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Exoplanets: Optimizing the Target List

Avi Loeb’s always interesting work has recently taken us into the realm of target selection for exoplanet surveys. Where should we be putting our time and money in the search for life elsewhere, and what can we do to maximize both the credibility of the investigation and the funding that it demands? These sound like pedestrian matters compared to the excitement of discovery — finding Proxima b was a lot more exciting than watching any congressional committee debate over NASA’s priorities.

But Proxima Centauri b, that fascinating world around the nearest star, fits neatly into this narrative, for reasons that contrast nicely with its own nearest neighbors, Centauri A and B. The Centauri stars are an obvious target, and one that Loeb has devoted considerable time to assessing, given his deep involvement with Breakthrough Starshot’s study of a mission there. If we find planets around Centauri A or B, are they our priority? Just where does Proxima b fit in?

And what of fascinating systems like TRAPPIST-1? We have an abundance of evidently rocky planets there, but should we prioritize this M-dwarf over G-class systems found by TESS?

Lacking infinite resources, we have to decide what to allocate where, and that means that target selection for upcoming observations, both in space and on the ground, is a priority. As SETI has demonstrated, the federal dollar can vanish, especially after years of null results. It would be disastrous if the search for non-technological life met a similar fate. All this has prompted Loeb and Manasvi Lingam, likewise at Harvard, to think about how we can maximize our chances for success, applying the kind of cost-benefit analyses often used in economics.

Their paper has just appeared in The Astrophysical Journal Letters. The search for biosignatures of necessity confronts the real world of financing and public perception. Assumptions are everything, here discussed in terms of the statistical ‘priors’ that go into making the calculation. If we had to make a choice between solar-mass stars and M-dwarfs and if (crucially) all other things were equal — i.e., if we were not constrained by our instruments and could observe each type of star with high fidelity — then Sun-like stars obviously win.

Why? Because we know of only one life-bearing world, and that is our own. Thus in a time of scarce resources, it would make sense to put our funds into searching around this kind of star. But we can’t do away with the observational problems that exist, and given these, M-dwarfs stand out as the better target because given the current state of the art, we can more readily hope to delve into their atmospheres with our rapidly evolving observatories, probing their chemistry and looking for biosignatures. Earth-class planets around M-dwarfs offer us deep transits and the earliest prospects for using transmission spectroscopy to study them.

We’re thus pulled in different directions, based on our priors. From the paper;

If we consider a flat prior, where the probability of life is independent of the choice of star, focusing on planets around M-dwarfs is more advantageous because the detection of biosignatures becomes much easier. On the other hand, there is mounting evidence, especially based on considerations of space weather, that the potential habitability of Earth-analogs around M-dwarfs might be much lower relative to their counterparts around G-type stars. Hence, if we adopt a prior where the habitability is selectively suppressed around low-mass stars, we conclude that it would be more advantageous to focus on the search for life on planets orbiting Sun-like stars relative to those around M-dwarfs.

What to do? Obviously, press on in our analysis of both kinds of stars as the possible home for life. We’ve looked many times in these pages at the problems M-dwarfs present, usually in terms of tidal locking and the stellar flare activity common especially to younger stars. As the paper notes, the growing concern about space weather, which can involve stripping of the atmosphere itself or its serious degradation, makes the question even more problematic.

Image: This diagram compares the planets of our inner solar system to Kepler-186, a five-planet system about 500 light-years from Earth in the constellation Cygnus. The planets of Kepler-186 orbit an M dwarf, a star that is half the size and mass of the Sun. What share of our resources should go into investigating M-dwarfs as opposed to solar-type stars? Credit: NASA Ames/SETI Institute/JPL-Caltech.

We have no final answers here, but as the analysis continues, Loeb and Lingam are pointing out that if habitability around M-dwarfs is effectively suppressed by factors like these, then prioritizing planets around solar-type stars makes the most sense. This is not to say that we would not, in an ideal world, press on with strong efforts in both directions. But I think the authors’ point is telling: To achieve the funding levels we need, the community must be conducting a search that is both credible and can demonstrate a measure of success.

