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To See a Habitable World

Video presentations from the recent Tennessee Valley Interstellar Workshop are beginning to appear online. It’s welcome news for those of us who believe all conferences should be available this way, and a chance for Centauri Dreams readers to home in on particular presentations of interest. I published my Closing Remarks at TVIW right after the meeting and will watch with interest as the complete 2017 videos now become available. There are a number of these I’d like to see again.

All of this gets me by round-about way to Project Blue, the ongoing attempt to construct a small space telescope capable of directly imaging an Earth-like planet around Centauri A or B, if one is indeed there. For the other talk I gave at TVIW 2017 (not yet online) had to do with biosignatures, and the question of whether we had the capability of detecting one in the near future with the kind of missions now approved and being prepared for launch. This was delivered as part of a presentation and panel discussion with Greg Benford and Angelle Tanner (Mississippi State), one of the conference’s ‘Sagan Meetings.’

The answer is yes, though a highly qualified yes, and Project Blue could be a part of that. When we talk about biosignatures, we are presuming the ability to analyze a planet’s atmosphere, and that is something we’re fully capable of doing today. The fact that such analyses have involved hot gas giants should not deter us, for we’re shaping the methods that will eventually get us down to Earth-mass worlds. These planets won’t necessarily be Earth-like, for the first ones we’ll see in this range will be orbiting red dwarf stars in the immediate neighborhood.

Image: Transmission spectroscopy illustrated. If we can find a transiting world, we can study both the primary eclipse as it moves across the face of the star and the secondary eclipse as the planet’s thermal radiation disappears behind it. Credit: Sara Seager (MIT).

There are reasons for that, most specifically the deep transit depth that an Earth-size planet would present when passing, in a habitable zone orbit, across the face of a nearby red dwarf. In the primary eclipse, some of the starlight we detect has passed through the planet’s atmosphere, giving us the chance to study its components. A biosignature gas is one that is produced by life and accumulates at detectable levels within an atmosphere. We’re especially interested in gases that are out of equilibrium, such as oxygen in the presence of methane.

The point here is that oxygen is unstable over geological timeframes, and would be depleted through reactions with volcanic gases and oxidation unless there existed a source to replace it. A combination of oxygen and methane would indicate a steady source of both, as James Lovelock noted in 1975. The combination is simply out of balance from what would occur through natural processes. Detecting it would indicate the probable presence of life on the planet.

False positive issues abound here, but let’s get into them tomorrow. Because I want to circle back to Project Blue at this point, to discuss the fact that beyond transmission spectroscopy, we have the capability of looking at planets through direct imaging. Coronagraphs are one way of suppressing the light of a central star, and so are starshades, the latter a possibility for the WFIRST mission, which could be constructed so as to be ‘starshade ready’ and thus capable of being joined in space by a later starshade mission that would fly tens of thousands of kilometers away and offer the prospect of detecting planets within 1 AU even of G-class stars like the Sun. In any case, WFIRST is being adapted for a coronagraph whether or not the starshade flies.

Image: Direct imaging at work. HR 8799 is a star a bit more massive than the Sun, and far younger, about 60 million years old. We see not one but three planets orbiting it. This is the first portrait of an exosolar system. It was announced at the same time as the Fomalhaut candidate, too, giving this system a tie as the first ever seen orbiting a Sun-like star. The planets, labeled b, c, and d, are about 7, 10, and 10 times the mass of Jupiter, respectively, and orbit their star at 68, 38, and 24 times the distance of the Earth from the Sun. It’s possible planets c and d are massive enough to qualify as brown dwarfs. Credit: Gemini Observatory.

I provided the background for Project Blue at the end of September, describing the small telescope originally called ACEsat that, in the hands of Eduardo Bendek and Ruslan Belikov (NASA Ames) later morphed into the crowd-funded Project Blue design. The designers believe that this telescope would be fully capable of photographing a ‘pale blue dot’ around one or both of the primary Alpha Centauri stars, and that would mean we could dispense with transmission spectroscopy and deal with light directly from the planet in question to study its atmosphere.

The Project Blue campaign has a week to run in its attempt to reach a $175,000 goal (it’s 73 percent of the way there as of this morning). Giving the campaign an additional boost is the action of a donor who has pledged to match contributions until the goal is reached, making every dollar people contribute turn into two. Check the project’s Indiegogo page for further information about its light suppression techniques, which include not only a coronagraph but a deformable mirror, low-order wavefront sensors and software control algorithms to manipulate incoming light, as well as post processing methods that substantially enhance image contrast.

Image: (L) Voyager’s ‘pale blue dot’ photo of Earth from the edge of our Solar System; (R) A simulated image of a photo Project Blue aims to take of exoplanets around one of the stars in the Alpha Centauri system. (Credit: NASA JPL / Jared Males).

