arXiv subjects

Steven J. Desch

Publications and source records attributed to Steven J. Desch.

At least 19 recordsLinked to original sources

arXiv2025

The Extent of Solar Energetic Particle Irradiation in the Sun's Protoplanetary Disk

Solar flares emit X rays and high-energy (MeV-GeV) ions (Solar Energetic Particles, or SEPs). Astronomical observations show solar mass-protostellar fluxes are a factor $\Phi \approx 3 \times 10^2 - 3 \times 10^3$ times higher than the present-day Sun. Constraining $\Phi$ in the early solar system is important for modeling ionization in the Sun's protoplanetary disk, the extent of magnetorotational instability or magnetocentrifugal outflows, or even production of short-lived radionuclides. Recent interpretations of meteoritic data -- cosmogenic Ne in hibonite grains, initial $({}^{10}{\rm Be}/{}^{9}{\rm Be})_0$ ratios in Ca-rich, Al-rich inclusions (CAIs), or even inferences of live ${}^{7}{\rm Be}$ in CAIs -- have suggested values $\Phi > 10^5$, even as large as $\Phi \approx 6 \times 10^6$, which would make the young Sun extraordinarily active, even for a protostar. We constrain $\Phi$ by re-examining these data. We conclude: cosmogenic Ne was produced in hibonite grains as they resided in the disk; ${}^{36}{\rm Cl}$ was created in Cl-poor grains after the disk dissipated; ${}^{10}{\rm Be}$ was inherited from the molecular cloud, with almost no ($< 1\%$) ${}^{10}{\rm Be}$ created in the disk; and there is no evidence whatsoever for any live ${}^{7}{\rm Be}$ in CAIs. We show these data are consistent with a value $\Phi \approx 3 \times 10^3$ for the first $> 5$ Myr of the solar nebula. The early Sun evidently emitted a flux of X rays and SEPs not atypical for a protostar.

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arXiv2025

Discovery of an icy and nitrogen-rich extrasolar planetesimal

White dwarfs accreting planetary debris provide detailed insight into the bulk composition of rocky exo-planetesimals. However, only one Kuiper-Belt analogue has been identified in that way so far. Here, we report the accretion of an icy extra-solar planetesimal onto white dwarf WD 1647+375 using ultraviolet spectroscopy from the Hubble Space Telescope. The accreted material is rich in the volatiles carbon, nitrogen, and sulphur, with a chemical composition analogous to Kuiper-belt objects (KBOs) in our solar system. It has a high nitrogen mass fraction ($5.1\pm1.6$ per cent) and large oxygen excess ($84\pm7$ per cent), indicating that the accreted planetesimal is water-rich (a water-to-rock ratio of $\simeq2.45$), corroborating a cometary- or dwarf planet-like composition. The white dwarf has been accreting at a rate of $\approx 2\times10^{8}$ g s$^{-1}$ for the past 13 years, implying a minimum mass of $\sim10^{17}$ g for the icy parent body. The actual mass could be several orders of magnitude larger if the accretion phase lasts $\sim10^5$ yr as estimated in the literature from debris disc studies. We argue that the accreted body is most likely a fragment of a KBO dwarf planet based on its nitrogen-rich composition. However, based on the chemical composition alone, it is difficult to discern whether this icy body is intrinsic to this planetary system, or may have an interstellar origin.

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arXiv2023

The first white dwarf debris disk observed by JWST

This letter reports the first JWST spectroscopy of a white dwarf debris disk, giving a preliminary assessment of the salient features, and recommendations for future observations. The polluted and dusty star WD 0145+234 experienced a major collisional event in its circumstellar disk in 2018, accompanied by an infrared outburst, and subsequently a gradual decrease in thermal emission. Time-series NIRSPEC observations demonstrate that the circumstellar disk is returning to a quiescent state with a T~1000 K infrared excess similar to the bulk of known dusty white dwarfs. MIRI spectroscopy reveals a 9-12 micron solid-state emission feature consistent with silicate minerals as observed in debris disks observed with Spitzer IRS. The strength and morphology of the silicate feature appear unchanged relative to the continuum in spectra taken over a year apart, consistent with steady-state collisional evolution of the circumstellar debris. A tentative emission feature around 7 microns may be due to carbonates, and if confirmed would indicate aqueous alteration in the parent body.