Will our search evolve in the direction of solar-type stars? Here Alpha Centauri makes an interesting bellwether. We have the aforementioned Proxima Centauri b, an Earth-mass planet in the habitable zone of a close red dwarf. But as the angular separation between the primary stars Centauri A and B continues to increase, we should finally be able to make a call on Earth-mass planets orbiting either or both of these stars, the G-class A and the K-class B.

Finding a planet in the habitable zone of one of the primary Centauri stars would present us with an abundance of targets closer to us than any other stellar system. The ongoing work on assessing M-dwarfs and the habitability issues they raise will help us decide where to concentrate our resources. Maximizing the chances for detection may eventually lead us to choices other than the most readily studied stars, and back to stars more like our own.

The paper is Loeb & Lingam, “Optimal Target Stars in the Search for Life,” The Astrophysical Journal Letters Vol. 857, No. 2 (20 April 2018). Full text.

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Comments on this entry are closed.

  • Gary Wilson May 1, 2018, 13:09

    My vote would be to go after the low hanging fruit. So first nearby G class stars, followed by nearby stars of other types that show some stability. I don’t think we reduce the priority of nearby M dwarfs too much because we don’t know under what conditions other than those of an earthlike world, life can survive. The Keplar 186 worlds are extremely interesting as an example and could yield some very important results so some effort is worthwhile, but at 560 light years away it seems extremely unlikely we would devote resources to send probes which would take 2 1/2 millennia to get there with a realistic technology. The Trappist system is much more worthy of greater expenditure at 35 light years distance. Surely distance is a huge factor in deciding where to allocate money? The best bet for getting detailed data and attracting greater funding lies nearby whenever possible. I don’t think you will every beat close up observation. Look at what the probes sent to the outer solar system have revealed.

    • Andrei May 3, 2018, 17:07

      I agree that G class stars should be given first priority, it is not only because we live on such a world ourselves.
      There’s a number of reasons to think planets around these stars have more benign environment. Fast rotation give internal dynamo, and if the planet have the right amount of water the rotation make wide oceans free of ice, which give a hydrological cycle, seasons and this water might lubricate continental drift.

      But I do think some teams will target the low mass stars even when they know what said above.
      I think Gary posted above thought of this problem with funding, the first team to publish the composition or atmosphere of several such planets will be rewarded the next time they apply for funds.
      So rocky planets who orbit fast around M-dwarf stars, (many have been found with transit methods), these can be more thoroughly studied since they pass often and give opportunities to do the studies faster and will remain a tempting target.

  • Spaceman May 1, 2018, 14:51

    I thought TESS will only be capable of finding planets in short period orbits around smaller stars and the only planned photometry mission capable of finding earth-sized planets in 1 AU orbits around G-type stars is PLATO?

    • Paul Gilster May 1, 2018, 16:00

      Spaceman, I’m looking at Bouma et al., “Planet Detection Simulations for Several Possible
      TESS Extended Missions,” available here: https://arxiv.org/pdf/1705.08891.pdf

      and saying this:

      “TESS will be sensitive to sub-Neptune sized transiting planets orbiting M dwarfs out to ∼ 200 pc and G dwarfs out to ∼ 1 kpc,” etc.

      • Antonio May 1, 2018, 16:53

        There is also this more recent paper on the same topic for the main mission: https://arxiv.org/abs/1804.05050

      • Antonio May 1, 2018, 17:06

        From the paper:

        Sullivan et al. (2015), Bouma et al. (2017), and Ballard (2018) have previously estimated the planet yield from TESS. These previous studies selected stars from a simulated Galactic model rather than real stars, and
        therefore we expect there are moderate differences between our predicted yields and previous studies. Additionally, we have different selection strategies for both 2-minute cadence targets and for FFI stars. We built a
        realistic 2-minute cadence star selection model that limits the stars observed at the pole cameras to just 6,000 stars per hemisphere, whereas the previous works assumed TESS can observe many more stars in the CVZ than is possible with the flight hardware configuration used. We also use a different prioritization metric than previous work, which is based on the metric used by the TESS Target Selection Working Group (Pepper et al. in preparation). For the FFI targets we primarily consider those within the CTL, whereas different cuts on brightness are made in earlier works. Therefore, we expect to see significant differences in the planet yield for giant planets.