Have a look at the Project Blue page and help if you can. The goals involved are to set the science requirements for the mission, complete the opto-mechanical design and request industry letters of interest, while developing a mission performance simulator to analyze different configurations (the latter will eventually become available online).

A mission like Project Blue could conceivably move the timetable forward in terms of biosignature detection, if absolutely everything falls into place and the right kind of planet is indeed orbiting Centauri A or B. Thus, between upcoming red dwarf investigations using the James Webb Space Telescope (aided by data from the Transiting Exoplanet Survey Satellite), and possible direct imaging of an Earth-like planet around the nearest star system, we could conceivably detect biosignatures within the next ten to fifteen years. It’s a stretch, and I’ll explain why tomorrow, but add a decade or two to the timeline and the odds begin to climb.


{ 17 comments… add one }
  • Douglas Loss October 30, 2017, 14:32

    Hello Paul, and thanks for the boost! We’re getting the TVIW presentations online as quickly as we can; I hope they’ll all be available sometime in November.

  • Ashley Baldwin October 30, 2017, 14:49

    This mission is potentially pivotal . For the obvious reason of hopefully finding habitable zone terrstrial planets around as the two nearest and Sun like stars . But more than this and less well known is that Project Blue would become a technological demonstrator , however small, of the hardware required to direct image exoplanets . Of all this technology the most important by far is the telescope coronagraph. Central to direct imaging .

    As stated this blocks out the light of the star and in doing so allows the much, much dimmer orbiting planets to be seen . Sounds easy in principle but boy is it hard in practice. Exoplanet imaging coronagraphs have been being researched at numerous high contrast test centres for well over a decade , nearly two in fact . Only now , thanks largely to WFIRST mission formulation work over the last eighteen months or so , are they beginning to approach the sort of performance required to image the sort of Earth like planets hoped for by Project Blue. The Project Blue telescope has heritage in ACESat not least through its choice of coronagraph type . The Phase Induced Amplitude Apodisation coronagraph , PIAA for short .

    This instrument is one of numerous high performance coronagraphs that have been extensively investigated and one of the two under consideration for WFIRST as back up to its Hybrid Lyot and Shaped Pupil Mask first choice .

    So why the PIAA ? Coronagraphs are all known to reduce the contrast difference between a parent star and an orbiting planet , allowing the latter to me imaged and characterised . The contrast difference between the Earth at 1 AU from the sun is about ten billion in visible light . 1 to the power of 10 , shortened by convention to 1e10. So that’s the sort of contrast required to image Earth mass stars around Sun like stars . Very hard , but not impossible and indeed already achieved in single wavelength ( monochromatic ) or narrow wavelength light on the imaging test beds at JPL, Lockheed Martin and NASA Ames ( where Bendek and Belikov hail from ) .

    If that was all that was necessary then why haven’t we had a direct mission already then ? Well the other key performance metric of a coronagraph is not just its contrast reduction but how close it can image planets to their stars . The angular separation or “inner working angle “, IWA. Habitable zone planets lie closer in to their stars and the dimmer the star the close this will be. Say jus 0.2 AU for an early M class star .( though being dimmer and emitting more in longer wavelength infrared such stars have less strenuous contrast requirements down to merely 1e8, 0ne hundred million. Important for the Earth based ELTs with their 1e8 atmospheric limited coronagraphs but the advantage of huge apertures allowing small IWAs ) . The IWA is generally expressed in terms of wavelength with the best potential achievable around 1.2 lambda though it is also dependent on telescope aperture as IWA = 1.2lambda/ D ( in m) – the D being important as seen above with huge ELTs. So better coronagraphs offer best IWA performance aided by bigger telescopes . But big telescopes are expensive . ( for conventional designs NASA works on a cost of about $44 million per 10cms aperture – do the math ) The cost of a coronagraph to date has largely been in developing them . Once operational they aren’t particularly expensive to build and run as seen by the Project Blue costs . Telescope aperture costs scale significantly with size as stated above – just think of $8.5 billion 6.5m JWST and even $3.5 billion 2.4m WFIRST.

    High contrast is harder to achieve the closer to the star you get . Although the many coronagraph designs have been shown to be capable of achieving good IWAs, it’s the combination of this with high contrast that has proved elusive . Of all coronagraphs the PIAA has been shown to hold the best theoretical limits on both though.

    Finally and equally crucial there is one particular area where the PIAA is the undisputed champion and which is especially important to the success of small telescopes like Project Blue and ACESat . That’s throughput . In other words how much of the light incident on the telescope reaches its focal plane and characterising sensor equipment . The PIAA has consistently achieves figures as high as 95% in unobscured telescopes ( Project Blue ) and it’s close cousin ,the PIAACMC ( Complex Mask Coronagraph) has even managed 70 % on obscured telescopes designs ( like WFIRST) . By way of comparison the current coronagraph selection for WFIRST achieves about 35 % throughput at maximum and this on an already 20 % obscured telescope ( by its secondary mirror ) . That’s a lot of light lost and lost light reduces sensitivity . Essentially an unobscured ( “coronagraph optimised “) telescope using a 95 % PIAA coronagraph could thus collect as much light as an obscured one with a mirror three times larger !