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arXiv2023

Planetesimals at DZ stars I: chondritic compositions and a massive accretion event

There is a wealth of evidence to suggest that planetary systems can survive beyond the main sequence. Most commonly, white dwarfs are found to be accreting material from tidally disrupted asteroids, whose bulk compositions are reflected by the metals polluting the stellar photospheres. While many examples are known, most lack the deep, high-resolution data required to detect multiple elements, and thus characterise the planetesimals that orbit them. Here, spectra of seven DZ white dwarfs observed with Keck HIRES are analysed, where up to nine metals are measured per star. Their compositions are compared against those of solar system objects, working in a Bayesian framework to infer or marginalise over the accretion history. All of the stars have been accreting primitive material, similar to chondrites, with hints of a Mercury-like composition at one star. The most polluted star is observed several Myr after its last major accretion episode, in which a Moon-sized object met its demise.

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arXiv2023

Origin of Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions in Meteorites

Most meteoritic calcium-rich, aluminum-rich inclusions (CAIs) formed from a reservoir with ${}^{26}{\rm Al}/{}^{27}{\rm Al} \approx 5 \times 10^{-5}$, but some record lower $({}^{26}{\rm Al}/{}^{27}{\rm Al})_0$, demanding they sampled a reservoir without live ${}^{26}{\rm Al}$. This has been interpreted as evidence for "late injection" of supernova material into our protoplanetary disk. We instead interpret the heterogeneity as chemical, demonstrating that these inclusions are strongly associated with the refractory phases corundum or hibonite. We name them "Low-${}^{26}{\rm Al}/{}^{27}{\rm Al}$ Corundum/Hibonite Inclusions" (LAACHIs). We present a detailed astrophysical model for LAACHI formation in which they derive their Al from presolar corundum, spinel or hibonite grains $0.5 - 2 \, μ{\rm m}$ in size with no live ${}^{26}{\rm Al}$; live ${}^{26}{\rm Al}$ is carried on smaller ($<$50 nm) presolar chromium spinel grains from recent nearby Wolf-Rayet stars or supernovae. In hot ($\approx$ 1350-1425 K) regions of the disk these grains, and perovskite grains, would be the only survivors. These negatively charged grains would grow to sizes $1 - 10^3 \, μ{\rm m}$, even incorporating positively charged perovskite grains, but not the small, negatively charged ${}^{26}{\rm Al}$-bearing grains. Chemical and isotopic fractionations due to grain charging was a significant process in hot regions of the disk. Our model explains the sizes, compositions, oxygen isotopic signatures, and the large, correlated ${}^{48}{\rm Ca}$ and ${}^{50}{\rm Ti}$ anomalies (if carried by presolar perovskite) of LAACHIs, and especially how they incorporated no ${}^{26}{\rm Al}$ in a solar nebula with uniform, canonical ${}^{26}{\rm Al}/{}^{27}{\rm Al}$. A late injection of supernova material is obviated, although formation of the Sun in a high-mass star-forming region is demanded.

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arXiv2023

Statistical Chronometry of Meteorites. II. Initial Abundances and Homogeneity of Short-lived Radionuclides

Astrophysical models of planet formation require accurate radiometric dating of meteoritic components by short-lived (Al-Mg, Mn-Cr, Hf-W) and long-lived (Pb-Pb) chronometers, to develop a timeline of such events in the solar nebula as formation of Ca-rich, Al-rich Inclusions (CAIs), chondrules, planetesimals, etc. CAIs formed mostly around a time ("t=0") when the short-lived radionuclide 26Al (t1/2 = 0.72 Myr) was present and presumably homogeneously distributed at a known level we define as (26Al/27Al)SS = 5.23 x10^-5. The time of formation after t=0 of another object can be found by determining its initial (26Al/27Al)0 ratio and comparing it to (26Al/27Al)SS. Dating of meteoritic objects using the Mn-Cr or Hf-W systems is hindered because the abundances (53Mn/55Mn)SS and (182Hf/180Hf)SS at t=0 are not known precisely. To constrain these quantities, we compile literature Al-Mg, Mn-Cr, Hf-W and Pb-Pb data for 14 achondrites and use novel statistical techniques to minimize the discrepancies between their times of formation across these systems. We find that for (53Mn/55Mn)SS = (8.09+/-0.65)x10^-6, (182Hf/180Hf)SS = (10.42+/-0.25)x10^-5, tSS = 4568.36+/-0.20 Myr, and a 53Mn half-life of 3.80+/-0.23 Myr, these four free parameters make concordant 37 formation times recorded by the different systems in 14 achondrites. These parameters also make concordant the ages derived for chondrules from CB/CH achondrites, formed simultaneously in an impact, and are apparently concordant with the I-Xe chronometer as well. Our findings provide very strong support for homogeneity of 26Al, 53Mn, and 182Hf in the solar nebula, and our approach offers a framework for more precise chronometry.

