What’s ahead for exoplanet telescopes in space? Ashley Baldwin, who tracks today’s exciting developments in telescope technology, today brings us a look at how a dark energy mission, WFIRST, may be adapted to perform exoplanet science of the highest order. One possibility is the use of a large starshade to remove excess starlight and reveal Earth-like planets many light years away. A plan is afoot to make starshades happen, as explained below. Dr. Baldwin is a consultant psychiatrist at the 5 Boroughs Partnership NHS Trust (Warrington, UK), a former lecturer at Liverpool and Manchester Universities and, needless to say, a serious amateur astronomer.

by Ashley Baldwin

Big things have small beginnings. Hopefully.

Many people will be aware of NASA’s proposed 2024 WFIRST mission. Centauri Dreams readers will also be aware that this mission was originally identified in the 2010 Decadal Survey roadmap as a mission with a $1.7 billion budget to explore and quantify “dark energy”. As an add-on, given that the mission involved prolonged viewing of large areas of space, an exoplanet “microlensing” element as a secondary goal, with no intrusion on the dark energy component, was also proposed.

Researchers believed that the scientific requirements would be best met by the use of a wide-field telescope, and various design proposals were put forward. All of these had to sit within a budget envelope that also included testing, 5 years of mission operations costs and a launch vehicle. This allowed a maximum aperture telescope of 1.5 m, a size that has been used many times before, thus reducing the risk and need to perform extensive and costly pre-mission testing, as had happened with JWST. As the budget fluctuated over the years, the aperture was reduced to 1.3 m.

Then suddenly in 2012 the National Reconnaissance Office (NRO) donated two 2.4 m telescopes to NASA. These were similar in size, design and quality to Hubble, so had lots of “heritage”, but critically they were also were wide-field, large, and worked largely but not exclusively in the infrared. What an opportunity: Such a scope would collect up to three times as much light as the 1.3 m telescope.

Tuning Up WFIRST

At this point a proposal was put forward to add in a coronagraph in order to increase the exoplanet and science return of WFIRST, always a big driver in prioritising missions. This coronagraph is a set of mirrors and masks, an “optical train” that is placed between the telescope aperture and the “focal plane” in order to remove starlight reaching the focal plane while allowing the much dimmer reflected light of any orbiting exoplanet to do so, allowing imaging and spectrographic analysis. Given the proposed savings presented by using a “free” mirror it was felt that this addition could be made at no extra cost to the mission itself.

The drawback is that the telescope architecture of the two NRO mirrors is not designed for this purpose. There are internal supporting struts or “spiders” that could get in the way of the complex apparatus of any type of coronagraph, impinging on its performance (this is what makes the spikes on images in reflecting telescopes). The task of fitting a coronagraph was not felt to be insurmountable, though, and the potential huge science return was deemed worth it so NASA funded an investigation into the best use of the new telescopes in terms of WFIRST.

The result was WFIRST-AFTA, ostensibly a new dark energy mission with all of its old requirements but with the addition of a coronagraph. Subsequent independent analyses by organisations such as the NRC (National Research Council – the auditors), however, suggested that the addition of a coronagraph could increase the overall mission cost to over $2 billion. This was largely driven by the use of “new” technology that might need protracted and costly testing. The spectre of JWST and its damaging costs still looms and now influences space telescope policy. Although the matter is disputed, it is based on the premise that despite over ten years of research, the technology is still thought to be somewhat immature and in need of more development. A research grant was gifted to coronagraph testbed centres to rectify this.

Image: An artist’s rendering of WFIRST-AFTA (Wide Field Infrared Survey Telescope – Astrophysics Focused Telescope Assets). From the cover of the WFIRST-AFTA Science Definition Team Final Report (2013-05-23). Credit: NASA / Goddard Space Flight Center WFIRST Project – Mark Melton, NASA/GSFC.

