Exoplanet Imaging from Space: EXCEDE & Expectations

We are entering the greatest era of discovery in human history, an age of exploration that the thousands of Kepler planets, both confirmed and candidate, only hint at. Today Ashley Baldwin looks at what lies ahead, in the form of several space-based observatories, including designs that can find and image Earth-class worlds in the habitable zones of their stars. A consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK), Dr. Baldwin is likewise an amateur astronomer of the first rank whose insights are shared with and appreciated by the professionals designing and building such instruments. As we push into atmospheric analysis of planets in nearby interstellar space, we’ll use tools of exquisite precision shaped around the principles described here.

by Ashley Baldwin

This review is going to look at the current state of play with respect to direct exoplanet imaging. To date this has only been done from ground-based telescopes, limited by atmospheric turbulence to wide orbit, luminous young gas giants. However, the imaging technology that has been developed on the ground can be adapted and massively improved for space-based imaging. The technology to do this has matured immeasurably over even the last 2-3 years and we stand on the edge of the next step in exoplanet science. Not least because of a disparate collection of “coronagraphs”, originally a simple physical block placed in the optical pathway of telescopes designed to image the corona of the Sun by French astronomer Bernard Lyot, who lends his name to one type of coronagraph.

This is an instrument that in combination with ground-based pioneering work on telescope “adaptive optics” systems and advanced infrared sensors in the late 1980s and early ’90s progressed in the last ten years or so to the design of space-based instruments – later generations of which have now progressed to the point of driving telescopes like 2.4m WFIRST, 0.7m EXCEDE and 4m HabEX. Different coronagraphs work in different ways, but the basic principle is the same. On-axis starlight is blocked out as much as possible, creating a “dark hole” in the telescope field of view where much dimmer off-axis exoplanets can then be imaged.

Detailed exoplanetary characterisation including formation and atmospheric characteristics is now within tantalising reach. Numerous flagship telescopes are at various stages of development awaiting only the eventual launch of the James Webb Space Telescope (JWST), and its cost overrun, before proceeding. Meantime I’ve taken the opportunity this provides to review where things are by looking at the science through the eyes of an elegant telescope concept called EXCEDE (Exoplanetary Circumstellar Environment & Disk Explorer), proposed for NASA’s Explorer program to observe circumstellar protoplanetary and debris discs and study planet formation around nearby stars of spectral classes M to B.

Image: French astronomer Bernard Lyot.

Although only a concept and not yet selected for development, I believe EXCEDE – or something like it – may yet fly in some iteration or other, bridging the gap between lab maturity and proof of concept in space and in so doing hastening the move to the bigger telescopes to come. Two of which, WFIRST (Wide Field Infrared Survey Telescope) and HabEX (Habitable Exoplanet Imaging Mission) also get coverage here.

Why was telescope segmented deployability so aggressively pursued for the JWST?

“Monolithic”, one-piece mirror telescopes are heavy and bulky – which gives them their convenient rigid stability, of course.

However, even a 4m monolithic mirror-based telescope would take up the full 8.4m fairing of the proposed SLS block 1b and with a starshade added would only just fit in lengthways if it had a partially deployable “scarfed” baffle. The telescope would mass around 20 tonnes built from conventional materials. Though if built with proven lightweight silicon carbide, already proven with the success of the ESA’s 3.5 m Herschel telescope, it would come in at about a quarter of this mass.

Big mirrors made out of much heavier glass ceramics like Zerodur have yet to be used in space beyond the 2.4m Hubble and would need construction of 4m-sized test “blanks” prior to incorporation in a space telescope. Bear in mind too that Herschel also had to carry four years worth of liquid coolant in addition to propellant. With minimal modification, such a similarly proportioned telescope might fit within the fairing of a modified New Glenn launcher too. If NASA shakes off its reticence about using silicon carbide in space telescope construction – something that may yet be driven – like JWST before it – by launcher availability. This given the uncertain future of the SLS and especially its later iterations.

