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Exoplanet Biosignatures Workshop-Without-Walls Review Papers - Discussion Forum

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19 Apr 2017

5. Exoplanet Biosignatures: Observational Prospects

  1. NExSS Manager

    Contact: Yuka Fujii, yuka.fujii.ebihara *at*
    Summary: While a variety of exoplanet biosignatures have been proposed, none of these has been detected or constrained. We provide an overview of the prospects for biosignature detection and general characterization of potentially habitable exoplanets, focusing on temperate Earth-size planets. We review the planned space-based and ground-based projects as well as the basic methods these missions will employ, and summarize what kind of properties maybe observable as the new facilities come on line. We distinguish reasonable expectations for the first constraints on spectroscopic features of atmospheres (and perhaps surfaces) of transiting and non-transiting planets obtainable before 2030, versus larger surveys to address statistical questions such as the occurrence rate of habitable environments, for future projects beyond 2030. The broad outlook which this paper presents is useful in considering new methodologies to characterize exoplanets of astrobiological interest, and in developing a framework to evaluate the possibility of biosphere based on the observables. 

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  2. Kevin Stevenson

    Section 6 discusses the prospects of detecting biosignatures beyond 2030 and, in particular, has nice sections on HabEx and LUVOIR.  However, the discussion on OST is relegated to a single sentence.  This gives the impression that detecting biosignatures through transmission and emission spectroscopy will not be feasible and is not worth discussing in this paper.

    I encourage the authors to include a section on OST that describes their observation and detection strategies in the mid-IR.  I would be happy to provide specific details on the subject matter and participate in further discussion.

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  3. Shawn D Domagal-Goldman


    Some help on that would be great!

    Yuka, can you work with Kevin on this?


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  4. Yuka Fujii

    Hi Kevin,

    Thanks for your feedback! We would love to have your inputs on OST for Section 6. 
    If you could draft the key aspects of the mission to be discussed here (quantitative prospects for biosignature detections etc.) and references, that would be very helpful. We can also discuss with Karl and Nick. 


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  5. Avi Mandell

    Hi Yuka - i was wondering how you determined the resolutions for JWST on pg 17 -- does this include photometry and spectroscopy?  my understanding was that NIRSpec only goes up to 2700, but maybe i'm missing something.

    You also state that it can get down to 10 ppm.  This may be possible, but at this time the absolute noise floor for both the NIR and MIR instruments is uncertain.  Current estimates suggest it could be 20 ppm for the NIR and 50 ppm for the MIR (Greene et al.), but these are only guesses based on HST and Spitzer, and the improved electronics and orbital location could result in lower noise floors than currently possible.

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  6. Stephen Kane

    Thank you Yuka for leading the effort on this great draft! I have a couple of last-minute comments for you. In section 4.2 you mention that the phase signature is essentially undetectable, but that depends on a lot of features regarding both the planets (radius, geometric albedo, and semi-major axis) and the orbital architecture. For example, planets in the HZ around M dwarfs will have significantly higher phase amplitudes since that goes with 1/a^2. Also, it was shown by Kane & Gelino, 2013, ApJ, 762, 129 and Gelino & Kane, 2014, ApJ, 787, 105 that orbital architectures that produce frequent alignments near superior conjunction (such as resonant orbits) have substantially improved phase detection prospects. Ultimately, it is usually the stellar activity (both intrinsic and instrumental) that limits the signal-to-noise.

    Another general comment I have is regarding orbital ephemerides. I've been working on calculating orbital ephemerides for many years and a critical piece of the follow-up prospects will be accurately determining the Keplerian orbital elements to predict when (for example) the maximum star-planet angular separation will occur. The TERMS project has been doing this for years and I'm now applying those techniques to missions such as WFIRST. There will need to be a concerted effort to ensure that sufficient RV measurements are acquired prior to observation with WFIRST/Luvoir/HabEx/etc since observing at the wrong time would be a devastating loss of valuable telescope time. There are various facets to designing optimal observing strategies based on precisely determined orbits and I'm happy to contribute a paragraph or subsection about this if you like.

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  7. Anonymous

    Dear Yuka,

    In section 5.1.2 please add a reference to the paragraph

    High-energy radiation towards the X-ray range, called XUV (roughly 1–1000 Å), drives atmospheric loss through thermal mechanisms (Jean escape and hydrodynamic escape) and non-thermal mechanisms (e.g., acceleration of ions by charge separation driven by XUV ionization). Thus, the XUV flux over the history of the star critically affects the stability of the atmosphere. 

    Airapetian et al. (2017) How Hospitable Are Space Weather Affected Habitable Zones? The Role of Ion EscapeApJ Let, 836



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  8. Sarah Rugheimer

    I unfortunately didn't get a chance to go through this whole paper before the comment period ends, but I have a few notes for the beginning. 

    Page 4 - in the 3rd paragraph the 4 other reviews are cited, but it is not called out that these are reviews, so the references to them seem to be missing all the other great literature in the field. I'd make sure for each of these put (see review by Schweiterman et al., 2017 and references therein) or (see Caitling et al., 2017 for a review) etc. 

    Page 5 - for the HZ sentence "....although the exact location of the HZ of a given star depends on many factors including the stellar spectrum, planetary rotation, ..." I'd cite Kasting 1993 and others, including the 3D modeled HZ by Leconte et al. 2013 which includes planetary rotation, for example. Also the dry inner edge HZ from Zsom et al. 2014. 

    Page 7 - I think there needs to be citations to Kreidberg et al. 2014's work on detecting H2O in the HST chemical analyses sentence as well as Knudson et al., 2007's paper on mapping heterogeneity. 

    Finally, there are quite a few grammar errors or incomplete sentences. I'd have a close read through it again. I'd be happy to point out those that I saw so far, but perhaps sending me the word document so I can do track-changes would be most efficient for everyone, if desired. 

    Sorry that's I got through before the end of today! All the best and I will continue to read the review for my own benefit as it seems to be a very useful one. :)

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  9. Yuka Fujii

    Hi Avi, in my understanding the maximum resolution of MRS1 goes up to 3250 and that is where this number comes from. 

    Re: 10ppm, you are referring to the following part, right? “and will provide a spectrophotometric precision of 10-100 ppm for time series observations spanning from hours to days.” We will revise this part including your points. (We have another paragraph on page 14 to discuss the noise floor.) Thanks for your comment!

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  10. Yuka Fujii

    Hi Stephen,

    Thanks for the feedback!

    As for the phase variation (in the reflected light), we focused on the rocky planets with moderate temperature, so there is a range of orbital separation and albedo. But it is true that the detectability depends on the planetary parameters as well as observational settings. We will elaborate this part taking it into account.  

    The significance of determining orbital ephemerides is an interesting discussion and certainly worth included in the text. I would appreciate your contribution on this. Thanks! 

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  11. Yuka Fujii

    Hi Vlad, I actually think we had this reference at some point but it is missing in the final version accidentally. Thanks for pointing it out!

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  12. Yuka Fujii

    Hi Sarah, thank you very much for the suggestions! We will revise accordingly. We will also make sure that the grammatical errors are corrected. 

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