  • Abelard Lindsey May 1, 2018, 15:02

    They should be looking at K’s. There are more of them than G’s and they do not have the flare issues as M’s. A habitable planet around a K would be far enough out from the primary that it can have a normal rotation rate, rather than the 3:2 (like Mercury) or completely tidally locked. I always thought M’s suck for these and other reasons.

    • J. Jason Wentworth May 3, 2018, 0:34

      I agree. I think (based on their abundance, “gentler” electromagnetic emissions, and Main Sequence life duration) that if we are ever inducted into a “Galactic Club” (as Ronald Bracewell called it), we may find that most civilizations’ suns are K class stars. If there are “Goldilocks stars,” I think they’re K dwarfs.

  • Geoffrey Hillend May 1, 2018, 16:11

    I don’t expect to see any Earth sized bodies in the life belt around Centari A or B due to the gravitational tidal disruption of a twin star system when they were protostars. Kepler 186f has an orbit comparable with Mecury, so it might not be tidally locked.

    I think our best bet for finding intelligent life is around an Earth twin with a Moon which was the result of a giant impact which gave the larger body the crucial angular momentum to make a magnetic field and large, iron liquid core. Consequently we would look at G class stars and some K stars. It can’t hurt to look at nearby M class stars since scientists always want to eliminate all doubt or examine all the evidence. They are a good test for new telescopes of the future.

  • Antonio May 1, 2018, 16:46

    “Assumptions are everything, here discussed in terms of the statistical ‘priors’ that go into making the calculation.”

    Astrobiology has too many assumptions, all of them geocentric assumptions. That’s why I stopped reading news about habitability long ago. Too much, hugely much, astronomically much generalization from a single example. Totally non-sense to me.

    • J. Jason Wentworth May 3, 2018, 19:44

      Our current ignorance leaves us little alternative, but we have to start *somewhere*–or just give up (which I certainly don’t advocate). There may be many quite un-Earthly star/planet situations in which life–even intelligent life–arises and thrives, but our own example is, for the present, the only viable example we know of. It is the one model that we *know* works, and which would work elsewhere, given the same circumstances. Astronomical observations (from the Earth’s surface and from spacecraft), SETI, SETA, and METI, and interstellar probes will–never soon enough!–provide us with knowledge about what circumstances will nurture life.

  • Michael C. Fidler May 1, 2018, 18:42

    “If the targets are poorly chosen and the search fails to detect signatures of life, there is a risk that the search for primitive (i.e., non-technological) forms of life will share the fate of SETI and lose its mainstream credibility and federal funding.”

    The problem is this list from the Planetary Habitability Laboratory (PHL) : Here is the list of the stars within 10 parsecs (32.6 light years) from the Sun.

    http://phl.upr.edu/projects/nearby-stars-catalog

    The ages of these stars is still a imprecise science and looking at the amount of time that planets have had to evolved around them will lead to the ones with a higher probability of “technological forms of life”. My bet would be on the early M Dwarfs (M1 to M4).

    • Michael C. Fidler May 1, 2018, 20:02

      Terraforming Wiki – M – type stars.