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arXiv2023

Statistical Chronometry of Meteorites. I. The Pb-Pb age of the Solar System's t=0

We use rapidly cooled achondrites to test the assumption of 26Al homogeneity in the solar nebula, by checking if there is a single value of tSS, the absolute "Pb-Pb" age of the Solar System's t=0, that makes concordant their ages from the Al-Mg and Pb-Pb systems. We find that values tSS = 4568.42 +/-0.24 Myr do make these ages concordant, and therefore the hypothesis of homogeneous 26Al is not falsified. This age, defined to be when the solar nebula had (26Al/27Al) = 5.23 x 10^-5, is significantly older than the ~4567.3 Myr inferred from direct measurements of Pb-Pb ages in CAIs. Discrepancies between the Al-Mg and Pb-Pb chronometers in chondrules and CAIs have previously been interpreted as arising from heterogeneities in 26Al, under the presumption that the Al-Mg and Pb-Pb systems in CAIs closed simultaneously. We examine this assumption and show that resetting is to be expected in CAIs. In particular, we quantitatively demonstrate that it is plausible that Pb-Pb ages of CAIs were reset at late times, without resetting the earlier Al-Mg ages, if they were transiently heated in the same manner as chondrules. We critically examine Pb-Pb isochrons, refining data and suggesting best practices for their calculation and reporting. We advocate reporting chronometry as times of formation after t=0 rather than absolute ages, as only the former is useful for astrophysical models of the solar nebula. We strongly advocate averaging of multiple samples, rather than anchoring to individual meteorites, to improve precision.

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arXiv2023

Short-Lived Radionuclides in Meteorites and the Sun's Birth Environment

The solar nebula contained a number of short-lived radionuclides (SLRs) with half-lives of tens of Myr or less, comparable to the timescales for formation of protostars and protoplanetary disks. Therefore, determining the origins of SLRs would provide insights into star formation and the Sun's astrophysical birth environment. In this chapter, we review how isotopic studies of meteorites reveal the existence and abundances of these now-extinct radionuclides; and the evidence that the SLR ${}^{10}{\rm Be}$, which uniquely among the SLRs is not produced during typical stellar nucleosynthesis, was distributed homogeneously in the solar nebula. We review the evidence that the SLRs ${}^{26}{\rm Al}$, ${}^{53}{\rm Mn}$, and ${}^{182}{\rm Hf}$, and other radionuclides, were also homogeneously distributed and can be used to date events during the Solar System's planet-forming epoch. The homogeneity of the SLRs, especially ${}^{10}{\rm Be}$, strongly suggests they were all inherited from the Sun's molecular cloud, and that production by irradiation within the solar nebula was very limited, except for ${}^{36}{\rm Cl}$. We review astrophysical models for the origin of ${}^{10}{\rm Be}$, showing that it requires that the Sun formed in a spiral arm of the Galaxy with higher star formation rate than the Galaxy-wide average. Likewise, we review the astrophysical models for the origins of the other SLRs and show that they likely arose from contamination of the Sun's molecular cloud by massive stars over tens of Myr, most likely dominated by ejecta from Wolf-Rayet stars. The other SLRs also demand formation of the Sun in a spiral arm of the Galaxy with a star formation rate as high as demanded by the Solar System initial ${}^{10}{\rm Be}$ abundance. We discuss the astrophysical implications, and suggest further tests of these models and future directions for the field.

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arXiv2022

The Nominal Range of Rocky Planet Masses, Radii, Surface Gravities and Bulk Densities