Another issue was whether the WFIRST budget will be delivered in the 2017 budget. This is far from certain, given the ongoing requirement to pay off the JWST “debt”. Those who remember the various TPF initiatives that followed a similar pathway and were ultimately scrapped at a late stage will shiver in apprehension. And that was before the huge overspend and delay of JWST, largely due to the use of numerous new technologies and their related testing. Congress was understandably wary of avoiding a repeat.

This spawned a proposal for the two cheaper “backup” concepts I have talked of before, EXO-S and EXO-C. These are probe-class concepts with a $1 billion budget cap. Primary and backup proposals will be submitted in 2015. EXO-S itself involved using a 1.1 m mirror with a 34 m starshade to block out light, rather than the coronagraphs of WFIRST-AFTA and EXO-C. Its back-up plan proposed using the starshade with a pre-existing telescope, which with JWST already having been irrevocably ruled out, left only WFIRST. Meanwhile, a research budget was allocated to bring coronagraph technology up to a higher “technology readiness level”.

Recently sources have suggested that the EXO-S back up option has come under closer scrutiny, with the possibility of combining WFIRST with a starshade becoming more realistic. At present that would only involve making WFIRST “starshade ready”, which involves the various two-way guidance systems necessary to link the telescope and a starshade to allow for their joint operation. The cost is only a few tens of millions and easily covered by some of the coronagraph research fund plus the assistance of the Japanese space agency JAXA (who may well fund the coronagraph too, taking the pressure off NASA). This doesn’t allow for the starshade itself however, which would need a separate, later funding source.

Rise of the Starshade

What is a starshade? The idea was originally put forward by astronomer Webster Cash from the University of Colorado in 2006; Cash is now part of the EXO-S team led by Professor Sara Seager from MIT. The starshade was proposed as a cheap alternative to the high-quality mirrors (up to twenty times smoother than Hubble) and advanced precision wavefront control of light entering the telescope. A starshade is essentially a large occulting device that acts as an independent satellite and is placed between any star being investigated for exoplanets and a telescope that is staring at them. In doing so, it blocks out the light of the star itself but allows any exoplanet light to pass (the name comes from the fact that starshades were pioneered to allow telescopic imaging of the solar corona). Exactly the same principle as a coronagraph only outside the telescope rather than inside it. The brightness of a planet is not determined by its star’s brightness so much as its own reflectivity and radius, which is why anyone viewing our solar system from a distance would see both Jupiter and Venus easier than Earth.

Image: A starshade blocks out light from the parent star, allowing the exoplanet under scrutiny to be revealed. Credit: University of Colorado/Northrup Grumman.

Any exoplanetary observatory has two critical parameters that determine its sensitivity. How close it can cut out starlight to the star, allowing nearby planets to be imaged, is the so-called Inner Working Angle, IWA. For the current WFIRST plus starshade mission, this is 100 milliarcseconds (mas), which for nearby stars would incorporate any planet orbiting at Earth’s distance from a sun. Remember that Earth sits within a “habitable zone”, HBZ, determined by a temperature that allows the presence of liquid water on the planetary surface. The HBZ is determined by star temperature and thus size, so 100 mas unfortunately would not allow viewing of planets much closer to a smaller star with a smaller HBZ, such as a K-class star. A slightly less important parameter is the Outer Working Angle, OWA, which determines how far out the imager can see from the star. This applies in the narrow field of coronagraphs but not to starshades, enabling planetary imaging for many astronomical units from the star.

The second key parameter is “contrast brightness” or fractional planetary brightness. Jupiter-sized planets are, on average, a billion times dimmer than their star in visible light, and this worsens by ten times for smaller Earth-sized planets. The difference drops by about 100-1000 times in infrared light. The WFIRST starshade is designed to work predominantly in visible light, just creeping into infrared at 825 nm. This is because the most common and cheap light detectors available for telescopes, CCDs, operate best in this range. Thus to image a planet, the light of a parent star needs reducing by at least a billion times to see gas giants and ten billion to see terrestrial planets. WFIRST reduces starlight by exactly ten billion times before its data is processed, with an increase to 3 to the power of minus 11 after processing. So WFIRST can view both terrestrial and gas giant planets in terms of its contrast imaging. The 100 mas IWA allows imaging of planets in the HBZ of Sunlike stars and brighter K-class stars (whose HBZ overlap with larger stars).