Meantime, at JWST conception there just wasn’t any suitable heavy lift/big fairing rocket available (or indeed now!) to get a single 6.5m mirror telescope into space. Especially not to the prime observation point at the Sun/Earth L2 Lagrange point 900 K miles away in deep space. And that was the aperture deemed necessary to be a worthy successor to Hubble.

An answer was found in a Keck-style segmented mirror which could be folded up for launch and then deployed after launch. Cosmic origami if you will (it may be urban myth but rumour has it origami experts were actually consulted).

The mistake was in thinking that transferring the well established principle of deployable space radio antennae to visible/IR telescopes would be (much) easier than it would turn out. The initially low cost “evolved”, but as it did so did the telescope and its descendants. From infrared cosmology telescope to “Hubble 2” and finally exoplanet characteriser as the new branch of astronomy arose in the late nineties.

A giant had been woken and filled with a terrible resolve.

The killer for JWST hasn’t been the optical telescope assembly itself, so much as folding up the huge attached sunshade for launch and then deploying it. That’s what went horribly wrong with “burst seams” in the latest round of tests and which continues to cause delays. Too many moving parts too – 168 if I recall. Moving parts and hard vacuums just don’t mix and the answer isn’t something as simple as lubricants, given conventional ones would evaporate in space, so that leaves powders, the limitations of which were seen with the failure of Kepler’s infamous reaction wheels. Cutting-edge a few years ago, these are now deemed obsolete for precision imaging telescopes, replaced instead by “microthrusters” – a technology that has matured quietly on the sidelines and will be employed on the upcoming ESA Euclid and then NASA’s HabEX.

From WFIRST to HabEX

The Wide Field IR Space Telescope, WFIRST is more by circumstance than design monolithic, and sadly committed to use reaction wheels, six instead of Kepler’s paltry four admittedly. I have written about this telescope before, but a lot of water as they say, has flowed under the bridge since then. An ocean’s worth indeed and with wider implications with the link as ever being exoplanet science.

To this end, any overview of exoplanet imaging cannot be attempted without starting with JWST and its ongoing travails, before revisiting WFIRST and segueing into HabEX. Then finally seeing how all this can be applied. I will do this by focusing an older but still robust and rather more humble telescope concept, EXCEDE.

Reaction wheels – so long the staple of telescope pointing. But now passé, and why? Exoplanet imaging. The vibration reaction the wheels cause, though slight, can impact on the imaging stability even at the larger 200mas inner working angle (IWA) of the WFIRST coronagraph, IWA being defined as the nearest to the star that maximum contrast can be maintained. In the case of the WFIRST coronagraph this is 6e10 contrast (which has significantly exceeded its original design parameters already.

The angular separation of a planet from its star, or “elongation”, e, can be expressed as e = a/d, where a is the planetary semi-major axis expressed in Astronomical Units (AUs) and d is the distance of the star from Earth in parsecs (3.26 light years). By way of illustration, the Earth as imaged from ten parsecs would thus appear to be 100mas from the Sun – but would require a minimum 3.1m aperture scope to capture enough light and provide enough angular resolution of its own. Angular resolution of a telescope is its ability to resolve two separate points and is expressed as the related ƛ / D, where ƛ is the observation wavelength and D is the aperture of the telescope – in meters. So the shorter the wavelength and bigger the aperture, the greater the angular resolution.

A coronagraph in the optical pathway will impact on this resolution according to the related equation n ƛ / D where n is a nominal integer set somewhere between 1 and 3 and dependent on coronagraph type, with a lower number giving a smaller inner working angle nearer to the resolution/diffraction limit of the parent telescope. In practice n=2 is the best currently theoretically possible for coronagraphs, with HabEX set at 2.4 ƛ / D. EXCEDE’s PIAA coronagraph rather optimistically aimed for 1 ƛ / D – currently unobtainable, though later VVD iterations or perhaps revised PIAA might yet achieve this and what better way to find out than via a small technological demonstrator mission?