      “The biggest challenge for settlers will more than likely be the powerful flares. Depending on the amount of flares, some red dwarfs (like UV Ceti) are unlikely to host habitable planets. Planets closely orbiting flare stars would almost certainly have no atmosphere as powerful flares would blown such away a long time ago. Even a strong magnetic field is not enough to stop these bursts. Their power is similar to solar flares, but because planets with liquid water are so close (20 times closer then the Earth), the effect is huge. Some stars, have such violent flares that their light can increase 100 times. Others, like Proxima Centauri, generate a flare every two hours. A more massive star will not have so many flares like a smaller star. Stars with spectral type M0, M1, M2 and M3, are much more safe. Stars with spectral types fainter than M4 (M5, M6, M7 and below) will be more unstable. Stars that rotate faster will also have more flares. Since older stars rotate slower, it can be said that older stars are more safe. In fact Barnard’s Star is older than our galaxy (and one of the most safe red dwarfs). The Internet Stellar Database lists safe red dwarfs with an additional n letter, while flare stars have an additional e attached to their spectral class.

      Nearby red dwarfs – complete list available here

      Proxima Centauri (flare star) – 4.24 LY
      Barnard’s Star (safe) – 5.98 LY
      Wolf 359 (flare star) – 7.78 LY
      Lalande 21185 (safe) – 8.29 LY
      UV Ceti & BL Ceti (flare stars) – 8.73 LY
      Ross 154 (safe) – 9.68 LY
      Ross 248 (flare star) – 10.32 LY
      Since red dwarfs are the longest lived among all main sequence stars, they might be the last refuge for civilizations that will exist closer to the end of the universe.”

      http://terraforming.wikia.com/wiki/M_-_type_stars

      • ljk May 2, 2018, 10:36

        We keep assuming humanity and ETI will want to set up in a star system and live permanently on some alien planet. It might make a lot more sense survival-wise to live on a WorldShip constructed to the needs of its makers and roam the galaxy searching for resources and the equivalent of tourism rather than be stuck on a single rock knowing that some day they would just have to pull up stakes and move again when the resident star starts dying or something goes south with the planet itself.

        • J. Jason Wentworth May 3, 2018, 20:33

          I agree. Several scientists (including Carl Sagan) have suggested that nomadic extrasolar civilizations may exist. If so, M class red dwarf stars would be ideal colony sites because of these slow-burning little suns’ extremely long lives. Also:

          While I think planets of K class dwarf stars are likely the most common homes of indigenous civilizations, I can think of at least three situations in which life–and even intelligence–might arise (although not commonly, I’d guess) on planets of red dwarf stars:

          [1] A terrestrial-type planet might form far enough away from the star to retain its atmosphere, then migrate inward later, after the star’s “vicious flare phase” was past (with these stars’ immensely long lives, life would have plenty of time to get going);

          [2] A large enough moon–perhaps something like a “warm Titan”–of a close-in Jovian-type planet (not necessarily a “Hot Jupiter,” but a warm one) might retain its atmosphere due to the protection afforded by the giant planet’s magnetosphere, and;

          [3] An Earth-like planet that formed within the ecosphere (habitable zone) of a red dwarf star might be able to hold onto its atmosphere if (A) the star wasn’t too flare-active (apparently, not all red dwarfs are) and (B) the planet had–like Uranus–a spin axis nearly parallel to the plane of its orbit, so that the planet’s rotation wasn’t tidally-locked to the star. So:

          As frustrating as it might be (due to the sheer numbers of them), examining M as well as K dwarf stars (along with all other stars–especially G and F ones–with slow rotations) may be worthwhile, in the search for possible abodes of extrasolar life. With so many M dwarfs in the Sun’s neighborhood, what a pity it would be if we overlooked other life–intelligent or not–just a relatively few light-years away. (Even just for gathering planetary population statistics, detecting red dwarfs’ planets would be worth doing, and such worlds might one day be new bases or homes for human beings.)