By way of comparison, WFIRST/AFTA can only reduce the star’s light by a billion times, which only allows gas giants to be viewed. Its IWA is just inside Mars’ orbit, so it can see the HBZ only for some Sun-like stars or larger, as light entering the telescope diminishes with distance. EXO-C can do better than this, and aims to view terrestrial planets for nearby stars. It is limited instead by the amount of light it can collect with its smaller 1.5 m aperture, less than half WFIRST and less still when passed through the extended “optical train” of the coronagraph and the deformable mirrors used in the critical precision wavefront control.

One thing that affects starshades and indeed all forms of exoplanet imaging is that other objects appearing in an imaging field can mimic a planet although they are in fact either nearer or further away. Nebulae, supernovae and quasars are a few of the many possibilities. In general, these can be separated out by spectroscopy, as they will have spectra different from exoplanets that mimic their parent star’s light in reflection. One significant exception is “exozodiacal light”. This phenomenon is caused by the reflection of starlight from free dust within its system.

The dust arises from collisions between comets and asteroids within the system. Exozodiacal light is expressed as a ratio of its luminosity to that of the star’s luminosity, the so-called “zodis”, where one zodi is equivalent to the Sun’s system. For our own system this ratio is only 10 ^-7. In general the age of the system determines how high the zodiacal light ratio is, with older systems like our own having less as their dust dissipates. Thus zodiacal light in the Sol system is lower than in many systems, some of which have up to a thousand times the level. This causes problems in imaging exoplanets, as exozodiacal light is reflected at the same wavelength as planets and can clump together to form planet-like masses. Detailed photometry is needed to separate planets from this exozodiacal confounder, prolonging integration time in the process.

Pros and Cons of Starshades

No one has ever launched a starshade, nor has anyone ever even built a full-scale one, although large-scale operational mock-ups have been built and deployed on the ground. A lot of their testing has been in simulation. The key is that being big, starshades would need to fold up inside their launch vehicle to “deploy” once in orbit. Similar things have been done with conventional antennae in orbit, so the principle is not totally new. The other issue is one of so-called “formation” flying. To work, the shade must cast an exact shadow over the viewing telescope throughout any imaging session, where sessions could be as long as days or weeks. This requires precision control for both telescope and starshade to an accuracy of plus or minus one metre. For WFIRST to operate, the starshade/telescope distance must be 35 thousand kilometres. Such precision has been achieved before, but only over much shorter distances of tens of metres. Even so, this is not perceived as a major obstacle.

Image: Schematic of a deployed starshade. TOMS: Thermo-Optical Micrometeorite Shield. Credit: NASA/JPL/Princeton University.

The other issue is “slew” time. Either the telescope or starshade must be self-propelled in order to move into the correct position for imaging. For the WFIRST mission this will be the starshade. To move from one viewing position to another can take days, with an average time between viewings of 11 days. Although moving the starshade is time-consuming, it isn’t a problem — recall that first and foremost WFIRST is a dark energy mission, so the in-between time can be used to conduct such operations.

What are the advantages and disadvantages of starshades? To some extent we have touched on the disadvantages already. They are bulky, out of necessity requiring larger and separate launch vehicles, with unfolding or “deployment” in orbit, factors that increase costs. They require precision flying. I mentioned earlier that at longer wavelengths the contrast between planet and star diminishes, which suggests that viewing in the infrared would be less demanding.