This also shows that searching for exoplanets is best done at shorter visible wavelengths between 0.4 and 0.55 microns, with telescope aperture determining how far out from Earth planets can be searched for at different angular distances from their star. This in turn will govern the requirements determining mission design. So for a habitable zone imager like HabEX where n=2.4 and whose 4m aperture can search habitable zones of sun like stars out to a distance of about 12 parsecs. Coronagraph contrast performance varies according to design and wavelength so higher values of n, for instance, might still allow precision imaging further out from a star, perhaps looking for Jupiter/Neptune analogies or exo-Kuiper belts. Coronagraphs also have outer working angles, the maximum angular separation that can be viewed between a star and planet or planetary system (cf starshades,whose outer working angle is limited only by the field of view of the host telescope and is thus large).

Any such telescope, be it WFIRST or HabEX, for success will require numerous imaging impediments to be adequately mitigated – so called “noise”. Noise from many sources: target star activity, stellar jitter, telescope pointing & drift. Optical aberrations. Erstwhile “low-order wavefront errors” – accounting for up to 90% of all telescope optical errors (ground and space) and including defocus, pointing errors like tip/tilt and telescope drift occurring as a target is tracked, due for instance to variations in exposure to sunlight at different angles. Then classical optical “higher order errors” such as astigmatism, coma, spherical aberration & trefoil – due to imperfections in telescope optics. Individually tiny but unavoidably cumulative.

It cannot be emphasised enough that for exoplanet imaging, especially of Earth-mass habitable zone planets, we are dealing with required precision levels down to hundredths of billionths of a meter. Picometers. Tiny fractions of even short optical wavelengths. Such wavefront errors are by far the biggest obstacle to be overcome in high-contrast imaging systems. The image above makes the whole process seem so simple, yet in practice this remains the biggest barrier to direct imaging from space and from the ground even more.

The delay between the (varying) wavefront error being picked up by the sensor, fed to the onboard computer and in turn the deformable modifying mirror to enable correction (along with parallel correction of pointing tip/tilt errors by a delicate “fast steering mirror”), and the precision of that correction – has been too lengthy. The central core of the adaptive optics (AO) system.

It has only been over the the last few years that there have been essential breakthroughs that should finally allow elegant theory to become pragmatic practice. This through a combination of wavefront correction via improved deformable mirrors and wavefront sensors and their enabling computer processing speed all working in tandem. This has led to creation of so-called “extreme adaptive optics” with the general rule that the shorter the observed wavelength, the greater the sensitivity “extremity” of the required AO. It is an even larger impediment on the ground where the atmosphere adds an extra layer of difficulty. These combine to allow a telescope to find and image tiny, faint exoplanets, and more importantly still, to maintain that image for the tens or even hundreds of hours necessary to locate and characterise them. Essentially a space telescope’s adaptive optics.

A word here. Deformable mirrors, fast steering mirrors, wavefront sensors, fine guidance sensors & computers, coronagraphs, microthrusters, software algorithms. All of these, and more, add up to a telescope’s adaptive optics – originally developed and then evolved on the ground, this instrumentation is now being adapted in turn for use in space. It all shares the feature of modifying and correcting any errors in wavefront of light entering a telescope pupil prior to reaching its focal plane and sensors.

Without it imaging via big telescopes would be severely hampered and the incredible precision imaging described here would be totally impossible.

That said, the smaller the IWA the greater the sensitivity to noise and especially vibration and line of sight “tip/tilt” pointing errors, and the greater the need for the highest performance, so called “extreme adaptive optics”. HabEX has a tiny IWA of 65 mas for its coronagraph (to allow imaging of at least 57% of all sun-like star hab zones out as far as 12 parsecs) and operates at a raw contrast as low as 1e11 – a hundred billionth of a metre!

Truly awesome. To be able to image at that kind of level is incredible frankly when this was just theory less than a decade ago.