The two primary observable quantities of an exoplanet--its mass and radius--alone are not sufficient to probe a rocky exoplanet's interior composition and mineralogy. To overcome this, host-star abundances of the primary planet-building elements (Mg, Si, Fe) are typically used as a proxy for the planet's bulk composition. The majority of small exoplanet hosts, however, do not have available abundance data. Here we present the open-source ExoPlex mass-radius-composition solver. Unlike previous open-source mass-radius solvers, ExoPlex calculates the core chemistry and equilibrium mantle mineralogy for a bulk composition, including effects of mantle FeO content, core light elements and surface water/ice. We utilize ExoPlex to calculate the planetary radii, surface gravities and bulk densities for 10$^6$ model planets up to 2 R$_\oplus$ across these geochemistries, adopting the distribution of FGK stellar abundances to estimate of the range of bulk exoplanet compositions. We outline the $99.7\%$ distribution of radii, surface gravity and bulk densities that define planets as "nominally rocky." Planets outside this range require compositions outside those expected from stellar abundance data, likely making them either Fe-enriched super-Mercuries, or volatile-enriched mini-Neptunes. We apply our classification scheme to a sample of 85 well-resolved exoplanets without available host-star abundances. We estimate only 9 planets are within the "nominally rocky planet zone" at $>70\%$ confidence, while $\sim20\%$ and $\sim30\%$ of this sample can be reasonably classified as super-Mercuries or volatile-rich, respectively. Our results provide observers with a self-consistent way to broadly classify a planet as likely rocky, Mercury-like or volatile-enriched, using mass and radius measurements alone.

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arXiv2022

A primordial atmospheric origin of hydrospheric deuterium enrichment on Mars

The deuterium-to-hydrogen (D/H or 2H/1H) ratio of Martian atmospheric water (~6x standard mean ocean water, SMOW) is higher than that of known sources, requiring planetary enrichment. A recent measurement by NASA's Mars Science Laboratory rover Curiosity of >3 Gyr clays yields a D/H ratio ~3x SMOW, demonstrating that most enrichment occurs early in Mars's history. As on Venus, Mars's D/H enrichment is thought to reflect preferential loss to space of 1H (protium) relative to 2H (deuterium), but the global environmental context of large and early hydrogen losses remain to be determined. Here, we apply a recent model of primordial atmosphere evolution to Mars, link the magma ocean of the accretion epoch with a subsequent water-ocean epoch, and calculate the behavior of deuterium for comparison with the observed record. We find that a ~2-3x hydrospheric deuterium-enrichment is produced if the Martian magma ocean is chemically reducing at last equilibration with the primordial atmosphere, making H2-CO the initially dominant species, with minor abundances of H2O-CO2. Reducing gases - in particular H2 - can cause greenhouse warming and prevent a water ocean from freezing immediately after the magma ocean epoch. Moreover, the pressure-temperature conditions are high enough to produce ocean-atmosphere H2O-H2 isotopic equilibrium such that surface H2O strongly concentrates deuterium relative to H2, which preferentially takes up protium and escapes from the primordial atmosphere. The proposed scenario of primordial H2-rich outgassing and escape suggests significant durations (>Myr) of chemical conditions on the Martian surface conducive to prebiotic chemistry immediately following Martian accretion.

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arXiv2022

Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets can Support Temperate Climates

The ideal exoplanets to search for life are those within a star's habitable zone. However, even within the habitable zone planets can still develop uninhabitable climate states. Sustaining a temperate climate over geologic ($\sim$Gyr) timescales requires a planet contain sufficient internal energy to power a planetary-scale carbon cycle. A major component of a rocky planet's energy budget is the heat produced by the decay of radioactive elements, especially $^{40}$K, $^{232}$Th, $^{235}$U and $^{238}$U. As the planet ages and these elements decay, this radiogenic energy source dwindles. Here we estimate the probability distribution of the amount of these heat producing elements (HPEs) that enter into rocky exoplanets through Galactic history, by combining the system-to-system variation seen in stellar abundance data with the results from Galactic chemical evolution models. Using these distributions, we perform Monte-Carlo thermal evolution models that maximize the mantle cooling rate. This allows us to create a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle and temperate climate as a function of its mass and when it in Galactic history. We apply this framework to a sample of 17 likely rocky exoplanets with measured ages, 7 of which we predict are likely to be actively degassing today despite our pessimistic assumptions. For the remaining planets, including those orbiting TRAPPIST-1, we cannot confidently assume they currently contain sufficient internal heat to support mantle degassing at a rate sufficient to sustain a global carbon cycle or temperate climate without additional tidal heating or undergoing plate tectonics.

