To Take the Best Direct Images of Exoplanets With Space Telescopes, we’re Going to Want Starshades

Between 2021 and 2024, the James Webb (JWST) and Nancy Grace Roman (RST) space telescopes will be launched to space. As the successors to multiple observatories (like Hubble, Kepler, Spitzer, and others), these missions will carry out some of the most ambitious astronomical surveys ever mounted. This will range from the discovery and characterization of extrasolar planets to investigating the mysteries of Dark Matter and Dark Energy.

In addition to advanced imaging capabilities and high sensitivity, both instruments also carry coronagraphs – instruments that suppress obscuring starlight so exoplanets can be detected and observed directly. According to a selection of papers recently published by the Journal of Astronomical Telescopes, Instruments, and Systems (JATIS), we’re going to need more of these instruments if we truly want to really study exoplanets in detail.

This Special Section on Starshades includes papers (released between January and June of 2021) that address the latest science, engineering, research, and programmatic advances made with coronographs. Also known as Starshades, these instruments address one of the greatest challenges in exoplanet detection and characterization. To summarize, the vast majority of known exoplanets (4,422 confirmed to date) have been discovered using indirect means.

Exoplanet Studies

Of these methods, the most widely-used and effective are the Transit Method (Transit Photometry) and the Radial Velocity Method (Doppler Spectroscopy). In the former, astronomers monitor stars for periodic dips in brightness, which are a possible indication that an orbiting exoplanet (or several) is passing in front of the parent star (aka. transiting) relative to the observer.

In the latter, astronomers measure how a star moves back and forth (and how fast) to measure the gravitational influence of any satellites in orbit. Separately, these methods are also effective at determining an exoplanet’s radius (Transit) and its mass (Radial Velocity). Used together, they are the most effective means of confirming and characterizing exoplanets, as well as placing constraints on their potential habitability.

In rare cases, astronomers have been able to directly observe exoplanets by detecting starlight reflected by their atmospheres – aka. the Direct Imaging Method. Unfortunately, the vast majority of these planets were gas giants or had a long-period orbit around their star (or both). For smaller, rocky planets with shorter orbits (where Earth-like planets are more likely to be found), any light reflected from their atmospheres is likely to be washed out by their sun.

Hence why coronagraphs have been the subject of considerable research and development in recent years. In addition to instruments that can be integrated with observatories, NASA also intends to build a spacecraft that can work in conjunction with space telescopes to suppress obscuring starlight. These efforts are part of NASA’s Starshade project, which envisions a spacecraft with a deployable flower-shaped light shield.

The Special Section

Despite the progress that has been made in recent years in the development of the Starshade (and related technologies), news of these advancements tends to be scattered. For this reason, the editors of the special section – all of whom are members of NASA’s Starshade Technology and Science Working Group – collected 19 research papers that represent the most recent research (from Jan to June of 2021).

As the editors they state in the introductory paper, titled “Special Section on Starshades: Overview and a Dialogue“:

“The starshade is a technology that has seen rapid development and wide interest at many institutions. Many of the advances in this field are spread over many journals and meetings. As a result, they are difficult to collect in a single location in order to get a good view of the state of starshades.”

“As interest in developing starshade-based missions grows, we hope that this special section will serve as a tutorial, providing enough of a background for potential investigators who are not familiar with starshades to have a current overview of the field in one location.”

These papers are divided into six categories that correspond to four different areas of research, all of which are presented in part two – Summary and Contributions – of the Special Section. It begins with a rundown of the Starshade program and the types of missions it will enable, followed by a series of technology-related papers that considers the challenges of deployments and rendezvous, followed by a series of science and mission-related papers.

Summary and Contributions

Many of the papers address the particular challenges that Starshade will face when paired with a space telescope. For instance, Part Two – Deployment and Formation Flying – presents papers that examine the technical challenges of sending a mission to space in stowed formation and then deploying it once it reaches its destination – similar to what the James Webb will do once launched (currently scheduled for Nov. 2021).

Remaining in formation with a space telescope is also a major challenge, especially when the telescope is transitioning from one target to another. Here, the papers presented consider different guidance, navigation, and propulsion systems, determining that a chemical propulsion system that doesn’t require ground tracking and a laser beacon would be the optimal arrangement.