That’s where the revolutionary Vector Vortex “charge” coronagraph (VVC) now comes in – the “charge 6” version still offers a tiny IWA but is less sensitive to all forms of noise – and especially the low wavefront errors described above – than other ultra high performance coronagraphs, noise arising from small but cumulative errors in the telescope optics.

This played a major if not pivotal role in the VVC 6 selection for HabEX. The downside (compromise) is that only 20% light incident on the telescope pupil gets through to the focal point instruments. This is where the unobscured largish 4m aperture of HabEX helps, to say nothing of removing superfluous causes of diffraction and additional noise in the optical path.

There are other VVC versions, the “charge 2” for instance (see illustration), that allows 70% throughput – but is so sensitive to noise as to be ineffectual at high contrast and low IWA. Always a trade off. That said, at the higher IWA (144mas) and lower contrast (1e8 raw) of a small imager telescope like the Small Explorer Programme concept EXCEDE, where throughput really matters, the charge 2 might work with suitable wavefront control. With a raw contrast (the contrast provided by the coronagraph alone) goal of < 1e8, "post-processing" would bring this down to the 1e9 required to meet the mission goals highlighted below. Post-processing involves increasing contrast post-imaging and includes a number of techniques with varying degrees of effectiveness that can increase contrast by up to an order of magnitude or more. For brevity I will mention only the main three here. Angular differential imaging involves rotating the image (and telescope) through 360 degrees. Stray starlight, so called "speckles", are artefacts and move with the image.

A target planet does not, allowing the speckles to be removed, thus increasing the contrast. This is the second most effective type of post-processing. Speckles tend to be wavelength-specific, so looking at different wavelengths in the spectrum once again allows them to be removed with a planetary target persisting through various wavelengths. So-called spectroscopic differential imaging.

Finally, light reflected from a target tends to be polarised as opposed to starlight, and thus polarised sources can be picked out from background, unpolarised leaked starlight speckles with the use of an imaging polarimeter (see below).

Polarimetric differential imaging. Of the three, the last is generally the most potent and is specifically exploited by EXCEDE. Taken together these processes can improve contrast by at least an order of magnitude. Enter the concept conceived by the Steward Observatory at the University of Arizona. EXCEDE.

EXCEDE: The Exoplanetary Circumstellar Environment & Disk Explorer

Using a PIAA coronagraph with a best IWA of 144 mas (ƛ/D) and a raw contrast of 1e8, the EXCEDE (see illustration) proposal consisted of a three year mission that would involve:

1/ Exploring the amount of dust in habitable zones

2/ Determining if said dust would interfere with future planet-finding missions – the amount of zodiacal dust in the Solar System is set at 1 “zodi”. Exozodiacal dust around other stars is expressed in multiples of this. Though a zodi of 1 appears atypically low, with most observed stellar systems having (far) higher values.

3/ Constraining the composition of material delivered to newly formed planets

4/ Investigating what fraction of stellar systems have large planets in wide orbits (Jupiter & Neptune analogues)

5/ Observing how protoplanetary disks make Solar System architectures and their relationship with protoplanets.

6/ Measuring the reflectivity of giant planets and constraining their compositions.

7/ Demonstrating advanced space coronagraphic imaging

A small and light telescope requiring only a small and cheap launcher to get it to its efficient but economic observation point in a 2000 Kms “sun synchronous” Low Earth Orbit – whereby the telescope would be in a near-polar orbit such that its position with respect to the Sun would remain the same at all points, allowing orientation of its solar panels and field of view to enable near continual viewing. Viewing up to 350 circumstellar & protoplanetary disks and related giant planets, visualised out to a hundred parsecs in 230 star systems.

The giant planets would be “cool” Jupiters and Neptunes located within seven to ten parsecs and orbiting between 0.5-7 AU from their host stars – often in the stellar habitable zone.