Unfortunately, longer wavelength light is more likely to “leak” by diffraction around the starshade, thus corrupting the image. This is an issue for starshades even in visible light and it is for this reason they are flower shaped, with anything from 12 to 28 petals, a design that has been found to reduce diffraction significantly. The starshade will have a finite amount of propellant, too, allowing up to a maximum of 100 points before running out. A coronagraph, conversely, should last as long the telescope it is working in.

Critically, because of the unpredictable thermal environment around Earth as well as the obstructive effect of both the Earth and Moon ( and Sun), starshades cannot be used in Earth orbit. This means either a Kepler-like “Earth trailing” orbit ( like EXO-C) or an orbit at the Sun-Earth-Lagrange 2 null point, 1.5 million kms outward from Earth. Such orbits are more complex and thus costly options than the geosynchronous orbit originally proposed. WFIRST is also the first telescope to be made service/repair friendly. This is likely to be possible through robotic or manned missions, but a mission to L2 is an order of magnitude harder and more expensive.

The benefits of starshades are numerous. We have talked of formation flying. For coronagraphs, the equivalent, and if anything more difficult issue, is wavefront control. Additional deformable mirrors are required to achieve this, which add yet more elements between the aperture and focal plane. Remember the coronagraph has up to thirty extra mirrors. No mirror is perfect and even with a 95% reflectivity over the length of the “optical train,’ a substantial amount of light is lost, light from a very dim target. Furthermore, coronagraphs demand the use of light-blocking masks, as part of the coronagraph blocks light in a non-circular way, so care has to be taken that unwanted star light outside of this area does not go on to pollute that reaching the telescope focal plane.

Even in bespoke EXO-C, only 37% light gets to the optical plane and WFIRST-AFTA will be worse. It is possible to alleviate this by improving the quality of the primary mirror but obviously with WFIRST this is not possible. In essence, a starshade takes the pressure off the telescope and the IWA and contrast ratio depend largely on it rather than the telescope. Thus starshades can work with most telescopes irrespective of type or quality, without recourse to the complex and expensive wavefront control necessary with a coronagraph.

The Best Way Forward

In summary, it is fair to say that of the various options for a WFIRST/Probe mission, the WFIRST telescope in combination with a starshade is the most potent in terms of its exoplanetary imaging, especially for potentially habitable planets. Once at the focal plane, any light will be analysed by a spectrograph that, depending on the amount of light it receives, will be able to characterise the atmospheres of the planets it views. The more light, the more sensitive the result, so with the 2.4 m WFIRST mirror as opposed to the 1.1 m telescope of EXO-S, discovery or “integration” times will be reduced by up to five times. Conversely and unsurprisingly, the further the target planet, the longer the time to integrate and characterise it. This climbs dramatically for targets further than ten parsecs or about 33 light years away, but there are still plenty of promising stars in and around this distance and certainly up to 50 light years away. Target choice will be important.

At present it is proposed that the mission would characterise up to 52 planets during the time allotted to it during the mission. The issue is which 52 planets to image? Spectroscopy tells us about planetary atmospheres and characterises them but there are many potential targets, some with known planets, others, including many nearer targets, with none. Which stars have planets most likely to reveal exciting “biosignatures”, potential signs of life ?

It is likely that astronomers will use a list of potentially favourable targets produced by respected exoplanet astronomer Margaret Turnbull in 2003. Dr. Turnbull is also part of the EXO-S team and first floated the idea of using a starshade for exoplanetary imaging with a concept paper on a 4 m telescope, New Worlds Observer, at the end of the last decade. This design proposed using a larger, 50 m starshade with an inner working angle of 65 mas, within Venus’ orbit so effective for planets in the habitable zone around smaller, cooler stars than the sun.