    So an optimal PIAA offers 1e10 raw contrast reduction at an IWA of just 1.2lambda and will offer a three times reduction in required telescope to achieve this . This mens 0.45m Project Blue with a PIAA coronagraph could achieve the same as a 0.8m obscured scope with hybrid Lyot Shaped pupil coronagraph . Scaled up by comparison , the 1.4m Exo-C concept telescope with a PIAA coronagraph would potentially get 25 % more light to its spectrograph than the current iteration of 2.4m WFIRST. EXO-C has a $1 billion price tag and is not currently envisaged as being able to see many if any habitable zone planets . However if the PIAA coronagraph can be developed and matured to its theoretical best – through missions like Project Blue – and incorporated into Exo-C then for a “cut price” we have a “flagship” performance telescope that can see habitable zone planets within 30 light years or so.

  • Ashley Baldwin October 30, 2017, 17:31

    It’s worth pointing out that transmission spectroscopy is limited by atmospheric refraction to higher altitudes. Not a problem for the Hot Jupiters observed so far with their thick envelopes . Potentially a limiting factor in the future for terrestrial planet observation ( TRAPPIST , LHS 1140b et al with JWST & ELTs ?) though . The refraction does however reduce at the sort of longer IR wavelengths seen with later M dwarfs though, possibly allowing spectra extending down into planetary tropospheres . The interesting ground layer atmosphere where Paul’s biosignatures might be found !

    The secondary eclipse or occultation illustrated so nicely here by Sara Seager often gets forgotten despite its central importance to transit spectroscopy . It is this that provides the baseline stellar spectrum from which all other transit spectra are derived . The difference between primary and secondary eclipses is used to calculate the exoplanetary period . For close in tidally locked planets this is also displays their day length and rotation rate . As the exoplanet enters or exits secondary eclipse it presents its “day side” spectrum , a combination of reflection and emission in comparison with its “night side” spectrum calculated via its primary transit at the same time as a transmission spectrum is observed as starlight passes through the exoplanet limb.

    Knowing the orbital period also allows a “phase curve” to be plotted as the exoplanet is differentially illuminated whilst orbiting its star , allowing brightness variations to be observed at different longitudes across the planetary surface rather than just one point . This can extend to different latitudes too during actual occultation ingress and egress. All of this could with sufficient spectrophotometric resolution allow observation of seasonal changes in spectra and wind velocities related to heat transfer . A technique pioneered already with Hubble for admittedly big, bright and close in hot Jupiters. Further extension to more temperate and smaller exoplanets might occur via the ELTs however with their ultra high apertures and spectrophotometric resolution – though it’s worth remembering the absolutely tiny number of photons involved for even the biggest of telescopes and the incredible technological achievement this work represents.

    Still early days and Sara Seager has recently pointed out the caveats of high false positive rates as the nascent science evolves , but things are moving as the science rapidly matures .

  • Ronald October 31, 2017, 8:30

    This question may have been asked before, but could this instrument, with some upgrading, be used on more stars than just the AC couple, with the same scientific goals? It could be so much more worthwhile and (cost-)effective to use such an instrument on more (solar type) stars in the vicinity, such as Tau Ceti, 82 Eridani, Delta Pavonis, etc., with a much greater chance of success. I cannot help thinking of the canceled Terrestrial Planet Finder. Could this mission be used as a kind of TPF light?

    • Ashley Baldwin October 31, 2017, 12:24

      I don’t know about Project Blue given additional imaging beyond the 2 years required for the Alpha Centuri stars would require further additional operations costs. When the similar ACESat was submitted as a Small Explorer concept it did have a one year extension mission option to view bright nearby stars Altair ( very forbidden planet ) and Sirius . Not close enough in to see their habitable zones though. It’s only the extreme close proximity of Alpha Centauri that allows such a small aperture telescope to view their hab zones.

      By way of comparison though , the Exo-S mission concept , using an external occulting Starshade to achieve similar performance and throughput to an optimised coronagraph could image down to Earth and Super Earth mass planets around favourable stars out to 30 light years with just a 1.1m scope .

      Shows just how important the precision of a telescope occulter is whether internal or external.

      • ljk November 1, 2017, 9:29

        If Altair has any planets, they probably are not going to have any life on them, or certainly nothing very advanced if they do: The star is already halfway through a roughly two billion year lifespan and spins on its axis once in just nine hours!