No big bandwidths, the coronagraph will image at just two wavelengths, 0.4 and 0.8 microns. Short optical wavelength to maximise coronagraph IWA and utilise an economic CCD sensor. The giant planets will be imaged for the first time (with a contrast well beyond any theoretical maximum from even a high performance ELT) with additional information provided via follow up RV spectroscopy studies – or Gaia astrometry for subsequent concepts. Circumstellar disks have been imaged before by Hubble but its older coronagraphs don’t allow anything like the same detail and are orders of magnitude short of the necessary contrast and inner working angle to view into the habitable zones of stars.

High contrast imaging in visual light is thus necessary to clearly view close-in circumstellar and protoplanetary disks around young and nearby stars, looking for their reaction with protoplanets and especially for the signature of water and organic molecules.

Exozodiacal light arises from starlight reflection from the dust and asteroid/cometary rubble within a star system, material that along with the disks above plays a big role in the development of planetary systems. It also acts as an inhibitor of exoplanetary imaging by acting as a contaminating light source in the dark field created around a star by a coronagraph with the goal of isolating planet targets. Especially warm dust close to a star, e.g in its habitable zone, a specific target for EXCEDE, whose findings could supplement ground-based studies in mapping nearby systems for this.

The Spitzer and Herschel space telescopes (with ALMA on the ground) both imaged exozodiacal light/circumstellar disks but at longer infrared wavelengths and thus much cooler and consequently further from their parent stars. More Kuiper belt than asteroid belt. Making later habitable planet imaging surveys more efficient as above a certain level of “zodis” imaging will be more difficult (larger telescope apertures allow for more zodis) with a median value of 26 zodis for a HabEX 4m scope. Yet another cause of background imaging noise – cf Solar System “zodiacal” light – which is essentially the same light visible within the Solar System (see illustration).

EXCEDE payload:

0.7m unobscured off-axis lightweight telescope

Fine steering mirror for precision pointing control

Low order wavefront sensor for focus and tip/tilt control

MEMs deformable mirror for wavefront error control (see below)

PIAA coronagraph

Two band imaging polarimeter

EXCEDE as originally envisaged used a Phase Induced Amplitude Apodisation PIAA coronagraph (see illustration), which also has a high throughput ideal for a small 0.7m off-axis telescope.

It was proposed to have an IWA of 144 mas at 5 parsecs in order to image in or around habitable zones – though not any terrestrial planets. However, this type of coronagraph has optics that are very difficult to manufacture and technological maturity has come slowly despite its great early promise (see illustration). To this end it has to be for the time being superseded by other less potent but more robust and testable coronagraphs such as the Hybrid Lyot (see illustration for comparison) earmarked for WFIRST and more recently the related VVC’s greater performance and flexibility. Illustrations of these are available for those who are interested in their design and also as a comparison. Ultimately though one way or the other they block or “reject” the light of the central star and in doing so create a dark hole in the telescope field of view in which dim objects like exoplanets can be imaged as point sources, mapped and then analysed by spectrometry. These are exceedingly faint. The dimmest magnitude star visible to the naked eye has a magnitude of about 6 in good viewing conditions. A nearby exoplanet might have a magnitude of 25 or less. Bear in mind that each successive magnitude is about 2.5 times fainter than its predecessor. Dim!

Returning to the VVC, a variant of it could be easily substituted instead, without impacting excessively on what remains a robust design and practical yet relevant mission concept. Off-axis silicon carbide telescopes of the type proposed for EXCEDE are readily available. Light, strong, cheap and being unobscured, these offer the same imaging benefits as HabEX on a smaller scale. EXCEDE’s three year primary mission should locate hundreds of circumstellar/protoplanetary discs and numerous nearby cool gas giants along with multiple protoplanets – revealing their all important interaction with the disks. The goal is quite unlike ACEsat, a similar concept telescope, which I have described in detail before [see ACEsat: Alpha Centauri and Direct Imaging]. The latter prioritized finding planets around the two principal Alpha Centauri stars.