To date, the plan is to view 13 known exoplanets and look at the HBZ in 19 more. The expectation is that 2-3 “Earths” or “Super-Earths” will be found from this sample. This is where the larger aperture of WFIRST comes into its own. The more light, the greater the likelihood that any finding or image is indeed a planet standing out from background light or “noise”. This is the so called Signal to Noise ratio, SNR, that is critical to all astronomical imaging. The higher the SNR, the better, eliminating the possibility of false results. A level of ten is set for the WFIRST starshade mission for spectroscopic findings that characterise exoplanetary atmospheres. Spectroscopic SNR is a related concept and is described by a spectrograph’s ability to separate spectral lines due, for example, to a biosignature gas like O₂ in an atmosphere. For WFIRST this is set at 70, which while sounding reasonable is tiny when compared to the value used for searching for exoplanets using the radial velocity method, which runs into the hundreds of thousands.

Given the lack of in-space testing, the 34 m of EXO-S is nearest to the scaled model deployed in Earth at Princeton University, and thus the lowest but effective risk-size. The overall idea was considered by NASA but rejected on grounds of cost at a time of JWST development. Apart from this, only “concept” exoplanet telescopes like ATLAST, an 8 m plus scope, have been proposed with vague and distant timelines.

So now it is up to whether NASA feels able to cover making WFIRST starshade-ready and willing to allow a shift of orbit for WFIRST from geosynchronous orbit to L2. That decision could come as early as 2016, when NASA decides if the mission can go forward to see if it can get its budget. Then Congress will need to be persuaded that the $1.7 billion budget will stay at that level and not escalate. Assuming WFIRST is starshade-ready, the battle to fund a starshade will hopefully have begun once 2016 clearance is given. Ironically, NASA bid processes don’t cover starshades, so there would need to be a rule change. There are various funding pools whose timeline would support a WFIRST starshade, however. No one has ever built one, but we do know that the costs run into the hundreds of millions, especially if a launch is included, as it would be unless the next generation of large, cheap launchers like the Falcon Heavy can manage shade and telescope in one go. Otherwise the idea would be for the telescope to go up in 2024, with starshade deployment the following a year once the telescope is operational.

Meanwhile there are private initiatives. The BoldyGo Institute proposes launching a telescope, ASTRO-1, with both a coronagraph and starshade. This telescope will be off-axis, where the light from the primary is directed to a secondary mirror outside of the telescope baffle before being redirected to a focal plane behind the telescope. This helps reduce the obstruction to the aperture caused by an on-axis telescope, where the secondary mirror sits in front of the primary and directs light to a focal plane through a hole in the primary itself. This hole reduces effective mirror size and light gathering even before the optical train is reached and is the feature of a bespoke coronagraphic telescope. For further information consult the website at boldlygo.org.

Exciting but uncertain times. Coronagraph or no coronagraph, starshade or no starshade? Or perhaps with both? Properly equipped, WFIRST will become the first true space exoplanet telescope — one actually capable of seeing the exoplanet as a point source — and a potent one despite its Hubble-sized aperture. Additionally, like Gaia, WFIRST will hunt for exoplanets using astrometry. Coronagraph, starshade, micro lensing and astronomy — four for the price of one! Moreover, this is possibly the only such telescope for some time, with no big U.S. telescopes even planned. Not so much ATLAST as last chance. Either way, it is important to keep this item in the public eye as its results will be exciting and could be revolutionary.

The next decade will see the U.S. resume manned spaceflight with the SpaceX V2 and NASA’s own ORION spacecraft. At the same time, missions like Gaia, TESS, CHEOPS, PLATO and WFIRST will discover literally tens of thousands of exoplanets, which can then be characterised by the new ELTs and JWST. Our exoplanetary knowledge base will expand enormously and we may even discover biosignatures of life on some of them. Such ground-breaking discoveries will lead to dedicated “New Worlds Observer” style telescopes that with the advent of new lightweight materials, autonomous software and cheap launch vehicles, will have larger apertures than ever before, allowing even more detailed atmospheric characterisation. In this way the pioneering work of CoRoT and Kepler will be fully realised.