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arXiv2021

The Fundamental Connections Between the Solar System and Exoplanetary Science

Over the past several decades, thousands of planets have been discovered outside of our Solar System. These planets exhibit enormous diversity, and their large numbers provide a statistical opportunity to place our Solar System within the broader context of planetary structure, atmospheres, architectures, formation, and evolution. Meanwhile, the field of exoplanetary science is rapidly forging onward towards a goal of atmospheric characterization, inferring surface conditions and interiors, and assessing the potential for habitability. However, the interpretation of exoplanet data requires the development and validation of exoplanet models that depend on in-situ data that, in the foreseeable future, are only obtainable from our Solar System. Thus, planetary and exoplanetary science would both greatly benefit from a symbiotic relationship with a two-way flow of information. Here, we describe the critical lessons and outstanding questions from planetary science, the study of which are essential for addressing fundamental aspects for a variety of exoplanetary topics. We outline these lessons and questions for the major categories of Solar System bodies, including the terrestrial planets, the giant planets, moons, and minor bodies. We provide a discussion of how many of these planetary science issues may be translated into exoplanet observables that will yield critical insight into current and future exoplanet discoveries.

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arXiv2021

Matters Arising on "Breakup of a long-period comet as the origin of the dinosaur extinction" by Siraj & Loeb

The recent publication by Siraj & Loeb (2021; Nature Scientific Reports 11, 3803) attempts to revive the debate over whether the Chicxulub impactor was a comet or an asteroid. They calculate that ~20% of long-period comets impacting Earth will have first been disrupted by passage inside the Sun's Roche limit, generating thousands of fragments, each the needed size of the Chicxulub impactor. This would increase the impact rate of comets by a factor ~15, making them as likely to hit the Earth as an asteroid. They also argue that a comet would be a factor of 10 more likely to match the geochemical constraints, which indicate the Chicxulub impactor was carbonaceous chondrite-like. These conclusions are based on misinterpretations of the literature. Siraj & Loeb [1] overestimate the number of fragments produced during tidal disruption of a comet: tens of fragments are produced, not thousands. They also conflate 'carbonaceous chondrite' with specific types of carbonaceous chondrite, and ignore the evidence of iridium, making comets seem more likely than asteroids to match the Chicxulub impactor, when in fact they likely can be ruled out. Rather than a comet, an asteroidal impactor similar to CM or CR carbonaceous chondrites is strongly favored.

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arXiv2021

1I/'Oumuamua as an N2 ice fragment of an exo-Pluto surface: I. size and compositional constraints

The origin of the interstellar object 1I/'Oumuamua has defied explanation. We perform calculations of the non-gravitational acceleration that would be experienced by bodies composed of a range of different ices and demonstrate that a body composed of N2 ice would satisfy the available constraints on the non-gravitational acceleration, size and albedo, and lack of detectable emission of CO or CO2 or dust. We find that 'Oumuamua was small, with dimensions 45 m x 44 m x 7.5 m at the time of observation at 1.42 au from the Sun, with a high albedo of 0.64. This albedo is consistent with the N2 surfaces of bodies like Pluto and Triton. We estimate 'Oumuamua was ejected about 0.4-0.5 Gyr ago from a young stellar system, possibly in the Perseus arm. Objects like 'Oumuamua may directly probe the surface compositions of a hitherto-unobserved type of exoplanet: "exo-plutos". In a companion paper (Desch & Jackson, 2021) we demonstrate that dynamical instabilities like the one experienced by the Kuiper belt, in other stellar systems, plausibly could generate and eject large numbers of N2 ice fragments. 'Oumuamua may be the first sample of an exoplanet brought to us.

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arXiv2021

1I/'Oumuamua as an N2 ice fragment of an exo-pluto surface. II. Generation of N2 ice fragments and the origin of 'Oumuamua

The origin of the interstellar object 1I/'Oumuamua, has defied explanation. In a companion paper (Jackson & Desch, 2021), we show that a body of N2 ice with axes 45 m x 44 m x 7.5 m at the time of observation would be consistent with its albedo, non-gravitational acceleration, and lack of observed CO or CO2 or dust. Here we demonstrate that impacts on the surfaces of Pluto-like Kuiper belt objects (KBOs) would have generated and ejected ~10^14 collisional fragments--roughly half of them H2O ice fragments and half of them N2 ice fragments--due to the dynamical instability that depleted the primordial Kuiper belt. We show consistency between these numbers and the frequency with which we would observe interstellar objects like 1I/'Oumuamua, and more comet-like objects like 2I/Borisov, if other stellar systems eject such objects with efficiency like that of the Sun; we infer that differentiated KBOs and dynamical instabilities that eject impact-generated fragments may be near-universal among extrasolar systems. Galactic cosmic rays would erode such fragments over 4.5 Gyr, so that fragments are a small fraction (~0.1%) of long-period Oort comets, but C/2016 R2 may be an example. We estimate 'Oumuamua was ejected about 0.4-0.5 Gyr ago, from a young (~10^8 yr) stellar system, which we speculate was in the Perseus arm. Objects like 'Oumuamua may directly probe the surface compositions of a hitherto-unobserved type of exoplanet: "exo-plutos". 'Oumuamua may be the first sample of an exoplanet brought to us.