The EXCEDE scope was made to fit a NASA small Explorer programme $170 million budget, but could easily be scaled according to funding. Northrop Grumman manufactures them up to an aperture of 1.2m. The limited budget excludes the use of a full spectrograph, but instead the concept is designed to look at narrow visual spectrum bandwidths within the coronagraph’s etendue [a property of light in an optical system, which characterizes how “spread out” the light is in area and angle] that coincide with emission of elements and molecules from with the planetary or disk targets, water in particular. All this with a cost effective CCD-based sensor.

Starlight reflected from an exoplanet or circumstellar disk tends to be polarised, unlike direct starlight, and the use of a compact and cheap imaging polarimeter helps pick the targets out of the image formed at the pupil after the coronagraph has removed some but not all of the light of the central star. Some of the starlight “rejected” by the coronagraph is directed to a sensor that links to computers that calculate the various wavefront errors and other sources of noise before sending compensatory instructions to the optical pathway deformable mirrors and fast steering mirror to correct.

The all important deformable mirrors (manipulated from beneath by multiple mobile actuators) and especially the cheap but efficient new MEMs (micro-electro-mechanical mirrors) – 2000 actuators per mirror for EXCEDE, climbing to over 4096, or more, for the more potent HabEX. But yet to be used in space. WFIRST is committed to an older, less efficient “piezoelectric” alternative (more expensive) deformable mirror.

So this might be an ideal opportunity to show that MEMs work on a smaller, less risky scale with a big science return. MEMs may remain untested in space and especially the later more sensitive multi-actuator variety, but the more actuators, the better the wavefront control.

EXCEDE was originally conceived and unsuccessfully submitted in 2011. This was largely due to the immaturity of its coronagraph and related technology like MEMs at that time. The concept remains sound but the technology has now moved forward apace thanks to the incredible development work done by numerous US centres (NASA Ames, JPL, Princeton, Steward Mirror Lab and the Subaru telescope) on the Coronagraphic Instrument, CGI, for WFIRST. I am not aware of any current plans to resurrect the concept.

However the need remains stronger than ever and the time would seem to be more propitious. Exozodiacal light is a major impediment to exoplanet imaging so surveying systems that both WFIRST & HabEX will look at might save time and effort to say nothing of the crucial understanding of planetary formation that imaging of circumstellar disks around young stars will bring. Perhaps via a future NASA Explorer programme round or even via the European Space Agency’s recent “F class” $170 million programme call for submissions. Possibly in collaboration with NASA – whose “missions of opportunity” programme allows materiel up to a value of $55 million to supplement international partner schemes. The next F class gets a “free” ride, too, on the launcher that sends exoplanet telescopes PLATO or ARIEL to L2 in 2026 or 2028. Add in EXCEDE class direct imager and you get an L2 exoplanet observatory.

Mauna Kea in space if you will. By way of comparison, the overall light throughput of obscured WFIRST is just 2%!

The 72m HabEX starshade has an IWA of 45 mas and a throughput of 100% (as does the smaller version proposed for WFIRST) and requires minimal telescopic mitigation/adaptive optics as for coronagraphs. This also makes it ideal for the prolonged observation periods required for spectroscopic analysis of prime exoplanetary targets, where every photon counts. Be it habitable zone planets with HabEX or a smaller-scale proof of concept for a starshade “rendezvous” mission with WFIRST.

By way of comparison, the proposed EXO-S Probe Class programme (circa $1 billion) included an option for a WFIRST/Starshade “rendezvous” mission. This whereby a HabEX-like 30-34m self-propelled Starshade joins WFIRST at the end of its five year primary mission to begin a very much deeper three year exoplanet survey. Though considerably smaller than the HabEX Starshade, it also possesses the like benefits of high optical throughput (even more important on a non-bespoke obscured & smaller 2.4m aperture), a small Inner Working Angle (much less than with the WFIRST coronagraph), significantly reduced star/planet contrast and most important of all as we have already seen above, vastly reduced constraints on telescope stability & related wavefront control.