The importance of proper scheduling is also addressed, which is true for Starshade as much as the proposed Remote Occulter – an orbiting Starshade concept designed to work with ground-based telescopes. In sections three and four, Starlight Suppression and Performance Modeling and Solar Glint, another major challenge is addressed, which is the possibility of interference caused by zodiacal light and solar light reflected by Starshade‘s petals.

Exploring the Opportunities

Of equal importance are papers that explore the benefits of a Starshade mission when paired with next-generation telescopes like the JWST and RST, as well as the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR). These proposed missions will be optimized for Direct Imaging and the characterization of exoplanet atmospheres, and are likely to be paired with a Starshade concept (or have coronagraph instruments of their own).

Also of interest are the many papers that detail the Starshade Program and NASA’s efforts to bring the Starshade to Technology Readiness Level 5 (TLR 5) – an effort that is being overseen by the S5 Project. This part also details the efforts of the Starshade Exoplanet Data Challenge and the Roman Exoplanet Imaging Data Challenge (EIDC), both of which aim to connect the science requirements of future Starshade missions with specific performance parameters.

In the case of the former, researchers at NASA JPL have relied on synthetic images of exoplanet systems generated by the Starshade Imaging Simulation Toolkit for Exoplanet Reconnaissance (SISTER). This versatile tool, which is maintained by Caltech, is designed to provide accurate models of what exoplanet systems would look like when observed with the assistance of a Starshade.

The EIDC, meanwhile, is a community engagement effort led by the Turnbull CGI Science Investigation Team, which in turn is led by Margaret C. Turnbull – the renowned astrophysicist and research scientist Margaret C. Turnbull of the SETI Institute. Launched in 2019, the Roman EIDC basically simulated what the Nancy Grace Mission Telescope will see with (and without) the assistance of a Starshade Rendezvous Mission.

This is picked up later in Sections Five and Six – Exoplanet Detection and Observations – where research papers once again discuss the kinds of exoplanet systems future telescopes will see with Starshade‘s help. In some of these papers, example images were provided and specific targets – nearby star systems that are considered optimal candidates – were discussed.

The Observations section closes with three papers authored by different teams, all of which were led by lead researcher Eliad Peretz of NASA Goddard. Here, the research teams examined how effective space telescopes with Starshade would be at detecting and characterizing potentially habitable exoplanets (based on various observational conditions) and how ground-based observatories would also benefit from the Remote Occulter.

Artist’s concept of the NASA HabEx space telescope paired with the Starshade. Credit: Gaudi et al.

Fostering a Dialogue

The Special Section wraps things up with a bit of tutorial titled Dialogue on Starshades. In this section, questions are raised and addressed in the form of a dialogue between a hypothetical student (named Morgan Nemandi) and a Starshade “expert” (Urania Sage). The questions were based on actual questions that have been asked by members of the amateur astronomy and space exploration enthusiast communities.

The dialogue is also an homage to Galileo’s Dialogue Concerning the Two Chief World Systems (It: Dialogo sopra i due massimi sistemi del mondo), a treatise he published in 1632 that presented arguments in favor of Copernicus’ heliocentric model of the Universe. It was also presented in a format that emphasizes how many of the questions people have about space exploration are things they are not always comfortable asking (i.e., “things I am afraid to ask”).

This final part of the Special Section is organized in a four-day format, reflecting the original Dialogue. While Day One involves Morgan and Urania covering the basics of Starshade and the history of the project, Day Two has them addressing questions related to engineering and technology, Day Three covers questions of science, and Day Four deals with program-related questions.

While a timetable has not yet been released for the Starshade’s development, it is clear that NASA and other space agencies intend to pursue this technology. In the coming years, starlight suppressors and coronographs are likely to become an integral part of next-generation astronomy and exoplanet research. When paired with the James Webb, Roman, TESS, HabEx, and LUVOIR space telescope, the number of known rocky exoplanets will increase exponentially.

Starshade technology will also aid missions like the ESA’s proposed CHaracterising ExOPlanets Satellite (CHEOPS), PLAnetary Transits and Oscillations of star (PLATO), and Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) space telescopes. These missions will also be dedicated to completing the census of terrestrial exoplanets, characterizing their atmospheres, and finding evidence of life beyond our Solar System.

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