          • Michael C. Fidler May 5, 2018, 10:27

            I noticed that Barnard’s Star has been listed with the e as a flare star, so went looking for the data and found that 20 years ago one flare was was observed for about an hour with no reports since. This seems to be a little harsh to put in the class of the often flaring red dwarfs, but there is something else that is interesting about this flare: it was over 8000 degrees Kelvin and blue in color. Barnard’s Star is a M4 dwarf with an age of over 10 billion years and a slow 130 days rotation which puts it in the class of an old non flaring red dwarf. After looking at what happens to red dwarfs after they leave the main sequence I see that they turn into blue dwarfs with temperatures at around 8000 degrees Kelvin. Could these flares just be an early burp in the stars life foretelling its future as a blue dwarf or could an ET civilization be dumping Hydrogen into it to forestall its eventual change to blue?

            Blue Dwarfs: Stars Yet To Be.

            http://beyondearthlyskies.blogspot.com/2013/09/blue-dwarfs-stars-yet-to-be.html

            http://2.bp.blogspot.com/-wfyjNG9FAiA/UiGs2WiRngI/AAAAAAAAF5Q/EC3m6b_rVLw/s640/Post+-+September+2013+(9)+-+2.jpg

  • Robin Datta May 1, 2018, 21:11

    For any given pranet, the issues should be boiled down thus:
    1. Incompatible with chemistry-based life as we understand or imagine it.
    2. Chemistry-based life not excluded, but no indications suggesting it.
    3. Chemistry-based life not excluded, but some suggestions of such life.
    4. Chemistry-based life suggested, with other indications of such life.
    5. Chemistry-based life not excluded, some possibility of intelligence (Tabby’s star).
    6. Chemistry-based life excluded, some possibility of intelligence (post- or non- biological intelligence).
    7. Indications of intelligence.
    7. Confirmation of intelligence.

  • Alex Tolley May 1, 2018, 23:01

    I agree with Antonio, there are far too many assumptions involved. Dressing them up with mathiness doesn’t really lead to better decision making.

    Depending on the number of stars in the search, it might be better to initially make no assumptions, and try to drive the search direction from the early results. The ability to determine an Archaean equivalent biosignature might be the key driver, unless the aim is to find a late Proterozoic equivalent world that looks like an Earth II.

    We really have no idea what range of biologies life in the universe can take. Looking for life that looks like terrestrial life because it is our only example may result in our missing the big picture. IMO, it is a mistake much like Nasa’s “follow the water” approach. It would be ironic if the ExoMars orbiter detects life from the analysis of CH4 in the Martian atmosphere, a very different approach than than looking for liquid surface (or subsurface) water.

  • AlexT May 2, 2018, 3:13

    May be we have to search for the planets where our (the Earth) life is possible , with target to use this planet for future colonizing and etc.?
    I suppose that existence ETI civilization or even any life form is less important that the task to find the new (additional, alternative) “ home” for homo sapiens (and companion live organisms).
    I suppose we have to be more egoistic, so finding of ET life or civilization will be additional bonus in those researchs.

    • ljk May 2, 2018, 10:31

      There is an issue that often gets side-stepped whenever we talk about finding exoplanets like Earth with the ultimate goal of eventually colonizing them — in addition to what could become a hoary old trope that we send our biological selves into the galaxy to live on planets that could support terrestrial organic creatures because that is what they’ve been doing in science fiction for decades now so it must be the way to go, right.

      The issue is: If an alien planet is like Earth in key ways, this would presumably include being inhabited. Whether by the equivalent of just fungus and groundhogs or something highly intelligent, the point is such worlds will already have their own life forms which helped to make them terrestrialish in the first place.

      So do we go marching in and just set up shop if there is nothing there we consider to be intelligent? Robert Zubrin and others who want to colonize Mars so badly have already said they don’t want Martian microbes stopping such plans, so we have at least one real-world example of what some would do if we found an exoworld inhabited by paramecium and stromatolites (and probably higher up on the food chain but lacking sufficient “brains” so far as we humans judge such things). And what about intelligent natives who “qualify” in the smarts department but are lacking in the technology realm? Maybe Avatar won’t be so far off.