Bear in mind that WFIRST will still be using vibration-inducing reaction wheels for fine pointing. Operating at closer distances to the telescope than HabEX, the “slew” times between imaging would be significantly reduced too. This addition would increase the exoplanet return (both number and characterisation) many fold, even to the point of a small chance of imaging potentially habitable exoplanets. The more so if there have been the expected advances in performance of the software algorithms required to increase contrast post-processing (see above) and also to allow multi-star wavefront control that permits imaging of promising nearby binary systems (see below). Just a few tens of millions of dollars are required to make WFIRST “starshade” ready prior to launch and would keep this option open for the duration.

The obvious drawback with this approach is the long time required to manoeuvre into position from one target to the next along with the precision “formation flying” (stationed tens of thousands of kms from the starshade according to observed wavelength) required between telescope and starshade. For HabEX, this has a 250 km error margin in the back or forwards axis, but just 1m laterally and just one degree of starshade tilt.

So the observation strategy is done in stages. First the coronagraph searches for planets in each target star system over multiple visits, “epochs”, over the orbital period of the erstwhile exoplanet. This helps map out the orbit and increases chances of discovery . The inclination of any exoplanetary system in relation to the solar system is unknown – unless it closely approaches 90 degrees (edge on) and displays exoplanetary transits. So unless the inclination is zero degrees (the system sits face on to the solar system and lies in the plane of the sky like a saucer seen face on), the apparent angular separation between an exoplanet and its parent star will also vary across the orbital period. This might include a period during which it lies interior to the IWA of the coronagraph – potentially giving rise to false negative results. Multiple observation visits helps compensate for this.

Once the exoplanet discovery and orbital characteristics are constrained, starshade-based observations follow up. With its far larger light throughput (near 100%) the extra light available allows detailed spectroscopy across a wide bandwidth and detailed characterisation of high priority targets. For HabEX, this will include up to 100 of the most promising habitability prospects and some representative other targets. Increasing or reducing the distance between the telescope and the starshade allows analysis across different wavelengths.

In essence “tuning in” the receiver, with smaller telescope/starshade separations for longer wavelengths. For HabEX, this extends from UV through to 1.8 microns in the NIR. The coronagraph can characterise too if required but is limited to multiple overlapping 20% bandwidths with much less resolution due to its heavily reduced light throughput.

Of note, presumed high priority targets like the Alpha Centauri, Eta Cassiopiae and both 70 and 36 Ophiuchi systems are excluded. They are all relatively close binaries and as both the coronagraph and especially the starshade have largish fields of view, the light from binary companions would contaminate the “dark hole” around the imaged star and mask any planet signal. (This is also an issue for background stars and galaxies too, though these are much fainter and easier to counteract.) It is an unavoidable hazard of the “fast” F2 telescope employed – F number being the ratio of focal length to aperture. A “slower”, higher F number scope would have a much smaller field of view, but would need to be longer and consequently even more bulky and expensive. F2 is another compromise, in this case driven largely by fairing size.

Are you beginning to see the logic behind JWST a bit better now? As we saw with ACEsat, NASA Ames are looking to perfect suitable software algorithms to work in conjunction with the telescope adaptive optics hardware (deformable mirrors and coronagraph) to compensate for this (contaminating starlight from the off-axis binary constituent).

This is only at an early stage of development in terms of contrast reduction, as can be seen in the diagram above, but proceeding fast and as software can be uploaded to any telescope mission at any time up to and beyond launch.

Watch that space.

So exoplanetary science finds itself at a crossroads. Its technology is now advancing rapidly but at a bad time for big space telescopes with the JWST languishing. I’m sure JWST will ultimately be a qualified success and its transit spectroscopy characterisation of planets like those around TRAPPIST-1 will open the way to habitable zone terrestrial planets and drive forward telescope concepts like HabEX. As will EXCEDE or something like it around the same time.

