planets have life that is affecting their ozone layer,this number would be smaller. By observing these sys-tems across several decades, the actual number of en-vironmental movements currently ongoing in the solarneighborhood, η Green Earth, can be inferred. By re-placing η ⊕ in the Drake Equation with η Green Earth,this form of galactic anthropology allows us to recovernot only the occurrence rate of advanced civilizations inthe galaxy, but the occurrence rate of civilizations thatwe might actually want to contact. DISCUSSION
Feasibility of observations
The observations proposed in this paper are likely notfeasible, and even if they were, the transient nature ofthe feature makes it incredibly unlikely to be observed.However, it is possible to test this avenue of thoughtby observing the ozone layer in the Earth using space-borne spectrometers such as CIRS on Cassini. If it wereto take a short break from doing actual useful scienceduring a transit of the Earth in front of the Sun, andinstead attempted to observe the ozone signature in theEarth’s atmosphere, it would showcase the potential ofthis method.Additionally, any observations searching for exo-ozoneholes will likely have to be conducted from space. TheEarth’s atmosphere is not transparent (yet) at 9 . µ mbecause of our own ozone, but by moving to space thisfeature of the atmosphere can be avoided. As we may notwant to wait until all ozone in the Earth’s atmospherehas been depleted, we could just use JWST , which isscheduled to be launched soon (Zackrisson 2011), as wellas its eventual successors.
Possible complications
Assuming a depletion in alien ozone is dominant at onepole, much like our own, this hole would only be observ-able in transit for favorable orientations: depending onthe planet’s axial tilt and the scale height of the atmo-sphere, it is possible that the hole would be fully hiddenby the shadow of the planet, leaving it unobservable toan Earthbound observer. Even if the planet has a unfa-vorable axial tilt, however, we should always be able todetect long-term evolution in the total atmospheric ozoneconcentration through reflected light spectroscopy.In the unlikely situation that variations in exo-planetary ozone levels are detected, their interpretationmight be complicated by a number of factors. Atmo-spheric ozone is affected by cosmic ray flux (Lu & Sanche2001; Tabataba-Vakili et al. 2016), so in order to in-terpret the observations we must also observe and un-derstand the activity of the host stars. A mission like
Kepler , which was built to measure precise transit pho-tometry, but with an additional photometric filter cen-tered on the wavelengths most sensitive to ozone in themid-infrared, would be ideal for such a purpose. Theoriginal
Kepler mission cost approximately 600 milliondollars, but given that we already have a Kepler , thistime we could save significantly on research and devel-opment costs (except, perhaps, for the development ofsturdier reaction wheels).Observations of ozone could also be confused by highlevels of volcanic activity (Prather 1992) or high lev-els of thermonuclear warfare on the planet in question(Stevens et al. 2015). However, given the sensitivity re-quired to detect extrasolar environmental movements, ei-ther of these would likely create time variable infraredfeatures that would be easily distinguishable from the signal considered here.
Future implications
If changes in an exo-ozone hole are detected, this wouldopen up additional avenues for galactic anthropology.The timescale between the initial decrement in ozoneand the eventual return to normalcy could provide in-sights into the level of scientific funding as well as thescientific literacy of the political leaders on that planet. Thanks to Ian Czekala for reading a draft of thismanuscript and providing valuable feedback. We alsothank both Sarah’s Market and Caf´e and Armando’sPizza and Subs, both of Cambridage, MA, for the beerand sandwiches which enabled much of the work inpreparing this manuscript. We would especially like toacknowledge the ozone layer on the Earth for shieldingus from UV radiation throughout the preparation of thismanuscript.
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