      I initially wanted to say all this may become moot if it is Artilects who explore and settle the Milky Way galaxy, but there is always the chance they may have needs and goals for star systems that go way past our rather limited thinking and use for alien worlds. And what will happen to the flora and fauna of those places then? If we can think about sending vessels to other worlds, then we can spare a few moments to think about the consequences too and how we should respond to them. And let us not shove it off to ambiguous future generations.

      • AlexT May 2, 2018, 14:12

        If I could accept your point of view, I should conclude that homo sapience should not make any eforts for space exploration or researches, opposit we will bring lor of wow to ET live.
        Without those steps (stop exploration) we cannot protect our Universe from human expansion. This approach allows to save money of tax-payers too!
        Ideal solution.

        • ljk May 2, 2018, 17:01

          I never said do not explore/colonize space and I am pretty sure you are aware of this. What I am asking is that we at least consider some detailed planning ahead now so we can avoid or at least lessen some inevitable issues along the way.

      • Stephen September 11, 2018, 12:47

        I think we should search out candidates for terraforming and colonization that are ideally uninhabited or as uninhabited as possible. By this I mean several types of planets: (1) lifeless worlds that need serious changes to be terraformed Earth analogs for colonization AND (2) worlds that are analogs to early Earth and have no life or microscopic life. 1 type worlds may need atmospheres installed or changed (perhaps drastically like Venus) and other changes to become habitable by humans and Earth flora and fauna. As for 2 type worlds, I would not have a problem settling with humans and Earth fauna/flora breathable lifeless worlds or worlds where the only indigenous life is xenobacteria of varying degrees of primitiveness and complexity. I think Zubrin is right about this.
        IF (and it may not be the case) there are planets with indigenous non-sentient fauna and flora, perhaps we could settle them with humans but keep earth plants and animals off these worlds in order to preserve their natural existence to the extent it doesn’t interfere with human dominance and expansion. IF (again there may be no such worlds in the Milky Way) a world like Pandora in Avatar exists with primitive sentient life, I think we should practice the prime directive and leave these beings alone. Perhaps if they advanced enough we could consider revealing ourselves to them but otherwise I say let’s keep them off limits to avoid contamination. (In effect, keeping their worlds as a type of nature preserve.)

    • Alex Tolley May 2, 2018, 10:43

      Colonizing a planet with terrestrial type biology may not be a good idea. The biosphere and the colony will be subject to unforeseen contamination problems, let alone the moral issue of potentially disrupting the biosphere. Far better to find suitable [sterile] worlds for long term terraforming or even building space colonies instead. Living worlds should perhaps be treated as tourist destinations with extreme care taken to allow observation without contamination.

      • AlexT May 2, 2018, 15:24

        I suppose that we should find those terristrial type planet at first, then after we know all parameters of this planet we can try to make conclusion what will be better colonize it or begin terraforming of alternative planete-candidate (that should be found at first too). Whitout existence of planet-candidate and basic knowledge about planets this discussion (colonization or terraforming) is groundless.
        I want to remind that the question is there life (intelligence) in our Galaxy/Universe (excluding our own life) – still open.

      • Michael C. Fidler May 3, 2018, 10:15

        We don’t have to go very far to find just such a place – Venus has been pretty well sterilized, unless your species is from hell. With what we know now about bioengineering how long would it take to change it to near earth like conditions? A few comets to stir the pot and it just might be the next Riviera!

      • Michael C. Fidler May 3, 2018, 10:18

        I think you just solved the “Fermi Paradox”!

        “Living worlds should perhaps be treated as tourist destinations with extreme care taken to allow observation without contamination.”

        • Michael May 4, 2018, 17:50

          So ‘Alien encounters’ are merely exotourists wondering from the tour agents path and having a cuddle, what about the probes man, what’s that about!