A delay that holds up its successors both in time but also in funding. But lessons have been learned, and are likely to be out to good use. Just at the time that exoplanet science is exploding thanks to Kepler, and with TESS only just started, PLATO to come and then the bespoke ARIEL transit spectroscopic imager telescope to follow on. No huge leaps so much as incremental but accumulating gains. ARIEL moving on from just counting exoplanets to provisional characterisation.

Then onto imaging via WFIRST before finally HabEX and characterisation proper. But that will be over a decade or more away and in the meantime expect to see smaller exploratory imaging concepts capitalising on falling technology and launch costs to help mature and refine the techniques required for HabEX. To say nothing of whetting the appetite and keeping exoplanets formally where they belong.

But to finish on a word of perspective. Just twenty five years or so ago, the first true exoplanet was discovered. Now not only do we have thousands with ten times that to come, but the technology is coming to actually see and characterise them. Make no mistake that is an incredible scientific achievement as indeed are all the things described here. The amount of light available for all exoplanet research is utterly minuscule and the pace of progress to stretch its use so far is incredible. All so quick too. Not to resolve them, for sure (that would take hundreds of scopes operating in tandem over hundreds of kms) but to see them and scrutinise their telltale light. Down to Earth-mass and below and most crucially in stellar habitable zones. Precision par excellence. Maybe to even find signs of life. Something philosophers have deliberated over for centuries & “imagined” at length, can now be “imaged” at length.

At the forefront of astronomy, the public consciousness and in the eye of the beholder.

References

“A white paper in support of exoplanet science strategy”, Crill et al: JPL, March 2018

“Technology update”, Exoplanet Exploration Program, Exopag 18, Siegler & Crill, JPL/Caltech, July 2018

HabEX Interim report, Gaudi et al, August 2018

EXCEDE: Science, mission technology development overview, Schneider et al 2011

EXCEDE technology development I, Belikov et al, Proceedings of SPIE, 2012

EXCEDE technology development II, Belikov et al, Proceedings of SPIE, 2013

EXCEDE technology development III, Belikov et al, Proceedings of SPIE, 2014

“The exozodiacal dust problem for direct imaging of ExoEarths”, Roberge et al, Publications of the Astronomical Society of the Pacific, March 2012

“Numerical modelling of proposed WFIRST-AFTA coronagraphs and their predicted performances”, Krist et al, Journal of Astronomical Telescopes, Instruments & Systems, 2015

“Combining high-dispersion spectroscopy with high contrast imaging. Probing rocky planets around our nearest stellar neighbours”, Snellen et al, Astronomy & Astrophysics, 2015

EXO-S study, Final report, Seager et al, June 2015

ACESat: Alpha Centauri and direct imaging, Baldwin, Centauri Dreams, Dec 2015

Atmospheric evolution on inhabited and lifeless worlds, Catling and Kasting, Cambridge University Press, 2017

WFIRST CGI update, NASA ExoPag July 2018

“Two decades of exoplanetary science with adaptive optics”, Chauvin: Proceedings of SPIE, Aug 2018

“Low order wavefront sensing and control for WFIRST coronagraph”, Shi et al Proceedings of SPIE 2016

“Low order wavefront sensing and control..for direct imaging of exoplanets”, Guyon, 2014

Optic aberration: Wikipedia, 2018

Tilt (optics): Wikipedia, 2018

“The Vector Vortex Coronagraph”, Mawet et al, Proceedings of SPIE, 2010

“Phase-induced amplitude apodisation complex mask coronagraph tolerancing and analysis”, Knight et al, Conference paper: Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, July 2018

“Review of high contrast imaging systems for current and future ground and space-based telescopes”, Ruane et al, Proceedings of SPIE 2018

“HabEX telescope WFE stability specification derived from starlight leakage”, Nemati, H Philip Stahl, Mark T Stahl et al, Proceedings of SPIE, 2018

Fast steering mirror: Wikipedia, 2018