        • Ashley Baldwin November 1, 2017, 14:33

          Yes. And not the reason why Altair was selected . Simply because it ( along with Sirius and maybe Procyon ) was bright and relatively nearby. The brightness compensates for the limited light gathering capacity of the small ACESat aperture over just a year. The further out orbits compensate for the limits of the raw coronagraph performance in terms of contrast and IWA .

          It all illustrates the unique nature of the Alpha Centauri system. Two bright ,Sun-like stars just 1 parsec from Earth , consequently allowing an unusually large angular separation for their habitable zones . So much so that a mere 45cm scope and medium performance coronagraph can image them. The focus on the hardware however detracts from the innovative “orbital differential imaging” post processing technique employed . It can’t be rated highly enough. Only possible thanks to extended observations of just the two stars for a whole year that then creates the small IWA and high contrast required. A great technique but only really viable for this particular system.

  • Harry R Ray November 2, 2017, 9:36

    Using a process called “EPSI”, ground based telescpoes like Colossus or ExoLifeFinder could achieve a resolution EQUIVALENT TO OR BETTER THAN the NASA proposed lensing telescope!!! “Surface Imaging of Proxima b and Other Exoplanets: Topography, Biosignatures, and Artificial Mega-Structures.” by Svetlana V Berdyunga, Jeff R Kuhn, now up on the Exoplanet.eu website.

    • Harry R Ray November 3, 2017, 9:41

      Would it be possible for “EPSI”AND “SPIDERMAN” to be somehow COMBINED with Beam Shaping Diffusers in some kind of “triple whammy” fashion to obtain solar lensing type resolution on a a telescope CURRENTLY IN OPERATION, such as SPHERE?

  • Michael Fidler November 3, 2017, 0:08

    SPIDERMAN: an open-source code to model phase curves and secondary eclipses.
    “We present SPIDERMAN, a fast code for calculating exoplanet phase curves and secondary eclipses with arbitrary surface brightness distributions in two dimensions.
    Using a geometrical algorithm, the code solves exactly the area of sections of the disc of the planet that are occulted by the star. The code is written in C with a user-friendly Python interface, and is optimised to run quickly, with no loss in numerical precision. Approximately 1000 models can be generated per second in typical use, making Markov Chain Monte Carlo analyses practicable. The modular nature of the code allows easy comparison of the effect of multiple different brightness distributions for the dataset.
    As a test case we apply the code to archival data on the phase curve of WASP-43b using a physically motivated analytical model for the two dimensional brightness map. The model provides a good fit to the data; however, it overpredicts the temperature of the nightside. We speculate that this could be due to the presence of clouds on the nightside of the planet, or additional reflected light from the dayside. When testing a simple cloud model we find that the best fitting model has a geometric albedo of 0.32±0.02 and does not require a hot nightside. We also test for variation of the map parameters as a function of wavelength and find no statistically significant correlations.”


  • ljk November 10, 2017, 11:15

    More problems for big space telescopes

    After years of staying on schedule for a 2018 launch, NASA has delayed the James Webb Space Telescope to the spring of 2019. Jeff Foust reports on the issues that led to this delay, as well as challenges facing the next big space telescope after JWST.

    Monday, October 30, 2017


  • ljk November 28, 2017, 11:42


    By Keith Cowing on November 22, 2017 11:19 AM.

    WFIRST Independent External Technical/Management/Cost Review (WIETR), NASA

    “This report responds to the questions asked in the Terms of Reference (TOR) that established the WIETR and includes recommendations and options for NASA to consider. This report is input to NASA in support of its formulation of the WFIRST implementation plan so that the mission is both 1) well understood in terms of scope and required resources (cost, funding profile, schedule) and 2) executable. The WIETR recognizes the scientific importance and timeliness of WFIRST.

    The objectives of this ambitious mission are driven by the goal of answering profound questions about the Universe beyond our solar system and planet Earth. This ambition comes with challenges that must be recognized and addressed – these are the focus of this report.”

  • Tom Mazanec December 25, 2017, 11:07

    Add Project Blue to your projects list?

  • ljk December 29, 2017, 11:53

    NASA’s next major telescope to see the big picture of the universe

    December 22, 2017

    by Claire Saravia, NASA’s Goddard Space Flight Center


    Scheduled to launch in the mid-2020s, the Wide Field Infrared Survey Telescope (WFIRST) will function as Hubble’s wide-eyed cousin. While just as sensitive as Hubble’s cameras, WFIRST’s 300-megapixel Wide Field Instrument will image a sky area 100 times larger. This means a single WFIRST image will hold the equivalent detail of 100 pictures from Hubble.

    “A picture from Hubble is a nice poster on the wall, while a WFIRST image will cover the entire wall of your house,” said David Spergel, co-chair of the WFIRST science working group and the Charles A. Young professor of astronomy at Princeton University in New Jersey.

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