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arXiv2021

Icy Exomoons Evidenced by Spallogenic Nuclides in Polluted White Dwarfs

We present evidence that excesses in Be in polluted white dwarfs (WDs) are the result of accretion of icy exomoons that formed in the radiation belts of giant exoplanets. Here we use excess Be in the white dwarf GALEX J2339-0424 as an example. We constrain the parent body abundances of rock-forming elements in GALEX J2339-0424 and show that the overabundance of beryllium in this WD cannot be accounted for by differences in diffusive fluxes through the WD outer envelope nor by chemical fractionations during typical rock-forming processes. We argue instead that the Be was produced by energetic proton irradiation of ice mixed with rock. We demonstrate that the MeV proton fluence required to form the high Be/O ratio in the accreted parent body is consistent with irradiation of ice in the rings of a giant planet within its radiation belt, followed by accretion of the ices to form a moon that is later accreted by the WD. The icy moons of Saturn serve as useful analogs. Our results provide an estimate of spallogenic nuclide excesses in icy moons formed by rings around giant planets in general, including those in the solar system. While excesses in Be have been detected in two polluted WDs to date, including the WD described here, we predict that excesses in the other spallogenic elements Li and B, although more difficult to detect, should also be observed, and that such detections would also indicate pollution by icy exomoons formed in the ring systems of giant planets.

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arXiv2020

The Dynamic Proto-atmospheres around Low-Mass Planets with Eccentric Orbits

Protoplanets are able to accrete primordial atmospheres when embedded in the gaseous protoplanetary disk. The formation and structure of the proto-atmosphere are subject to the planet--disk environment and orbital effects. Especially, when planets are on eccentric orbits, their relative velocities to the gas can exceed the sound speed. The planets generate atmosphere-stripping bow shocks. We investigate the proto-atmospheres on low-mass planets with eccentric orbits with radiation-hydrodynamics simulations. A 2D radiative model of the proto-atmosphere is established with tabulated opacities for the gas and dust. The solutions reveal large-scale gas recycling inside a bow shock structure. The atmospheres on eccentric planets are typically three to four orders of magnitude less massive than those of planets with circular orbits. Overall, however, a supersonic environment is favorable for planets to keep an early stable atmosphere, rather than harmful, due to the steady gas supply through the recycling flow. We also quantitatively explore how such atmospheres are affected by the relative velocity of the planet to the gas, the planet mass, and the background gas density. Our time-dependent simulations track the orbital evolution of the proto-atmosphere with the planet--disk parameters changing throughout the orbit. Atmospheric properties show oscillatory patterns as the planet travels on an eccentric orbit, with a lag in phase. To sum up, low-mass eccentric planets can retain small proto-atmospheres despite the stripping effects of bow shocks. The atmospheres are always connected to and interacting with the disk gas. These findings provide important insights into the impacts of migration and scattering on planetary proto-atmospheres.

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arXiv2020

A Geologically Robust Procedure For Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen is an Earth-Like Biosignature

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet$'$s atmosphere signifies life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature, to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from Earth$'$s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: semi-major axis and stellar luminosity to ensure residence in the habitable zone; stellar XUV flux, to ensure an exoplanet can retain a secondary (outgassed) atmosphere. Next, high-precision mass and radius information should be combined with high-precision stellar abundance data, to constrain the exoplanet$'$s water content; those incompatible with less than 0.1 wt % H$_{2}$O can be deprioritized. Then, reflectance photometry or low-resolution transmission spectroscopy should confirm an optically thin atmosphere. Subsequent long-duration, high-resolution transmission spectroscopy should search for oxygen and ensure that water vapor and CO$_{2}$ are present only at low (10$^{2}$-10$^{4}$ ppm levels). Assuming oxygen is found, attribution to life requires the difficult acquisition of a detailed, multispectral light curve of the exoplanet to ensure both surface land and water. Exoplanets failing some of these steps might be habitable, even have observable biogenic oxygen, but should be deprioritized because oxygen could not be attributed unambiguously to life. We show how this is the case for the Solar System, the 55 Cnc System, and the TRAPPIST-1 System, in which only the Earth and TRAPPIST-1e successfully pass through our procedure.

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