        • J. Jason Wentworth May 5, 2018, 10:39

          Maybe that also explains the manna from heaven that’s mentioned in Exodus, as well as the Fermi Paradox–the Earth isn’t just a zoo, but a petting zoo! :-)

          • ljk May 8, 2018, 8:41

            Or perhaps we are like the episode of South Park where Earth is the galaxy’s version of a reality show. That would actually make a lot of sense, especially these days.

            http://southpark.wikia.com/wiki/Cancelled

  • ljk May 2, 2018, 9:30

    Since the whole point of this exercise is to find alien life and places to colonize some day, we should be looking for “rogue planets” that are actually giant WorldShips heading towards or already in our Sol system grabbing resources and such from our outer comet belts, or in the main planetoid belt if they dare.

    Oh of course they would dare: Most humans wouldn’t notice and even if they did figure out there was an interloper in our celestial backyard, it’s not like we could do anything about it anyway, certainly not quickly. No wonder there are so many UFO reports of objects buzzing aircraft and people on lonely roads at night, they know all we can do is point and scream Look!

    And infrared signatures where these are no visible objects.

    We are going to send our first interstellar vessels to the very nearest star systems whether they are viable for life or not, so if we are talking about what we should be focusing on first then they are it.

    Of course it is important to get data on everything that we can, but unless someone suddenly comes up with an FTL drive we won’t be going to Kepler 186 or most other such places any time soon. We also have to realize and accept that while Breakthrough Starshot is great in terms of someone actually planning a real interstellar mission instead of just another in an endless series of white papers with no serious monetary backing, the project won’t be happening for decades if it does happen at all. So you can add that to the 20 years travel time to Alpha Centauri, which means most of us reading this won’t be around to see what is there. Which is okay because we need to make and leave something positive for our future, but I know how many want to see such thing happen while they are still breathing.

  • Jim Early May 2, 2018, 12:04

    When one is trying to decide between options on where to invest limited ressearch resources, the usual choice is to invest in what you believe is the best guess. Sometimes it is cheaper to initially identify the poor choice so that later resources can be targeted correctly.

    Initially it appears to be easier to evaluate local m class stars. If you can show that there are no live signs or there are major problems with planets in the life zones of m class stars, then you can make your later expensive investments (satellites and telescopes) on studies of G and K star systems. This approach likely yields poor short term results, but it may givebetter long term results.

  • Ashley Baldwin May 2, 2018, 13:40

    The strategy ? In terms of biosignatures couldn’t be more simple.

    It should be , can only be , based around the limitations of the available technology . Which in the short to medium term is , in no particular order and potentially in combination , Spitzer, Hubble , JWST , ARIEL , possibly FINESSE and Metis on the E-ELT. The first five of these depend on transit spectroscopy which to have any chance of building up any kind of biosignature SNR requires plentiful and deep transits . Which means mid- late M dwarf system “hab zone” planets such as TRAPPIST -1 d, e and f, LHS1140b and anything else TESS kicks up over the next few years .

    Metis / E-ELT also does transit spectroscopy though with greater observation limitations ,but unlike the others has the resolution and light gathering capacity to do direct imaging of nearby planets . Mostly neighbouring M dwarfs again (Proxima b ,Ross 128 b ) but maybe including Tau Ceti – and any other neighbouring sun like stars ( e.g 61 Cygni , 40 Eridani , Eta Cassiopiae ?) that are sufficiently old ( according to Tarter and Turnbull 3 Giga years plus ) to have evolved atleast Earth like microbial life and that have been found to have hab zone planets by next gen RV spectroscopy searches like upcoming 100Earths . With an absolute maximum best contrast ratio of 1e7 from the ground , only spectroscopic biosignatures in the near to mid I.R could be seen however.

    So there it is and space weather be damned.( if M dwarf activity is that bad how come the TRAPPIST planets have already been shown to retain so many of their “volatiles” ?)

    Anything else is aspirational and decades away.

    • Harry R Ray May 3, 2018, 10:21

      One of the “volatiles” the TRAPPIST-1 planets MAY turn out to have is helium(helium is NOT a volatile PER SE, but astronomers like to CATEGORIZE all elements that are NOT H and He as “metals” so, I am doing the same thing here.), although Ramses Ramirez seems NOT to think so. I Acquiesed to his opinion on this until today when I came across THIS: “Helium in the eroding atmosphere of an exoplanet.” by Spake J., Sing D., Evans T., Oklopcic A., Bourrier V., et al(one of these authors was the LEAD author of the November 2017 paper ArXiv: 1711.05269; A New Windo into Escaping Exoplanet Atmospheres: 10830 A line of Helium.” by Antonjia Oklopcic, Christopher Hirata). What made me chance my mind is that WASP 107b’s irradiation is not that much different than TRAPPIST-1b’s(Wasp 107b is a 61 CygniA/61 CygniB-like K6 star and WASP 107b’ orbital period is 5.724149 days). The problem here is that helium can only be detected ONLY if it is in the excited metastable state. However, I would RISK precious HST time to observe TRAPPIST-1b at the 10830 angstrom line to find out.

  • Geoffrey Hillend May 2, 2018, 16:57

    Volatiles are not enough for the TRAPPIST-1 exoplanets could still be sterile with volatiles. There has to be more than water, or biosignature gases in the atmosphere like CH4, O2, N2, etc. There also has to be general principles of physics and astrophysics involved, not just astrobiology. The predictive ability of science is based on invariant, general principles. One has to have a knowledge base or foundation for science to work, otherwise one is only guessing which has a less probable chance of predicting an unknown. We should be able to find out whether or not M class dwarf stars have biosignatures within the next ten years. I agree that we should look at them to rule them out which is what science does. I predict they will be ruled out.

  • Geoffrey Hillend May 2, 2018, 17:00

    I meant exoplanets around M class dwarf stars.

  • P May 2, 2018, 18:22

    25 comments and almost as many different opinions.
    My conclusion is that we just search the entire parameter space in the most unbiased way possible and see what eventually emerges.
    That includes exotic objects like white and brown dwarfs, and so called ‘rouge’ objects.

    Either way its going to be fun, and I bet we will be surprised.

    P

    • ljk May 3, 2018, 8:28

      SETI has had six decades to figure out that G class stars aren’t exactly blasting out radio waves from earthlike planets. Only recently have they started getting outside their paradigm zone, though Radio SETI still dominates technically as well as in the culture.

      Optical SETI has been rising since the late 1990s when it became “acceptable” after initially being sidelined in the early 1960s, but radio telescopes are always seen as “sexier” and still dominate the image of SETI in people’s minds.

      As I agreed with you above in this thread and said in so many words, it is time we stopped looking for our car keys under the lamp post and started expanding our search areas. Yes I know it is happening but not fast enough. Ironically the general public may be more prepared for a cultural shift when it comes to finding and accepting ETI signaling or arriving in ways different than what the astronomical community has declared since circa 1959. The professional community is still hampered by the decades of ridicule caused by the UFO fringe/cult and their own peers who looked down upon aliens thanks to our juvenile and trashy culture perspectives on the subject.

      Let us hope a Russian billionaire can lead us out of this intellectual darkness.

  • andy May 2, 2018, 18:38

    Target the stars that your observational technique is good for. Trying to image a planet in an M-dwarf habitable zone is going to be extremely challenging due to the small scale. Transits of planets in an F-dwarf habitable zone are going to be pretty unlikely and the long orbital periods will make follow-up transit spectroscopy frustratingly slow.

    I think there is more value in trying to cover a wider parameter space than trying to just find Earth-twins, so I think the transit method’s preference for M- and K-dwarfs is a good thing – it forces us to explore non-Earthlike conditions. Once imaging gets to the point of being able to detect HZ planets, I hope that F- and A-type stars won’t be neglected.

  • ljk May 4, 2018, 9:42

    Protecting Mars From What – And Why Bother? Our Solar System Inheritance Of Unopened Treasure Chests

    By Robert Walker | May 4th 2018 02:27 AM

    http://www.science20.com/robert_walker/protecting_mars_from_what_and_why_bother_our_solar_system_inheritance_of_unopened_treasure_chests-232194